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1
Varri, A. L. "DYNAMICS OF GLOBULAR CLUSTERS." Doctoral thesis, Università degli Studi di Milano, 2012. http://hdl.handle.net/2434/203358.
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Context and motivation: Globular star clusters have long been considered the ideal astrophysical systems for the study of stellar dynamics. For such stellar systems, the relevant two-body relaxation times are typically shorter than their age, so that it can be argued that they are close to a thermodynamically relaxed state. Indeed, as a zeroth-order dynamical description, the class of models defined as a truncated Maxwellian distribution function (King models), supplemented by the assumption of spherical symmetry, have had remarkable success in the application to observed globular clusters. In fact, the great progress recently made in the acquisition of detailed photometric and kinematic information on the structure of globular clusters as well as the improvements in computational speed of the codes for performing N-body simulations and the availability of accelerator hardware call for a renewed effort in theoretical modeling.
Main results: Driven by these motivations, the present Thesis is devoted to the study of such quasi-relaxed stellar systems, with the aim of providing a more realistic dynamical paradigm in which fundamental physical ingredients such as the external tidal field, internal rotation, and weak anisotropy in the velocity space are properly taken into account. The main results can be summarized as follows:
(i) Self-consistent triaxial tidal models: As a generalization of the above mentioned spherical King models, we constructed a family of triaxial models in which the deviations from sphericity are induced by the presence of an external tidal field, taken into account self-consistently. By considering the simple case of a cluster in circular orbit within a host galaxy, the equilibrium distribution function is obtained from the one describing the spherical models by replacing the energy integral with the relevant Jacobi integral in the presence of the stationary tidal field. The construction of the models requires the solution of a singular perturbation problem for the relevant Poisson equation. A full characterization of the resulting configurations in terms of the relevant intrinsic and projected properties has been given and the range of the predicted flattening is consistent with that observed in most Galactic globular clusters.
(ii) Self-consistent axisymmetric rotating models: By following general statistical mechanics considerations, we constructed a family of rigidly rotating models defined as an extension of the King models to the case of axisymmetric equilibria, flattened by solid-body rotation. The relevant distribution function depends only on the Jacobi integral associated to the internal rotation; the structure of the models is determined by solving the relevant Poisson equation with the same perturbation method discussed for the tidal models, since the corresponding singular perturbation problem is formally equivalent.
In addition, we also considered a second family of models characterized by differential rotation, designed to be rigid in the central regions and to vanish in the outer parts. In this case the relevant Poisson equation is solved by a spectral iteration method, based on the Legendre expansion of the density and the potential. A full description of the photometric and kinematic observables has been provided and the models in the moderate rotation regime seem particularly suited to the description of the observed rotating star clusters. For general interest in stellar dynamics, we also studied the models in the strong rotation regime, which tend to show a central toroidal structure.
(iii) Dynamical stability of rotating stellar systems: By means of specifically designed N-body simulations with a direct numerical code (Starlab), a full stability analysis of the family of differentially rotating models has been performed. Configurations in the rigid and moderate differential rotation regime are found to be dynamically stable; curiously, there also exists an intermediate rotation regime in which the systems exhibit a central toroidal structure and are dynamically stable. In turn, a new dynamical instability, characterized by a variety of unstable Fourier modes of the density distribution, is observed in models with strong rotation and high degree of shear, in striking analogy with recent stability analyses of differentially rotating fluids with polytropic equations of state. The excitation of an unstable mode seems to be triggered by the presence of the relevant corotation point inside the rotating configuration. This result may help to clarify the physical motivation of the ``empirical'' Ostriker & Peebles stability criterion for rotating stellar systems.
(iv) Long-term evolution of rotating stellar systems: The long-term dynamical evolution of the differentially rotating models, studied as isolated systems, has been investigated by means of a comprehensive survey of N-body simulations. This study clarifies how the presence of global angular momentum affects the evolution of stellar systems with respect to the traditional paradigm for the dynamical evolution of nonrotating models and enriches the results obtained in the context of Fokker-Planck evolutionary models with rotation. In particular, by comparing the evolution of several rotating models with selected nonrotating models, characterized by the same initial structural properties, we found that rotating configurations reach core collapse more rapidly. Following early investigations, we also interpreted the evolution of a rotating system by distinguishing between a (short) initial phase, in which the gravo-gyro instability takes place and subsequently levels off, and a second phase in which the residual rotation no longer affects the dynamical evolution of the system, which experiences the gravothermal catastrophe and reaches core collapse, as it happens for nonrotating configurations.
(v) Observational signatures of internal rotation in Galactic globular clusters: We successfully applied the family of differentially rotating models to the interpretation of the structure and kinematics of three Galactic globular clusters, characterized by the presence of internal rotation, namely omega Cen, 47 Tuc, and M15. The selection of the relevant models has been performed by a method which combines a number of physically-based kinematic criteria with a statistically rigorous best-fit procedure for the determination of the relevant dimensionless parameters and physical scales of the configuration, respectively.
(vi) Pressure anisotropy as signature of partial relaxation in Galactic globular clusters: We have carried out a photometric and kinematic study of sample of Galactic globular clusters in different relaxation conditions, by means of King and f_nu models. The latter is a family of radially-biased spherical models, explicitly constructed for violently relaxed elliptical galaxies. The study suggests that less relaxed clusters tend to conform to the picture of formation via incomplete ``violent relaxation'', that is, the process associated to the rapid fluctuations of the gravitational potential during the early collapse phase of a self-gravitating system.
2
Masters, Craig Eugene Ashman Keith M. "The low metallicity globular clusters." Diss., UMK access, 2005.
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Thesis (M.S.)--Dept. of Physics. University of Missouri--Kansas City, 2005."A thesis in physics." Typescript. Advisor: K. M. Ashman Vita. Title from "catalog record" of the print edition Description based on contents viewed June 26, 2006. Includes bibliographical references (leaves 54-56). Online version of the print edition.
3
Hopwood, Madelaine E. L. "Interstellar matter in globular clusters." Thesis, Keele University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.323681.
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Goldsbury, Ryan. "White dwarf populations in globular clusters." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/57366.
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This thesis details three distinct projects that explore stellar populations in Milky Way globular clusters. In the first, a method of modelling mass segregation in clusters is presented. The model is fit to 54 clusters and the best fit parameters are presented in tabular form. The newly derived parameter that indicates the amount of mass segregation correlates strongly with other dynamical cluster parameters. In the second study, white dwarf data in the cluster 47 Tucanae are used to construct an empirical relation between temperature and time for these stars. The modified data are compared to theoretical cooling models from four different research groups. We find disagreement between all of the models and the data. The models are also inconsistent with each other. In the
third investigation, new UV white dwarf data in 47 Tuc is used to constrain the hydrogen mass fraction and neutrino production rates in cooling white dwarfs. A much different approach from the second project is used. The data are left untouched and the model is transformed to the space in which the data exist. Using the unbinned maximum likelihood statistic, the model’s parameter space is explored with MCMC sampling. A constraint on the rate of neutrino production in white dwarfs comes from this analysis.Science, Faculty of
Physics and Astronomy, Department of
Graduate
5
Dixon, Richard Igor John Dmitri Asoka Thomas. "Infra-red observations of globular clusters." Thesis, University of Edinburgh, 1992. http://hdl.handle.net/1842/27915.
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Near-infra-red imaging observations of the two highly reddened globular clusters M4 and M71 are presented. These data represent the deepest K observations yet published for any cluster. The photometric accuracy and repeatability of the infra-red camera IRCAM is investigated. Observations made at wavelengths of 1.25 μm (J) and 2.2 μm (K) show clearly that the camera-array combination is capable of a photometric accuracy of better than 1% on bright stars, with no evidence of residual non-linearity of > 2% over 6 magnitudes. The reduction procedure is discussed in detail. The profile fitting package DAOPHOT produces accurate and repeatable results from these small frames with large pixels. Zero-point calibrations are examined, and the data are combined with optical data. M71: Fitting new (V-K)-V and (V-K)-K isochrones finds for the four free parameters: E(B-V)=0.26 ± 0.03; (m-M)o=12.85 ± 0.15; age=13 ± 1 Gyr and [Fe/H]=-0.78 ± 0.3, which agree very well with the adopted values from the literature. Combining these with the unified literature results gives the best yet estimates of the cluster parameters: E(B-V)=0.266 ± 0.015; (m-M)o=12.87 ± 0.07; age=14.5 ± 2 Gyr and [Fe/H]=-0.7 ± 0.4. No significant binary fraction is detected on the main sequence. M4: An extensive re-assessment of the literature concludes that a value of R to 4.0 ± 0.2 is appropriate and that the 'best' literature parameters are: E(B-V)=0.37 ± 0.01; (m-M)o=11.22 ± 0.11 ; age=16 ± 2 Gyr and [Fe/H]=-1.1 ± 0.25. The Alcaino and Liller (1984) V magnitudes are found to contain a zero-point error. Allowing for this, isochrone fitting finds an age of 16 ± 3 Gyr and [Fe/H]=-1.0 ± 0.4.
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Pierce, Michael John, and n/a. "Spectroscopy of extra-galactic globular clusters." Swinburne University of Technology, 2006. http://adt.lib.swin.edu.au./public/adt-VSWT20070731.104253.
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The focus of this thesis is the study of stellar populations of extra-galactic glob-
ular clusters (GCs) by measuring spectral indices and comparing them to simple
stellar population models. We present the study of GCs in the context of tracing
elliptical galaxy star formation, chemical enrichment and mass assembly.
In this thesis we set out to test how can be determined about a galaxy's formation
history by studying the spectra of a small sample of GCs. Are the stellar population
parameters of the GCs strongly linked with the formation history of the host galaxy?
We present spectra and Lick index measurements for GCs associated with 3 el-
liptical galaxies, NGC 1052, NGC 3379 and NGC 4649. We derive ages, metallicities
and a-element abundance ratios for these GCs using the x2 minimisation approach
of Proctor & Sansom (2002).
The metallicities we derive are quite consistent, for old GCs, with those derived
by empirical calibrations such as Brodie & Huchra (1990) and Strader & Brodie
(2004). For each galaxy the GCs observed span a large range in metallicity from
approximately [Z/H]=�2 to solar.
We find that the majority of GCs are more than 10 Gyrs old and that we can-
not distinguish any finer, age details amongst the old GC populations. However,
amongst our three samples we find two age distributions contrary to our expecta-
tions. From our sample of 16 GCs associated with the 1-2 Gyr old merger remnant
NGC 1052, we find no young GCs. If a significant population of GCs formed during
this merger we would expect those GCs to have low mass-to-light ratios and be
included in our sample of bright GCs. We find 4 young GCs in our sample of 38
around NGC 4649, an old massive cluster elliptical. There are no signs of recent
star formation and therefore we do not expect any GCs to have formed within the
galaxy. These results seem to indicate that the GC systems of elliptical galaxies are
not strongly associated with recent field star formation.
We find a correlation between the α-element abundance ratio and the metallicity
for all three samples. Using Thomas, Maraston & Korn (2004) models, we measure
much higher α abundance ratios for low metallicity GCs than high metallicity GCs.
With current data and models we are limited in both the accuracy and the detail
with which we can probe this relationship.
We suggest that there are some difficulties reconciling measured GC parameters
with our expectations and propose some future work which could help to resolve
these and other issues.
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Pierce, Michael John. "Spectroscopy of extra-galactic globular clusters." Australasian Digital Thesis Program, 2006. http://adt.lib.swin.edu.au/public/adt-VSWT20070731.104253/index.html.
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Thesis (Ph.D) - Swinburne University of Technology, Faculty of Information & Communication Technologies, 2006.A dissertation presented in fulfilment of the requirements of for the degree of Doctor of Philosophy, Faculty of ICT, Swinburne University of Technology, 2006. Typescript. Bibliography p. 90-99.
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McDonald, Iain. "Stellar mass loss in globular clusters." Thesis, Keele University, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.505654.
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This work investigates stellar mass loss in globular clusters. It comprises of optical and infra-red photometric imaging and spectroscopy, plus radio interferometry observations. I present mid-infrared spectroscopic observations of stars in the globular clusters 47 Tucanae and u Centauri, finding 47 Tuc VI (and possibly V18) and ω Cen V6 surrounded by circumstellar silicate dust. ω Cen V42 may also be surrounded by carbon-rich dust. Much of this work is devoted to finding the threshold for dust production and the mass-loss rates from cluster stars with both chromospherically- and dust- or pulsation driven winds. Using very-high-resolution optical photometry, I have identified the transition between the two driving regimes as being at earlier spectral types than in solar-metallicity stars, suggesting that pulsation and continuum-driving become the dominant wind drivers at around K5~M3, or ~1500 L.
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Noble, Richard Gareth. "Precision CCD photometry of globular clusters." Thesis, University of Leeds, 1987. http://etheses.whiterose.ac.uk/980/.
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The first part of this thesis presents a review of colour-magnitude photometry of Galactic globular clusters. A survey has been made of recently-published colourmagnitude photometry of globular clusters, and a self-consistent distance scale established for these data, using the method of subdwarf matching to the observed main sequences. The distances thus derived have been combined with other published colour-magnitude data to infer cluster ages and horizontal branch magnitudes. The relationship between cluster horizontal branch magnitude and metallicity is dicussed. In Chapter 2, a review is made of the photometric applications of charge coupled devices, giving particular attention to the aquisition and analysis of crowded field CCD images within globular clusters. Digital photometric techniques are discussed, including analytic profile fitting to stellar images. The major experimental part of this thesis concerns observations of the globular cluster w Centauri. These observations, their subsequent reduction, and photometric analysis, are discussed in Chapter 3. This chapter also contains a discussion of previous work on this cluster, and highlights its extremely unusual nature. The significant discovery of geometrically varying stellar profiles in the programme CCD images, and the techniques for their analysis, are discussed. The colour-magnitude data from these observations are discussed in Chapter 4. The distance to the cluster is determined, and it is shown that there exists an intrinsic spread in colour upon the cluster main sequence. Theoretical isochrones are employed to show that this spread in colour is consistent with a main sequence metallicity distribution similar to that seen in more highly-evolved stars within the cluster. The age of the cluster is determined as 16 +3 Gyr, by comparison with theoretical isochrones. This result is consistent with recent determinations for other clusters, and supports the theory that the Galactic globular cluster system is essentially coeval. The cluster main sequence luminosity function has been constructed, and is compared with recent observations and theory. In Chapter 5, a preliminary colour-magnitude study of the SMC cluster Lindsay 11 is presented.
10
Milone, Antonino. "Multiple stellar populations in globular clusters." Doctoral thesis, Università degli studi di Padova, 2009. http://hdl.handle.net/11577/3426620.
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Over the last few years HST observations have resulted in one of the most exciting and unexpected developments in stellar population studies: the photometric discovery of multiple generations of stars in several globular clusters (GC).
It began with the discovery that the main sequence (MS) of Omega Centauri splits for several magnitudes into two distinct branches (Anderson 1997, Bedin et al.2004) . Since, surprisingly the blue branch was found to have higher metallicity (Piotto et al. 2005), the only isochrones that are able to fit this combination of color and metallicity are highly enriched in helium (Y 0.38), relative to the dominant population that is presumed to have near primordial helium.
During my PhD, I contributed to a series of discoveries, mainly based on HST data of unparalleled photometric accuracy, that demonstrated that Omega Centauri is far to be unique among globular clusters. Many other GCs were found to have features in their colour-magnitude diagram indicating the presence of multiple stellar populations. The first of this series of globulars was NGC-2808 that manifests the presence of different populations of stars with a split of the MS into three well defined sequences, even thought its sub-giant branch (SGB) is very narrow, with no dispersion in the abundance of iron-peak elements (Chapter 2, Piotto et al.2007). Evidence of multiple populations has been found also in NGC-1851 (Milone et al.2008), this time in the form of a split in the SGB region. Moreover, the presence of a group of RGB stars with enhanced Sr and Ba and strong CN bands (Yong/Grundahl 2008), and the presence of a bimodal horizontal branch, agrees with the hypothesisof two stellar generations inferred by the observed SGB split.
In addition, several other massive globulars have splitted SGB:
NGC-6388, NGC-6715 (M54), NGC-6656 (M22), NGC-5286, NGC-7089 and NGC-362. Notably, each of these clusters has a different color magnitude diagram and chemical properties. Interestingly enough, in M22, two groups of stars, characterized by different -element and slightly different iron content (Marino et al. in preparation), were found along the RGB, in the same percentage of the two groups observed in the SGB.
The high accuracy ACS photometry has also revealed that the multiple stellar population phenomenon is not confined only to Galactic GCs. Mackey/Broby Nielsen (2007) suggest the presence of two populations in Large Magellanic Cloud (LMC) globular NGC-1846. Moreover, by studying a large number of intermediate-age clusters in the LMC, we found that 70% of the entire analized sample hosts multiple stellar populations.
All these findings are introducing the intriguing idea that the multiple stellar populations are a common feature among Gcs, challenging the paradigm of globulars hosting a unique population of stars. In this Thesis I will discuss our observational findings in the context of multiple populations in GCs found during my PhD and their implications in the theoretical modelson the formation and evolution of these objects.Le recenti osservazioni condotte da HST negli ultimi anni hanno portato a uno dei piu' eccitanti e sorprendenti sviluppi negli studi sulle popolazioni stellari: la scoperta di generazioni multiple di stelle in molti ammassi globulari. La prima di queste scoperte fu la sequenza principale di Omega Centauri che si divide in due distinti rami al livello di varie magnitudini. Poiche', inaspettatamente e' stato trovato che il ramo blu ha metallicita' maggiore (Piotto et al.2005), le sole isocrone in grado di riprodurre questa combinazione di colore e metallicita' , sono arricchite in elio (Y0.38) rispetto alla popolazione dominante che si suppone avere elio primordiale. Durante la mia tesi di dottorato, ho contribuito a una serie di queste scoperte, principalmente basate su dati HST di incomparabile precisione fotometrica, che hanno mostrato che Omega Centauri non e' cosi' peculiare tra gli ammassi globulari. Sono stati trovati molti altri ammassi globulari che mostrano evidenze, nel loro diagramma colore magnitudine, della presenza di diverse popolazioni di stelle. Il primo di questa serie di globulari e' stato NGC-2808 che manifesta la presenza di diverse popolazioni stellari con una separazione della sequenza principale in tre sequenze ben definite, anche se il ramo delle sub giganti e' molto stretto, e non ci sono evidenze per una dispersione nell'abbondanza degli elementi pesanti. Evidenze di popolazioni multiple sono state trovate anche in NGC -1851 (Milone et al.2008), questa volta nella forma di una separazione nella regione del ramo delle sub giganti. Inoltre, la presenza di un gruppo di giganti rosse sovrabbondanti in Sr e Ba e caratterizzate da bande del CN forti, e la presenza di un ramo orizzontale bimodale, sono in accordo con l'ipotesi di due generazioni stellari come indicato dalla bimodalita' del ramo delle sub-giganti. L'elevata precisione della fotometria ACS ha inoltre rivelato che il fenomeno delle popolazioni multiple non e' confinato solo agli ammassi globulari della nostra Galassia. Mackey /Broby Nielsen (2007) suggeriscono la presenza di due popolazioni nell'ammasso NGC-1846 nella Grande Nube di Magellano. Inoltre, dallo studio di un elevato numero di ammassi ad eta' intermedia nella Grande Nube, abbiamo trovato che il 70% del campione analizzato ospita popolazioni multiple. Tutte queste scoperte stanno affermando l'affascinante idea per cui la presenza di diverse popolazioni stellari sia una caratteristica comune tra gli ammassi globulari, abbattendo il paradigma per cui questi oggetti ospiterebbero un'unica popolazione di stelle. In questa Tesi discutera' le nostre scoperte osservative, nel contesto delle popolazioni multiple in ammassi globulari, trovate durante il mio dottorato e le loro implicazioni sui modelli teorici di formazione e l'evoluzione di questi oggetti.
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Holland, Stephen. "The globular clusters an halo of M31." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0029/NQ27163.pdf.
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Masters, Craig Eugene Ashman Keith M. "The formation of low metallicity globular clusters." Diss., UMK access, 2007.
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Thesis (Ph. D.)--Dept. of Physics and Dept. of Mathematics. University of Missouri--Kansas City, 2007."A dissertation in physics and mathematics." Advisor: Keith M. Ashman. Typescript. Vita. Title from "catalog record" of the print edition Description based on contents viewed Nov. 13, 2007. Includes bibliographical references (leaves 67-70). Online version of the print edition.
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Lützgendorf, Nora. "Intermediate-mass black holes in globular clusters." Diss., Ludwig-Maximilians-Universität München, 2013. http://nbn-resolving.de/urn:nbn:de:bvb:19-161011.
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This work is focused on the search for intermediate-mass black holes (IMBHs) in the centers of globular clusters. It has been demonstrated that supermassive black holes (SMBHs) at the centers of galaxies show a tight correlation between their mass (M•) and the velocity dispersion (σ) of the galaxy. Investigating this M• − σ and similar correlations is crucial to constrain scenarios of galaxy formation and evolution. If they formed by runaway collisions of massive stars in young and dense stellar clusters, IMBHs could still be present in the centers of globular clusters, today.
We measured the inner kinematic profiles for a sample of 9 galactic globular clusters using integral-field spectroscopy and combined them with existing outer kinematics and photom- etry obtained form HST archive images. In order to constrain the mass of a possible black hole we applied analytical Jeans models in combinations with varying M/LV profiles to each of the clusters. The results of these fits range from strong hints towards an IMBH (e.g. NGC 6388) to globular clusters which do not show any indications of a rising velocity dis- persion profile in their center (e.g. NGC 2808). Furthermore, the discovery of two high velocity stars in NGC 2808 opened another opportunity to study the internal kinematics of this particular cluster and indicates a high number of stellar-mass black holes in NGC 2808. We finally combined our results with measurements from the literature and investigated known scaling relations for SMBHs in galaxies (e.g. M• − σ) at the low-mass end by plac- ing the results and upper limits of IMBH measurements on these correlations. We found that IMBHs follow similar, but more shallow correlations of their mass and the properties of their host systems. This might be caused with the severe mass-loss the cluster suffers during its life time.
In addition we ran numerical N-body simulations and compared globular clusters with dif- ferent black-hole retention fractions, IMBH masses and binary fractions. We found that IMBHs lead to a higher ejection rate of massive stars so that clusters with less depleted mass functions might therefore be good candidates to host IMBHs at their centers. In the future more N-body simulations will be performed in order to reproduce our observations in a more sophisticated way and perform crucial tests to our observing and analysis methods. The search for IMBHs requires both high spatial and spectral resolution and will remain at the edge of feasibility. However, it is crucial to continue the investigations in order to shed light on black-hole formation and growth.Die vorliegende Arbeit befasst sich mit der Suche nach mittelschweren Schwarzen Löch- ern in Kugelsternhaufen. In den letzten Jahren gab es bei zahlreichen Beobachtungen von supermassiven schwarzen Löchern Hinweise auf einen engen Zusammenhang zwis- chen ihren Massen (M•) und den charakteristischen Eigenschaften der Galaxien (z.B. Geschwindigkeitsdispersion σ ). Die Betrachtung dieser Zusammenhänge trägt erheblich dazu bei, Bildung und Entwicklung massiver Schwarzer Löcher zu verstehen. Die Er- forschung masseärmerer Schwarzer Löcher hat dabei eine zunehmende Bedeutung erlangt, um die Gültigkeit der für supermassive Schwarze Löcher entdeckten Zusammenhänge an ihnen zu testen. Desweiteren gelten sie als hervoragende Kandidaten als sogenannte Keime für den Wachstum supermassiver Schwarzer Löcher zu fungieren und das Rätsel ihrer rapider Massenzunahme zu lösen. In den dieser Arbeit zugrunde liegenden Beobachtungen wurden kinematische Profile von neun galaktischen Kugelsternhaufen untersucht. Dabei wurde die integral-field spectroscopy verwendet, die zu den modernsten spektroskopischen Techniken gehört. Darüber hinaus vereinten wir photometrische Profile dieser Kugelsternhaufen, die mit den Hubble Space Telescope aufgenommen wurden, mit den spektroskopischen Aufnahmen und verglichen die Ergebnisse mit analytischen Modellen. Durch dieses Vorgehen wurde es möglich, für je- den Kugelsternhaufen die Masse oder die Obergrenze der Masse des zentralen Schwarzen Loches zu bestimmen. Steigt das kinematische Profil zum Zentrum des Haufens stärker an als aufgrund des photometrischen Profis zu erwarten wäre, so weist dies auf ein dunkles Objekt im Zentrum hin - ein Schwarzes Loch. Im Ergebnis zeigt sich, dass es sowohl Kugel- sternhaufen mit starken Anzeichen für ein zentrales Schwarzes Loch (wie zum Beispiel NGC 6388) als auch Kugelsternhaufen mit sehr flachem Profil (NGC 2808) und damit kaum Anzeichen für zusätzliche Masse im Zentrum des Haufens gibt. Schließlich haben wir die Ergebnisse unserer Messungen mit Werten aus der Literatur zur M• − σ - Relation superschwerer Schwarzer Löcher verglichen. Dabei konnte nachgewiesen werden, dass mit- telschwere Schwarze Löcher eine ähnliche, allerdings flachere Korrelation aufweisen. Diese Abflachung könnte ihre Ursache im starken Masseverlust der Kugelsternhaufen während ihrer Lebensdauer haben. Neben den Beobachtungen wurden sogenannte N-Körper Simulationen durchgeführt. Sie ermöglichen einen tiefen Einblick in die internen dynamischen Prozesse in Kugelsternhaufen. Sie geben Hinweise auf mögliche Fehlerquellen bei unseren Beobachtungsmethoden und bieten einen Vergleich zu den erhobenen Daten. Die Zukunft der Erforschung mittelschwerer Schwarzer Löcher liegt in der Entwicklung im- mer hochauflösender und sensiblerer Instrumente, die es ermöglichen, noch weiter entfernte Objekte auf Schwarze Löcher zu untersuchen.
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Brierley, Mita Leela. "High Resolution Spectral Models for Globular Clusters." Thesis, University of Canterbury. Physics and Astronomy, 2010. http://hdl.handle.net/10092/4963.
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This thesis covers the development of high-resolution model spectra of simple-stellar-populations (SSP) to be used in the measurement of the ages, metallicities and chemical abundances of unresolved extragalactic globular clusters (GCs). The models are compared to low- and high-resolution spectra of GCs in the Milky Way and M31 galaxies, whose properties are already known, to establish the effectiveness of both the SSP spectral grid and of the direct spectral fitting procedure employed in this work.
The model SSP spectra were created using Dotter et al. (2007) isochrones, populated using the flux derived from a grid of stellar spectra, weighted by the Kroupa (2001) mass function. Models with varying mass loss from the red giant branch and varying numbers of He-burning stars were generated. The spectral grid currently covers a parameter range of 2 to 15 Gyrs in age, and -2.5 to 0 dex in [Fe/H] at an [alpha/Fe] of +0.4 dex. Metallicities derived for Milky Way GCs from Lick index comparisons to the model grid are in good agreement with values in the literature.
The stellar spectral grid, from which the GC spectra are generated, has been created using ATLAS9 and SYNTHE. The spectra are at a resolution of R = 100,000 and cover a wavelength range from 3000 - 9000 Angstroms. Extensive work was undertaken in creating appropriate lists of atomic and molecular transition oscillator strength (log gf) values for this spectral grid. An automated program was created to alter the strengths of millions of atomic transition lines in the Kurucz atomic line lists to fit a model spectrum of appropriate parameters to that of the red-giant star Arcturus and to the Solar spectrum at shorter wavelengths (3000 - 3727 Angstroms). Comparisons to these observed spectra were made manually for several molecular lines and band-heads, and log gf values changed en-mass for all the lines of a given molecular species.
The SSP spectra were compared to low-resolution spectra of Milky Way GCs. Integrated-light spectra of a large number of Galactic GCs were obtained from three sources: the Schiavon et al. (2005) Library of Integrated Spectra of Galactic Globular Clusters, taken using the Ritchey-Chretien spectrograph on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory; spectra obtained through private communication with M. Bessell using the Double Beam Spectrograph on the 2.3m telescope at Siding Springs Observatory; and spectra obtained using the Robert Stobie Spectrograph on the 11m diameter Southern African Large Telescope. With resolutions of 1500 to 2800, abundances of individual elements could not be determined, but overall ages and metallicities were derived. The model spectra were fitted to the observed spectra using a Chi^2 minimisation procedure over large wavelength regions to fully utilise the information available in the spectra. Derived metallicity values were in agreement with literature values. However, age determinations were not consistent with those derived from photometric methods and had large associated uncertainties. The lack of age information in the spectra at such resolutions is a similar result to that found by other studies using the Schiavon data (eg. Mendel et al., 2007; Koleva et al., 2008).
The SSP spectral grid was used to determine ages, metallicities and individual elemental abundances of three clusters (GCM06, GC5 and GC10) in the outer halo of M31. High-resolution spectra from Keck-HIRES were obtained through private communication with D. Forbes. Age and metallicity determinations were made simultaneously by fitting un-blended FeI lines and the H-beta and H-gamma lines. Diagnostic analysis (such as that done by Colucci et al., 2009) and simultaneous fitting of the FeI lines alone gave unrealistic age values that tended towards the lower limits (2 Gyrs) of the age grid. The age and metallicities derived in this work for these clusters are consistent with those found by Alves-Brito et al. (2009) using the same data.
Abundances of a number of elements were derived from the high-resolution spectra. An overall enhancement of alpha-elements (from measurements of Ca, Si and Ti) was seen in all three clusters ([alpha/Fe] = 0.67 +/- 0.2, 0.63 +/- 0.2 and 0.5 +/- 0.2 dex for clusters GCM06, GC5 and GC10 respectively) which is greater than that found for other M31 GCs (Puzia et al., 2005; Colucci et al., 2009). A depletion in Mg compared to the other alpha-elements is seen, in accordance with patterns seen in both Milky Way and M31 GCs (Gratton et al., 2004; Colucci et al., 2009). All three clusters show varying levels of enhancements and depletion in the other measured elements (C, Sc, V, Cr, Mn, Ni, Ba), none of which follow the trends seen in Milky Way clusters. Comparisons to high-resolution spectra of Milky Way GCs, for which abundance ratios are known from the measurement of individual stars, need to be made to establish the accuracy of this elemental-abundance analysis.
Overall, the system presented in this thesis is well designed to be used in the analysis of integrated-light spectra from distant, unresolved GCs. The uncertainties in the derived ages are still larger than desired, but the metallicity determination is very consistent when tested against clusters of known metallicities.
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Beasley, Michael Andrew. "Globular clusters as probes of galaxy formation." Thesis, Durham University, 2001. http://etheses.dur.ac.uk/4949/.
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Observations and analysis of globular cluster systems associated with three galaxy types are presented. Spectroscopy of globular cluster (GC) candidates in the Sculptor spirals NGC 253 and NGC 55 has identified 15 GCs in these galaxies. This spectroscopic sample, combined with plate scans, indicates total GC populations consistent with that expected for their luminosity and morphological type. From these data, we define new GC samples for spectroscopy. Radial velocities of 87 GCs in the Virgo elliptical NGC 4472 have been obtained, yielding data for 144 GCs when combined with previous studies. We find the blue GCs have significantly higher velocity dispersion than the red GCs, with little rotation in either population. The GCs dispersion profile declines slowly, yielding mass profiles consistent with X-ray data. We find a steeply rising M/L ratio, indicative of a massive dark halo surrounding this galaxy. From line-strengths of the GCs, we derive ages and metallicities for the GCs using simple stellar population (SSP) models. We find that the GCs are old and coeval and the bimodality seen in then- colours reflects metallicity rather than age differences. The GCs exhibit solar abundance ratios and both subpopulations show evidence for radial metallicity gradients. We have obtained high S/N spectra for 64 star clusters in the Large Magellanic Cloud. We measure their Lick indices to test the age and metallicity calibration of SSP models by comparison with literature values. We find our metallicities are consistent, although the values from our integrated spectra are slightly higher. The agreement of the ages for the old GCs is good, but is somewhat poorer for the youngest clusters. We obtain an age-metallicity relation for the clusters consistent with the galaxy's field stars. We show first results of a project to investigate the age and metallicity distributions of globular cluster systems using semi-analytic models of galaxy formation.
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Goodwin, S. P. "The early dynamical evolution of globular clusters." Thesis, University of Sussex, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360496.
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Tudor, Vlad Andrei. "The Radio Source Population of Globular Clusters." Thesis, Curtin University, 2018. http://hdl.handle.net/20.500.11937/75837.
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Globular clusters host a myriad of dynamically-formed objects, some of which may be black hole X-ray binaries. The existence of such systems in globular clusters has yet to be confirmed, but deep radio observations may efficiently find them. The MAVERIC survey was conceived with this goal in mind. In this thesis I present its results, highlighting black hole candidates, contrasting them with neutron star systems, and gauging their nature.
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Weldrake, David Thomas Fredrick, and weldrake@mpia-hd mpg de. "Giant Planets and Variable Stars in Globular Clusters." The Australian National University. Research School of Astronomy and Astrophysics, 2005. http://thesis.anu.edu.au./public/adt-ANU20050616.191315.
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Over the last decade, 135 extrasolar planets have been discovered, the vast majority found by ongoing radial velocity searches. Of the stars sampled in these searches, 1% have `Hot Jupiter' planets associated with them. Having masses equivalent to Jupiter yet orbital periods of only a few days, this new class of planet is clearly unlike anything in our Solar System.¶
Hot Jupiters present us with an intriguing prospect. If the orientation of the planetary orbit is close to edge-on, the planet will periodically transit across the face of its star, resulting in a small drop in brightness. This transit phenomenon has been successfully used for planet detection over the last couple of years, allowing determination of the planetary radius and accurate mass estimates when coupled with radial velocity observations.¶
To aid understanding of the effect stellar environment plays on Hot Jupiter formation and survivability, this thesis presents the results of a wide-field search for transiting Hot Jupiters in the globular cluster 47 Tucanae. This cluster presents many thousands of stars in a moderate field of view and provides the perfect target for a search of this nature. One previous transit search has been made in the central core of 47 Tuc; using the HST for 8.3 continuous days, Gilliland et.al (2000) expected 17 transits yet found none. This null result suggests that either system metallicity or stellar density may be inhibiting Hot Jupiter formation or survivability in the cluster.¶
This thesis presents a search for transits with a field of view 250 times larger than the HST search and samples the uncrowded outer halo of the cluster (previously unsampled for transits), providing important constraints on the effect of environment on Hot Jupiter formation. If planets are found, then stellar density would seem responsible for the Gilliland et.al (2000) core null result. If no planets are found to a significant level, the survey would provide strong evidence that system metallicity is the dominant factor. Using the ANU 40'' (1m) telescope at Siding Spring Observatory, a 30.4 night observing run was executed and photometry was derived via differential imaging. The dataset numbers 109,000 cluster (and field) stars for photometric analysis, of which 22,000 are suitable for the transit search. With a custom-written transit detection algorithm and extensive Monte Carlo simulations to model the dataset, seven planets should be detectable if the occurrence rate of Hot Jupiters is the same in the cluster as in the Solar Neighbourhood.¶
Despite a detailed search, no transit signatures were identified. This result strongly indicates that the low metallicity of the cluster is the dominant factor inhibiting planet formation in 47 Tuc. Current results in the Solar Neighbourhood show that planet frequency is strongly biased towards stars of high metallicity. This thesis shows that the metallicity trend is likely a universal phenomenon, not only limited to the immediate Solar Neighbourhood and raises questions of whether planets were much rarer in the earlier Universe.¶
As a side result of the search, 100 variable stars were identified in the field, 69 of which are new discoveries. Subsequent analysis reveals a strong period segregation among the cluster eclipsing binaries, indicating previously unobserved dynamical effects in the cluster. Distance estimates for both 47 Tuc and the SMC are in agreement with previously published values and an independent identification of the binary period-colour relation was observed. Two binaries seem to have low-luminosity companions worthy of followup and one variable is likely a star in the early phases of planetary nebula formation. All of the results presented in this thesis have been published in three separately refereed research papers.
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McLaughlin, Dean E. "Star formation in molecular clouds and globular clusters." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp03/NQ30104.pdf.
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Goldsbury, Ryan. "An analysis of stellar populations in globular clusters." Thesis, University of British Columbia, 2011. http://hdl.handle.net/2429/37991.
Full textAbstract:
This thesis is composed of three chapters, as well as an introduction, which describe three distinct projects.
In Chapter 2 we present new measurements of the centers for 65 Milky Way globular clusters. Centers were determined by fitting ellipses to the density distribution as well as the symmetry of the clusters. All of the determinations were done with stellar positions derived from a combination of two single-orbit Advanced Camera for Surveys images of the core of the cluster. We find that the ellipse-fitting method provides remarkable accuracy over a wide range of core sizes and density distributions, while the symmetry method is difficult to use on clusters with very large cores, or low density, requiring a larger field, or a very sharply peaked density distribution.
Chapter 3 deals with a re-analysis of previous work on white dwarf natal kicks, and expands on this to analyze the radial distributions of stellar populations in globular clusters at earlier stages of stellar evolution (earlier referring to pre-white dwarf). The effects of stellar incompleteness, and a method to account for this are discussed. Finally, the results of a statistical analysis of completeness corrected radial distributions in 56 globular clusters are presented. No significant evidence of kicks is found, however multiple clusters show evidence that stars along the horizontal branch have not relaxed since undergoing mass loss after leaving the main sequence.
In Chapter 4, we present a novel method for determining the distance to a star cluster by fitting spectral energy models to the spectral energy distributions of cluster white dwarfs in multiple filters. The statistics of our fitting method are discussed in detail. This approach results in a true distance modulus of (m-M)₀ = 13.35 ± 0.02 ± 0.06, which corresponds to a physical distance of 4.67 ± 0.04 ± 0.13 kpc. The first error given is random, and the second is systematic.
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Weldrake, David Thomas Frederick. "Giant planets and variable stars in globular clusters /." View thesis entry in Australian Digital Theses Program, 2005. http://thesis.anu.edu.au/public/adt-ANU20050616.191315/index.html.
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Johnston, Helen Margaret Kulkarni S. R. "Compact objects in the disk and globular clusters /." Diss., Pasadena, Calif. : California Institute of Technology, 1992. http://resolver.caltech.edu/CaltechETD:etd-09172008-152100.
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Anderson, Stuart Bruce Prince Thomas A. "A study of recycled pulsars in globular clusters /." Diss., Pasadena, Calif. : California Institute of Technology, 1993. http://resolver.caltech.edu/CaltechETD:etd-08282008-094151.
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Begin, Steve. "A search for fast pulsars in globular clusters." Thesis, Link to full text, 2006. http://hdl.handle.net/2429/69.
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Marino, Anna. "Multiple stellar populations in Globular Clusters: spectroscopic evidence." Doctoral thesis, Università degli studi di Padova, 2010. http://hdl.handle.net/11577/3427339.
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Globular cluster (GC) stars have occupied a fundamental role in our understanding of stellar evolution thanks to the assumption that their stars can be idealized as a simple stellar population, i.e. coeval and chemically homogeneous.
However, there is a growing body of observational facts which challenge this traditional view, revealing the presence of multiple stellar populations in GCs.
In this thesis I present my contribution to the study of multiple populations in GCs mainly from the spectroscopic point of view.
By taking advantage of high and mid-resolution spectra collected at various telescopes, I studied the chemical abundances for large sample of stars in the GCs, M4, M22, and Omega Centauri.
All the three studied cases show evidence of multiple populations, but they reveal themselves in different ways, indicating that these GCs have experienced different enrichment histories.
Thanks to the analysis of chemical content of stars belonging to distinct stellar populations, important informations have been obtained on the nature of the progenitors that enriched the intracluster medium from which the second generation formed.Il ruolo fondamentale che gli ammassi globulari (AG) hanno storicamente occupato negli studi di evoluzione stellare si deve all'assunzione per cui tutte le stelle di un ammasso costituiscano una popolazione stellare semplice, cioe' siano coeve e chimicamente omogenee. Tuttavia, una crescente quantita' di dati osservativi sta mettendo in crisi questa visione tradizionale, rivelando la presenza di popolazioni stellari multiple all'interno di uno stesso AG. In questa tesi discutero' il mio contributo, principalmente spettroscopico, allo studio delle popolazioni multiple in AG. Dall'analisi di spettri ad alta e media risoluzione ottenuti in diversi telescopi, ho determinato le abbondanze chimiche per vasti campioni di stelle negli AG M4, M22 e Omega Centauri. Tutti e tre i casi di AG studiati mostrano evidenze di popolazioni multiple, che si manifestano pero' in maniera differente, indicando che questi oggetti hanno subito una diversa evoluzione di arricchimento chimico. Grazie all'analisi del contenuto chimico di stelle appartenenti a popolazioni distinte, sono state ottenute importanti informazioni sulla natura dei progenitori che hanno arricchito il mezzo dal quale si e' formata la seconda generazione di stelle.
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ZOCCHI, ALICE. "OBSERVATIONAL TESTS ON THE DYNAMICS OF GLOBULAR CLUSTERS." Doctoral thesis, Università degli Studi di Milano, 2014. http://hdl.handle.net/2434/235122.
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Galactic globular clusters are nearly gas-free, self-gravitating stellar systems characterized by an apparently simple geometry, with finite size probably determined by tidal truncation. These unique properties make them excellent laboratories for studies of stellar dynamics, and ideal targets for N-body simulations. For a long time, they have been treated as spherically symmetric, nonrotating, isotropic systems. The spherical King (1966) models, constructed to match this physical picture, are usually considered as the correct zeroth-order dynamical reference model, and are sometimes successful in representing the observed characteristics of these systems.
In reality, this simple physical picture suffers from a number of limitations, which become more evident now that much improved observations have become available. In particular, deviations from sphericity have been observed and should be explained. Three physical ingredients are expected to affect the observed morphologies of stellar systems: internal rotation, pressure anisotropy, and external tides. A proper identification of the physical ingredients that shape the internal dynamics of globular clusters will lead to draw conclusions on their origin, and on the origin of their host systems.
In particular, a detailed characterization of the role played by internal rotation and pressure anisotropy in present-day globular clusters would be a crucial element to discriminate among different formation scenarios for this class of stellar systems; indeed, the main goal of this Thesis is to clarify the role of these two important dynamical factors.
For the purpose of giving a detailed and more realistic description of globular clusters, dynamical studies are an important counterpart to the stellar populations analyses often carried out for these systems. Dynamical studies are meaningful only when both photometric and kinematic data are taken into account, but unfortunately for globular clusters the application of dynamical models is frequently carried out only in relation to the available photometric profiles. This Thesis addresses this issue, and strongly supports the view that accurate kinematic data are crucial to provide a satisfactory description of these systems.
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Harris, William E., Stephanie M. Ciccone, Gwendolyn M. Eadie, Oleg Y. Gnedin, Douglas Geisler, Barry Rothberg, and Jeremy Bailin. "GLOBULAR CLUSTER SYSTEMS IN BRIGHTEST CLUSTER GALAXIES. III. BEYOND BIMODALITY." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/622870.
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We present new deep photometry of the rich globular cluster (GC) systems around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143 (Abell 2147), obtained with the Hubble Space Telescope (HST) ACS and WFC3 cameras. For comparison, we also present new reductions of similar HST/ACS data for the Coma supergiants NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to the radial limits of our data, comprising from 12,000 to 23,000 clusters per galaxy). The metallicity distribution functions (MDFs) of the GCs can still be matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes are separated by similar or equal to 0.8 dex, but the internal dispersions of each mode are so large that the total MDF becomes very broad and nearly continuous from [Fe/H] similar or equal to-2.4 to solar. There are, however, significant differences between galaxies in the relative numbers of metal-rich clusters, suggesting that they underwent significantly different histories of mergers with massive gas-rich halos. Last, the proportion of metal-poor GCs rises especially rapidly outside projected radii R >= 4 R-eff, suggesting the importance of accreted dwarf satellites in the outer halo. Comprehensive models for the formation of GCs as part of the hierarchical formation of their parent galaxies will be needed to trace the systematic change in structure of the MDF with galaxy mass, from the distinctly bimodal form in smaller galaxies up to the broad continuum that we see in the very largest systems.
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Lind, Karin. "Chemical analysis of globular star clusters: theory and observation." Diss., lmu, 2010. http://nbn-resolving.de/urn:nbn:de:bvb:19-120977.
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Bolte, Michael John. "New insights into globular clusters from main-sequence photometry /." Thesis, Connect to this title online; UW restricted, 1987. http://hdl.handle.net/1773/5424.
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Souza, Juliana Crestani Ribeiro de. "Spatial distribution of galactic globular clusters : distance uncertainties and dynamical effects." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2017. http://hdl.handle.net/10183/156776.
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Fornecemos uma amostra de 170 Aglomerados Globulares Galácticos (GCs) e analisamos as propriedades de sua distribuição espacial. Utilizando um vasto catálogo de nuvens escuras identificadas, listamos os GCs que estão atrás de uma ou mais delas e que podem estar submetidos a uma extinção mais complexa do que a considerada por mapas de extinção. Valores de incerteza em distância são obtidos da literatura recente e comparados com valores derivados de uma fórmula de propagação de erro. GCs são agrupados de acordo com características inusitadas, tais como idades relativamente jovens ou possível conexão com núcleos de galáxias anãs, de forma que o efeito desses grupos pode ser isolado na distribuição espacial geral. Adicionalmente, computamos o centróide da distribuição de GCs e estudamos como esse se relaciona com a distância ao centro da Galáxia. Considerando que uma formação galáctica via colapso monolítico é supostamente simétrica, investigamos assimetrias e como os valores de incerteza das distâncias as modificam. Velocidades espaciais e um potencial Galáctico são empregados para verificar se quaisquer assimetrias na distribuição espacial são devidas a objetos em movimento coerente, ou se são somente efeitos transientes.We provide a sample of 170 Galactic Globular Clusters (GCs) and analyse its spatial distribution properties. Using a comprehensive dust cloud catalogue, we list the GCs that are behind one or more identified dust clouds and could be subjected to a more complex extinction curve than extinction catalogues consider. Distance uncertainty values are gathered from recent literature and compared to values derived from an error propagation formula. GCs are grouped according to unusual characteristics, such as relatively young age or possible connection to dwarf galaxy nuclei, so that their effect on the general distribution can be isolated. Additionally, we compute the centroid of the GC distribution and study how it relates to the distance to the centre of the Galaxy. Considering that galactic formation via monolithic collapse is expected to be symmetrical, we probe asymmetries and how distance uncertainty values modify them. Spatial velocities and a Galactic potential are used to verify if any asymmetries in the spatial distribution are due to co-moving objects, or if they are merely transient effects.
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Johnson, Christian I., R. Michael Rich, Nelson Caldwell, Mario Mateo, John I. Bailey, Edward W. Olszewski, and Matthew G. Walker. "Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569." IOP PUBLISHING LTD, 2018. http://hdl.handle.net/10150/626525.
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Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high- resolution spectra from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch and HB stars in NGC 6569. We found the cluster to have a mean heliocentric radial velocity of -48.8 km s(-1) (sigma = 5.3 km s(-1); 148 stars) and <[Fe/H]> = -0.87 dex (19 stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant metallicity spread. NGC 6569 exhibits light- and heavy-element distributions that are common among old bulge/inner Galaxy globular clusters, including clear (anti) correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data suggest that NGC 6569 may be composed of at least two distinct populations, and the cluster's low <[La/Eu]> = -0.11 dex indicates significant pollution with r- process material. We confirm that both HBs contain cluster members, but metallicity and lightelement variations are largely ruled out as sources for the luminosity difference. However, He mass fraction differences as small as Delta Y similar to 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.
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Johnson, Christian I., Nelson Caldwell, R. Michael Rich, Mario Mateo, III John I. Bailey, Edward W. Olszewski, and Matthew G. Walker. "Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624431.
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NGC 5986 is a poorly studied but relatively massive Galactic globular cluster that shares several physical and morphological characteristics with "iron-complex" clusters known to exhibit significant metallicity and heavy-element dispersions. In order to determine whether NGC 5986 joins the iron-complex cluster class, we investigated the chemical composition of 25 red giant branch and asymptotic giant branch cluster stars using high-resolution spectra obtained with the Magellan-M2FS instrument. Cluster membership was verified using a combination of radial velocity and [Fe/H] measurements, and we found the cluster to have a mean heliocentric radial velocity of +99.76 km s(-1) (sigma = 7.44 km s(-1)). We derived a mean metallicity of [Fe/H] = -1.54 dex (sigma = 0.08 dex), but the cluster's small dispersion in [Fe/H] and low [La/Eu] abundance preclude it from being an iron-complex cluster. NGC 5986 has <[Eu/Fe]> = +0.76 dex (sigma = 0.08 dex), which is among the highest ratios detected in a Galactic cluster, but the small [Eu/Fe] dispersion is puzzling because such high values near [Fe/H] similar to -1.5 are typically only found in dwarf galaxies exhibiting large [Eu/Fe] variations. NGC 5986 exhibits classical globular cluster characteristics, such as uniformly enhanced [alpha/Fe] ratios, a small dispersion in Fe-peak abundances, and (anti) correlated light-element variations. Similar to NGC 2808, we find evidence that NGC 5986 may host at least four to five populations with distinct light-element compositions, and the presence of a clear Mg-Al anticorrelation along with an Al-Si correlation suggests that the cluster gas experienced processing at temperatures. greater than or similar to 65-70 MK. However, the current data do not support burning temperatures exceeding similar to 100 MK. We find some evidence that the first-and second-generation stars in NGC 5986 may be fully spatially mixed, which could indicate that the cluster has lost a significant fraction of its original mass.
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Nardiello, Domenico. "HST and ground-based analysis of multiple stellar populations in globular clusters." Doctoral thesis, Università degli studi di Padova, 2015. http://hdl.handle.net/11577/3424717.
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In the last decades spectroscopic and photometric evidence has proven that globular clusters host multiple stellar populations characterized by different
chemical abundances.
The aim of my thesis is to give a contribution in the analysis of multiple stellar populations in Galactic globular clusters, using observations collected both with ground-based telescopes and with the Hubble Space Telescope.
The first part of the thesis presents an overview regarding the topic of multiple stellar populations in Galactic globular clusters, from the first photometric and spectroscopic discoveries to the various theories developed to describe the formation and evolution of different stellar generations in a cluster.
Then a description of my work regarding multiple stellar populations in the globular clusters NGC 6121, NGC 6397, and NGC 6752 is given. Using ground-based FORS2@VLT observations of external regions of these three globular clusters, we identified multiple stellar populations in the main sequence of NGC 6121 and NGC 6752. This work has been possible thanks to high-precision photometry and
appropriate combinations of colors and magnitudes. We presented the radial distribution of the two stellar populations hosted by each globular cluster, combining our result for the external regions with the fraction of first and second stellar generation measured in the central regions using Hubble Space Telescope data. Both for NGC 6121 and for NGC 6752, we found that the radial distribution of the number ratio of the blue main sequence to the red main sequence is
almost flat inside ~17 arcmin from the center of each cluster. Hydrodynamical and N-body simulations for the formation and evolution of multiple stellar populations predict that second generation stars form in the inner regions of the cluster and are initially more concentrated than first generation stars. The subsequent long-term dynamical evolution, driven by two-body relaxation, gradually erases the initial differences in the first and second generation spatial distributions; this is the reason why, for long relaxation-time systems (e.g. omega Cen), second generation stars are concentrated in the globular cluster inner regions and retain some memory of the initial predicted spatial segregation. The relaxation-times of the globular clusters NGC 6121 and NGC 6752 are quite short (less then 1 Gyr), and therefore the two populations hosted by each cluster are today well-mixed, as demonstrated by their flat distribution.
In this work, we have also given an estimate of how much the second generation is enriched in helium with respect to the first generation, finding a mild (Delta Y ~ 0.02) difference between the two sequences for both the clusters.
I am a Co-I of the Hubble Space Telescope Treasury programme ``UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation'' (GO-13297, PI: G. Piotto). The second part of the thesis is based on this project. The aim of the Hubble Space Telescope treasury programme GO-13297 is the characterization and analysis of multiple stellar populations in a
sample containing 56 globular clusters, using UV/blue and optical filters.
Using the Hubble Space Telescope treasury programme data, we analyzed the multiple stellar populations of the metal-rich globular cluster NGC 6352. The combination of UV/blue and optical observations has made possible to split and follow the two stellar populations across all the evolutionary sequences of all color-magnitude diagrams. We estimated the enrichment in helium of the second stellar generation, finding Delta Y~0.03. Moreover, we developed an innovative technique to put an upper limit on the relative age between the two stellar generations, with a careful discussion of the impact on relative age of all possible sources of uncertainty in the cluster parameters, such as the error on \Delta Y or variations of metallicity and alpha-enhancement. Considering all these uncertainties together, we found that the two stellar populations of NGC 6352 are coeval within ~300 Myr. This result will be useful to constraint theoretical models of formation and evolution of multiple stellar populations in globular clusters.
Finally, in the last part of the thesis, I summarize the results presented in this work and present a list of works that I would like to realize in future.Negli ultimi decenni, prove spettroscopiche e fotometriche hanno dimostrato che gli ammassi globulari ospitano popolazioni stellari multiple caratterizzate da abbondanze chimiche differenti. Lo scopo della mia tesi e` dare un contributo nell'analisi delle popolazioni stellari multiple negli ammassi globulari della nostra Galassia, usando osservazioni acquisite sia con telescopi da terra che con l'Hubble Space Telescope. La prima parte della tesi e` una panoramica sul tema delle popolazioni stellari multiple negli ammassi globulari galattici, dalle prime scoperte fotometriche e spettroscopiche alle varie teorie sviluppate per descrivere la formazione ed l'evoluzione delle differenti generazioni stellari in un ammasso. In seguito, viene data una descrizione del mio lavoro sulle popolazioni stellari multiple negli ammassi globulari NGC 6121, NGC 6397 e NGC 6752. Usando le osservazioni da terra effettuate con FORS2@VLT delle regioni esterne di questi tre ammassi globulari, abbiamo identificato popolazioni stellari multiple nella sequenza principale di NGC 6121 e NGC 6752. Questo lavoro e` stato possible grazie a fotometria di alta precisione e combinazioni appropriate di colori e magnitudini. Abbiamo mostrato la distribuzione radiale delle due popolazioni stellari ospitate da ciascuno ammasso globulare, combinando il nostro risultato per le regioni esterne con la frazione di stelle di prima e seconda generazione misurate nelle regioni centrali con i dati dell'Hubble Space Telescope. Per entrambi gli ammassi NGC 6121 e NGC 6752, abbiamo trovato che la distribuzione radiale dei rapporti fra il numero di stelle nella sequenza principale blu e in quella rossa e` abbastanza piatta entro ~17 arcmin dal centro di ogni ammasso. Simulazioni idrodinamiche e a N-corpi riguardanti la formazione ed evoluzione delle popolazioni stellari multiple, predicono che le stelle di seconda generazione si formano nelle regioni interne dell'ammasso e sono inizialmente piu` concentrate rispetto alla prima generazione. La successiva evoluzione dinamica a lungo termine, guidata dal rilassamento a due corpi, cancella gradualmente le differenze iniziali tra le distribuzioni spaziali della prima e seconda generazione; questa e` la ragione per cui, in sistemi caratterizzati da tempi di rilassamento lunghi (per esempio omega Cen), le stelle di seconda generazione sono concentrate nelle regioni interne e conservano la memoria della segregazione spaziale iniziale predetta. I tempi di rilassamento degli ammassi globulari NGC 6121 e NGC 6752 sono abbastanza corti (meno di 1 Gyr), e percio` le due popolazioni ospitate da ogni ammasso sono oggi ben mescolate, come dimostrato dalla loro distribuzione spaziale piatta. In questo lavoro, diamo anche una stima di quanto la seconda generazione e` arricchita in elio rispetto alla prima generazione, trovando una leggera differenza (Delta Y ~0.02) tra le due sequenze per entrambi gli ammassi. Io sono uno dei Co-I del programma Treasury dell'Hubble Space Telescope "UV Legacy Survey of Galactic Globular Clusters: Shedding Light on Their Populations and Formation'' (GO-13297, PI: G. Piotto). La seconda parte della tesi e` basata su questo progetto. Lo scopo del programma Treasury GO-13297 dell'Hubble Space Telescope e` la caratterizzazione e analisi di popolazioni stellari multiple in un campione contenente 56 ammassi globulari, usando filtri UV/blu e ottici. Sfruttando i dati del programma Treasury dell'Hubble Space Telescope, abbiamo analizzato le popolazioni stellari multiple dell'ammasso globulare, caratterizzato da alta metallicita`, NGC 6352. La combinazione di osservazioni UV/blu e ottiche ha reso possibile individuare e seguire le due popolazioni stellari lungo tutte le sequenze evolutive dei diagrammi colore-magnitudine. Abbiamo stimato l'arricchimento in elio della seconda generazione, trovando Delta Y~0.03. Inoltre, abbiamo sviluppato una tecnica innovativa per porre un limite superiore sull'eta` relativa tra le due generazioni stellari, considerando attentamente l'impatto sull'eta` relativa di tutte le possibili sorgenti di incertezza nei parametri dell'ammasso, come l'errore su Delta Y o le variazioni di metallicita` e alpha-enhancement. Considerando tutte queste incertezze insieme, abbiamo trovato che le popolazioni stellari si NGC 6352 sono coeve entro ~300 Myr. Questo risultato sara` utile per vincolare i modelli teorici di formazione ed evoluzione di popolazioni stellari multiple negli ammassi globulari. Infine, nell'ultima parte della tesi, riassumo i risultati presentati in questo lavoro e presento una lista di lavori che vorrei realizzare in futuro.
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Huang, Hsiu-Hui. "X-ray and Optical observations of Globular Clusters and Pulsars." Diss., lmu, 2010. http://nbn-resolving.de/urn:nbn:de:bvb:19-123830.
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Buckley, David R. V. "Distances and ages of galactic globular clusters via infrared photometry." Thesis, University of Edinburgh, 1994. http://hdl.handle.net/1842/27479.
Full textAbstract:
One of the most promising routes to the formation of a quantitative model of the processes and timescales involved in transforming primordial, pre-galactic material into the galaxy as we know it today is the study of the 'fossil record', one important component of which is the galactic globular-cluster system. It is therefore vital to have information of the basic parameters - reddening, chemical composition, distance, age and internal and orbital dynamics - of many globular clusters. This thesis is concerned with the derivation of the distances and ages of the four galactic globular clusters M13 (NGC6205), M30 (NGC7099), NGC6752 and 47 Tuc (NGC104). To this end we have obtained infrared photometry of respectively 21, 344, 123 and 688 stars in these clusters. We have matched this photometry with existing optical photometry to form the optical-infrared colour magnitude diagrams which are the basic tool for this study. These colour-magnitude diagrams are to the best of our knowledge the deepest of their kind in existence, and in NGC6752 and 47 Tuc reach to some five V magnitudes below the main-sequence turn-off. The distances are derived using a sample of six metal-poor field subdwarf stars with accurate trigonometric parallax measurements, and a discussion is given of the sensitivity of these distances to various sources of error. We find that distances derived using the optical-infrared colour-magnitude planes are less sensitive to some systematic errors in the photometry than purely optical colour-magnitude diagrams, while distances determined in K, (B-V) are least sensitive to errors in the adopted ratio of total-to-selective extinction. Other sources of error considered are small photometric errors and errors in the adopted values of E[B-V] and [Fe/H]. For M13, M30, NGC6752 and 47 Tuc we derive distances of 7.1, 8.1, 4.0 and 4.4 kpc without Lutz-Kelker corrections.
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Apple, Rosemary K. "Homologous evolution in the post-collapse expansion of globular clusters." Thesis, University of Edinburgh, 2010. http://hdl.handle.net/1842/5720.
Full textAbstract:
We examine the evolution of globular star clusters, modelled as spherically symmetric stellar systems, using various techniques. Such clusters possess a central region of approximately uniform density which is referred to as the core. We concentrate our analysis on the evolution of the cluster after the core has undergone core collapse; a process where its radius decreases and its density increases. After this collapse, the system as a whole can expand in a self-similar fashion (homologous post-collapse evolution) which has long been thought to be due to gravitational interactions between different populations of single stars and binary stars in the core. We confirm this assumption by constructing a simple analytical model which combines much of the theoretical knowledge of previous research in the field. This model consists of two stellar populations, each defined by the mass of the individual stars, and a separate core. Our simple model is itself constructed from two simpler models – a twocomponent model without a core and a single mass model with a core – and takes into account the main gravitational interactions thought to drive the post-collapse evolution. To ensure that no important mechanisms have been neglected in our simple model, we will compare it with an N-body simulation. We compute our N-body models with NBODY6 (using a GPU version for large N). When we compare the N-body model with the simple model, we find qualitative agreement between them for most cases. Even though some mechanisms (e.g. escape of stars) are neglected in our simple model, we find that both models show homologous post-collapse evolution. We also review the homologous post-collapse Fokker-Planck model in the case of equal stellar masses derived by H´enon (1961) with the intention of extending this for the two-component case. We present our numerical solutions for H´enon’s model and find that our numerical solutions are in satisfactory agreement with the results shown in this paper. When we extend this work for a general two-component model (i.e. with no restriction on the number of heavier stars), we find that a homologous solution cannot be found with this approach. By contrast, we suggest that it would be possible to find a homologous two-component solution by extending the one-component solution published later by H´enon (1965), which differs from the earlier model by neglecting the external tidal field of the parent galaxy. Much of the work shown in this thesis would be relevant for such future study.
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Ridolfi, Alessandro [Verfasser]. "Long-term timing of pulsars in globular clusters / Alessandro Ridolfi." Bonn : Universitäts- und Landesbibliothek Bonn, 2017. http://d-nb.info/1149744871/34.
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Kennedy, David Colm John. "Optical and radio observations of interstellar gas towards globular clusters." Thesis, Queen's University Belfast, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263400.
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Zaritsky, Dennis, Denija Crnojević, and David J. Sand. "ARE SOME MILKY WAY GLOBULAR CLUSTERS HOSTED BY UNDISCOVERED GALAXIES?" IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621403.
Full textAbstract:
The confirmation of a globular cluster (GC) in the recently discovered ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question posed in the title. After estimating the halo mass of Eri II using a published stellar mass-halo mass relation, the one GC in this galaxy supports extending the relationship between the number of GCs hosted by a galaxy and the galaxy's total mass about two orders of magnitude in stellar mass below the previous limit. For this empirically determined specific frequency of between 0.06 and 0.39 GCs per 10(9)M(circle dot) of total mass, the surviving Milky Way (MW) subhalos with masses smaller than 10(10)M(circle dot) could host as many as 5-31 GCs, broadly consistent with the actual population of outer halo MW GCs, although matching the radial distribution in detail remains a challenge. Using a subhalo mass function from published high-resolution numerical simulations and a Poissonian model for populating those halos with the aforementioned empirically constrained frequency, we find that about 90% of these GCs lie in lower-mass subhalos than that of Eri II. From what we know about the stellar mass-halo mass function, the subhalo mass function, and the mass-normalized GC specific frequency, we conclude that some of the MW's outer halo GCs are likely to be hosted by undetected subhalos with extremely modest stellar populations.
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Dalessandro, Emanuele <1981>. "A panchromatic study of stellar populations in Galactic Globular Clusters." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2009. http://amsdottorato.unibo.it/1730/1/dalessandro_emanuele_tesi.pdf.
Full textAbstract:
The aim of this Thesis is to investigate (i) how common the bimodal Blue Straggler Stars (BSS)
radial distribution is in stellar clusters and (ii) which are the physical processes that can produce
this bimodality.
We discuss possible relations between the properties of the BSS radial distribution and the
dynamical state of the hosting clusters by making use of dynamical models and simulations.
When relevant, we also discuss the possible links with some
cluster "anomalies" and the effects of a massive object (like Imtermediate Mass Black Hole)
in the cluster center.
To this purpose we present the observational multiwavelength studies of the BSS populations and
their radial distributions in 5 GGCs: M5, M55, M2, NGC 2419 and NGC 6388.
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Dalessandro, Emanuele <1981>. "A panchromatic study of stellar populations in Galactic Globular Clusters." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2009. http://amsdottorato.unibo.it/1730/.
Full textAbstract:
The aim of this Thesis is to investigate (i) how common the bimodal Blue Straggler Stars (BSS)
radial distribution is in stellar clusters and (ii) which are the physical processes that can produce
this bimodality.
We discuss possible relations between the properties of the BSS radial distribution and the
dynamical state of the hosting clusters by making use of dynamical models and simulations.
When relevant, we also discuss the possible links with some
cluster "anomalies" and the effects of a massive object (like Imtermediate Mass Black Hole)
in the cluster center.
To this purpose we present the observational multiwavelength studies of the BSS populations and
their radial distributions in 5 GGCs: M5, M55, M2, NGC 2419 and NGC 6388.
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Saracino, Sara <1989>. "A Multi-Conjugate Adaptive Optics view of Bulge Globular Clusters." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amsdottorato.unibo.it/8631/1/Thesis_SSaracino_mar18.pdf.
Full textAbstract:
The Galactic bulge is one of the most massive and inaccessible regions of the Galaxy, where thick clouds of dust along the line of sight almost totally absorb the optical light. Bulge globular clusters (GCs) are useful tools to constrain the properties of this system, as they share the same kinematics and chemical composition with bulge field stars. However, these systems have been widely excluded from large surveys due to the huge extinction, and they remain in many cases only poorly known so far. The aim of this Thesis is to contribute to fill this gap by studying the stellar populations and structural parameters of bulge GCs by means of state-of- the-art high resolution near-infrared (NIR) instruments. We exploited the capabilities offered by the Multi-Conjugate Adaptive Optics system GeMS combined with the GSAOI imager at the Gemini South Telescope, in Chile. We mainly focused on two extreme cases. The first is Liller 1, one of the most obscured and therefore least studied systems in the Galaxy. It probe the benefit of using an Adaptive Optics system on an 8 meter telescope. The second is NGC 6624, which is well-studied and therefore allows a detailed comparison and assessment of the GeMS+GSAOI performance. Based on these data we performed an in-depth characterization of the instruments in terms of Point Spread Function modeling, Strehl Ratio and Encircled Energy distribution variations within the field of view. We derived also the first analytic solution to correct the geometric distortions of this system. Taking advantage of the exquisite quality and depth of the NGC 6624 images, in combination with deep HST NIR observations of 47 Tucanae, we performed a detailed analysis aimed at probing reliability and limits of the so-called "MS-knee" as a tool to derive accurate absolute ages in NIR CMDs of GC.
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Alexander, Poul Edwin Rennie. "The origin and evolution of the galactic globular cluster system." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708452.
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Hempel, Maren. "Early-Type Galaxies And Their Sometimes Not So Old Globular Clusters." Diss., lmu, 2004. http://nbn-resolving.de/urn:nbn:de:bvb:19-26974.
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Kavelaars, J. J. "Globular clusters as dynamical probes of the S0 galaxy NGC 3115." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape15/PQDD_0007/NQ27832.pdf.
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Simpson, Jeffrey. "Spectral Matching for Elemental Abundances of Evolved Stars of Globular Clusters." Thesis, University of Canterbury. Physics and Astronomy, 2013. http://hdl.handle.net/10092/8460.
Full textAbstract:
In order to understand the origin of globular clusters, large samples of their stars need to be observed and analyzed for their chemical composition. This is especially true for the complex, multimetallic cluster ω Centauri, with its large range of iron, carbon, nitrogen, oxygen, sodium and barium abundances. In order to accomplish this, an automated spectral matching pipeline was developed to determine these abundances. This thesis made use of photometry and low resolution spectroscopy to analyze the chemical composition of evolved stars in three clusters: ω Cen, 47 Tuc and NGC 6752. The latter two clusters are monometallic and selected due to their similar metallicities to the metal-rich and metal-poor stars in ω Cen. This allowed them to be used as test-cases for the spectral matching pipeline.
For ω Cen, two analyses were performed. In the first, 221 giant branch stars were selected that had known [O/Fe]. These stars showed the expected anticorrelation in [C/Fe]to [N/Fe]. In the second, spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen-rich or oxygen-poor. From this a catalogue of abundances of iron, carbon and barium of 848 giant branch stars were determined, of which 557 also had well-defined nitrogen abundances. k-means clustering analysis was used to group the stars in ω Cen into four homogeneous groups based upon these abundances. These groups suggest that there were at least four main periods of star formation in the cluster. The exact order of these star formation events is not yet understood, with some models predicting the groups formed from iron-poorest to iron-richest, while others suggest the potential for iron-poorer groups to form after iron-rich groups.
These results compare well with those found from higher resolution studies and show the value of more extensive lower resolution spectral surveys. They also highlight the need for large samples of stars when working with a complex object like ω Cen.
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Machin, Graham. "Cataclysmic variables in globular clusters and low mass X-ray binaries." Thesis, University of Oxford, 1990. http://ora.ox.ac.uk/objects/uuid:9cddbf39-034d-4d33-ad98-eecbe9bd60a7.
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Bradnick, Benjamin Thomas George. "On the dynamics in planetary systems, globular clusters and galactic nuclei." Thesis, University of Birmingham, 2017. http://etheses.bham.ac.uk//id/eprint/7826/.
Full textAbstract:
N-body simulations are used to investigate the dynamics of planetary systems based on the observed period-radius distribution by Kepler. The stability of the distri- bution is tested using integrations of 2,000 systems and with the addition of a Jupiter-like perturber in an aligned and inclined configuration sufficient for Lidov- Kozai (LK) oscillations. ∼ 67% of planetary systems are found stable, falling to ∼ 62% and ∼ 48% with an aligned or inclined giant perturber. Planet ejections are rare. Instability timescales of systems are predicted by spacing and multiplicity of planets, but exceptions are common. Evolution of select individual systems are investigated and classified. The dynamics of stellar binaries on eccentric orbits around a massive black hole (MBH) in the empty loss cone (LC) are also explored. The LK mechanism is sup- pressed by two-body relaxation from stars in galactic nuclei whilst tidal perturba- tions from the MBH excite the eccentricity of the binary to produce mergers in ∼ 75% of simulations. Stellar tides circularise the binaries and produce low velocity mergers. Enhanced magnetic fields in merger products could explain relativistic jet formation in tidal disruption events (TDEs). A method is presented for rapidly calculating the stellar evolution of stars with masses \[m=8.0-300.0M_\odot\] and metallicities \[-4.0\leq [Z/H]\leq 0.5\] that can be incorporated into future n-body simulations.
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Veljanoski, Jovan. "Globular clusters in the Local Group as probes of galaxy assembly." Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/9643.
Full textAbstract:
Understanding the formation and evolution of galaxies is one of the most active areas of research in astrophysics. Hierarchical merging of proto-galactic fragments to build more massive galaxies is the current preferred model. A key prediction of this theory is that haloes of nearby galaxies should contain remnants of this assembly process in the form of tidal debris. Found in all but the smallest of dwarf galaxies, globular clusters (GC) are excellent probes of galaxy haloes. Having high luminosities, they are favourable targets in the outer regions of galaxies where the associated stellar surface brightness is low. GCs are thought to be amongst the oldest stellar systems in the Universe, and are likely born in the most significant phases of galaxy formation. Their metallicities, ages, spatial distributions and kinematics can be used to constrain the assembly history of their host galaxy. In this thesis, I explore the photometric and kinematic properties of several GC systems in our cosmological backyard, the Local Group of galaxies. The work is based on a major spectroscopic campaign, follow-up to the photometric Pan- Andromeda Archaeological Survey (PAndAS), as well as additional optical and near-IR data sets. Radial velocities are obtained for 78 GCs in the halo ofM31, 63 of which had no previous spectroscopic information. The GCs have projected radii between ∼ 20 and 140 kpc, thus sampling the true outer halo of this galaxy. In addition, GCs in the dwarf galaxies NGC 147, NGC 185 and NGC 6822 are also spectroscopically observed. By conducting a detailed kinematic analysis, I find that GCs in the outer halo of M31 exhibit coherent rotation around the minor optical axis, in the same direction as their more centrally located counterparts, but with a smaller amplitude of 86 ± 17 km s−1. There is also evidence that the velocity dispersion of the outer halo GC system decreases as a function of projected radius from theM31 centre, and this relation can be well described by a power lawof index ≈ −0.5. I detect and discuss various velocity correlations amongst GCs that lie on stellar streams in the M31 halo. Simple Monte Carlo tests show that such configurations are unlikely to form by chance, implying that significant fraction of the GCs in the M31 halo have been accreted alongside their parent dwarf galaxies. I also estimate the dynamical mass of M31 within 200 kpc to be (1.2 − 1.6) ± 0.2 × 1012 M⊙. I also characterize the GC systems of three dwarf galaxies in the Local Group: the dwarf elliptical satellites of M31, NGC 147 and NGC 185, and the isolated dwarf irregular NGC 6822. Using uniform optical and near-IR photometry, I constrain the ages and metallicities of their constituent GCs. The metallicities of the GCs around NGC 147 and NGC 185 are found to be metal-poor ([Fe/H]. −1.25 dex), while their ages are more difficult to constrain. The GCs hosted by NGC 6822 are found to be old (>9 Gyr) and to have a spread of metallicities (−1.6 . [Fe/H] . −0.4). I find close similarity between the mean optical (V − I)0 colours of the GCs hosted by these three dwarf galaxies to those located in the M31 outer halo, consistent with the idea that dwarf galaxies akin to them might have contributed toward the assembly of the M31 outer halo GC population. Analysing their kinematics, I find no evidence for systemic rotation in either of these three GC systems. Finally, I use the available GC kinematic data to calculate the dynamical masses of NGC 147, NGC 185 and NGC 6822.
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Thomson, Grace. "A multi-wavelength, Hubble space telescope study of two globular clusters." Thesis, University of Southampton, 2013. https://eprints.soton.ac.uk/349262/.
Full textAbstract:
Globular clusters (GCs) are among the densest and oldest stellar aggregates in the Galaxy, and are thought to date from around the time that the Galaxy first formed. The high central densities that characterise GCs lead to frequent stellar interactions and the formation of exotic stellar populations, making GCs excellent laboratories for studying the stellar dynamics of dense environments. The ability to observe many stars which are equidistant and (approximately) the same age makes GCs invaluable in understanding stellar structure and evolution. This thesis describes surveys of two Galactic GCs: far- and near-ultraviolet data from the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope (HST) were used to study the core region of M80, and far-ultraviolet to Iband data from the Space Telescope Imaging Spectrograph (STIS), ACS and the Wide Field Camera 3 (WFC3) on-board HST were used to carry out an in-depth, multi-wavelength survey of NGC6752. In both studies, the properties of key stellar populations resident in globular clusters are investigated. In M80, it was discovered that the fainter (redder) blue straggler stars are more centrally concentrated than the brighter (bluer) ones. This is contrary to expectations, and suggests that blue stragglers might get a ‘kick’ at formation, before settling back towards the core. In a search for counterparts to known X-ray sources in M80, one X-ray source was shown to be the remnant of the classical nova T Scorpii. This source was undergoing a dwarf nova outburst during the observations. A variability study of the GC also revealed three variable sources, including an RR Lyrae that was observed around maximum brightness, an SXPhoenicis star with a ≈ 55minute period, and a longer period variable which might be another RR Lyrae or a Cepheid variable. In NGC6752, two known cataclysmic variables were revealed to be dwarf novae (DNe), which underwent outbursts during the observations. This takes the number of known DNe in NGC6752 to three, more than any other cluster. Some of the global parameters describing NGC6752 were also investigated. A new estimate of the cluster’s centre position was determined and used to show that the stellar density profile cannot be fit using a single ‘King’ profile, indicating that the cluster is in a core-collapsed state. Finally, a search for broadening in the main-sequence of the colour-magnitude diagram found evidence of small-scale broadening, suggesting the presence of multiple populations. A radial trend in the level of observed broadening was also suggested, with more broadening found in the core than the outer parts of the cluster.