Dissertations / Theses on the topic 'Large magellanic cloud'

Includes bibliographical references (leaves 67-69).
This work gives an account of the study of the metallicity [Fe/H] distribution (gradient) in the oldest population in the Large Magellanic Cloud (LMC), by making use of the available RR Lyrae data from the Optical Gravitational Lensing Experiment III (OGLE III). RR Lyrae stars are amongst the oldest objects in the universe and they have a range in element (metal) abundances. Measuring the distribution of metallicities of RR Lyrae stars in a galaxy gives one clues to the origin of galaxies. It is known that the pulsation periods of RR Lyraes is broadly correlated with their metallicity. This fact has been used for investigating the metallicity distribution of RR Lyrae stars in the LMC. I have found an indication that the proportion of metal poor RR Lyrae stars increases with distance from the centre of the LMC. In addition, an attempt was made to improve the metallicity-period relation by introducing the Fourier parameters, but this was unsuccessful. Lastly, a comparison is made with estimates of metallicity gradients of other LMC populations.

The Large and Small Magellanic Clouds are unique local laboratories for studying the formation and evolution of small galaxies in exquisite detail. The Survey of the MAgellanic Stellar History (SMASH) is an NOAO community Dark Energy Camera (DECam) survey of the Clouds mapping 480 deg2 (distributed over similar to 2400 square degrees at similar to 20% filling factor) to similar to 24th. mag in ugriz. The primary goals of SMASH are to identify low surface brightness stellar populations associated with the stellar halos and tidal debris of the Clouds, and to derive spatially resolved star formation histories. Here, we present a summary of the survey, its data reduction, and a description of the first public Data Release (DR1). The SMASH DECam data have been reduced with a combination of the NOAO Community Pipeline, the PHOTRED automated point-spread-function photometry pipeline, and custom calibration software. The astrometric precision is similar to 15 mas and the accuracy is similar to 2 mas with respect to the Gaia reference frame. The photometric precision is similar to 0.5%-0.7% in griz and similar to 1% in u with a calibration accuracy of similar to 1.3% in all bands. The median 5s point source depths in ugriz are 23.9, 24.8, 24.5, 24.2, and 23.5 mag. The SMASH data have already been used to discover the Hydra II Milky Way satellite, the SMASH 1 old globular cluster likely associated with the LMC, and extended stellar populations around the LMC out to R. similar to. 18.4 kpc. SMASH DR1 contains measurements of similar to 100 million objects distributed in 61 fields. A prototype version of the NOAO Data Lab provides data access and exploration tools.

The interaction between massive stars and the ISM is a fundamental process determining the structure and composition of the ISM. This work examines the stellar content and resulting dynamics of superbubbles in the LMC. We first show analytically that for 2 single-O star bubbles in M33, the evolution of wind power as the stars evolve is important in the bubble evolution. In a second prototype study, we find that the LMC superbubble DEM 152 shows evidence for sequential star formation, based on differing ages between the stars interior and exterior to the shell. We construct a numerical form of the standard Weaver et al. (1977) evolutionary model for wind-driven bubbles, and use the stellar census to compare the predicted shell evolution with the observed kinematics. There is a substantial discrepancy: shell's observed expansion velocity too large relative to its radius. I then find that the CMDs of the associations within 7 LMC superbubbles and 5 classical H II regions are indistinguishable. The HRDs, constructed with spectral types for 6 superbubble clusters, also appear similar to those in classical H II regions, implying that the shell formation timescale is shorter than the cluster evolutionary timescale. The stellar winds of the 1-2 most massive stars must therefore dominate the shell formation. The star-forming events for the superbubble associations are also no more extended in duration than that of other OB associations. The IMF slopes appear normal. Numerical modeling of the 6 superbubbles shows results falling into two distinct categories: "high-velocity" objects showing anomalous kinematics like DEM 152 and "low-velocity" objects which appear fairly consistent with the model. X-ray evidence suggests that the high-velocity objects have been accelerated by SNR impacts. Results for both categories imply an overestimate in the growth rate equivalent to an effective input power of up to an order of magnitude too large. I find that the superbubbles are likely to be struck and "burst" by such SNR impacts if the prior stellar wind power is log L(w) ≲ 37.8 erg s⁻¹. The interior coronal gas is then expelled by the pressure differential with the environment, which could greatly enhance the dispersal and distribution of the hot ionized medium. A minority of superbubbles with stellar wind power above the threshold are more likely to grow to the sizes of supergiant shells.

Classical Cepheids are key probes of both stellar astrophysics and cosmology as standard candles and pulsating variable stars. It is important to understand Cepheids in unprecedented detail in preparation for upcoming Gaia, James Webb Space Telescope (JWST) and extremely-large telescope observations. Cepheid eclipsing binary stars are ideal tools for achieving this goal, however there are currently only three known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions about the evolution of classical Cepheids because of an apparent age discrepancy between the Cepheid and its red giant companion. We show that the Cepheid component is actually the product of a stellar merger of two main sequence stars that has since evolved across the Hertzsprung gap of the HR diagram. This post-merger product appears younger than the companion, hence the apparent age discrepancy is resolved. We discuss this idea and consequences for understanding Cepheid evolution.

The Large Magellanic Cloud (LMC) is widely considered as the first step of the cosmological distance ladder, since it contains many different distance indicators. An accurate determination of the distance to the LMC allows one to calibrate these distance indicators that are then used to measure the distance to far objects. The main goal of this thesis is to study the distance and structure of the LMC, as traced by different distance indicators. For these purposes three types of distance indicators were chosen: Classical Cepheids,``hot'' eclipsing binaries and RR Lyrae stars. These objects belong to different stellar populations tracing, in turn, different sub-structures of the LMC. The RR Lyrae stars (age >10 Gyr) are distributed smoothly and likely trace the halo of the LMC. Classical Cepheids are young objects (age 50-200 Myr), mainly located in the bar and spiral arm of the galaxy, while ``hot'' eclipsing binaries mainly trace the star forming regions of the LMC. Furthermore, we have chosen these distance indicators for our study, since the calibration of their zero-points is based on fundamental geometric methods. The ESA cornerstone mission Gaia, launched on 19 December 2013, will measure trigonometric parallaxes for one billion stars with an accuracy of 20 micro-arcsec at V=15 mag, and 200 micro-arcsec at V=20 mag, thus will allow us to calibrate the zero-points of Classical Cepheids, eclipsing binaries and RR Lyrae stars with an unprecedented precision.

Four regions of massive star formation in the Large Magellanic Cloud (LMC) were observed for water and methanol maser emission and radio continuum emission. A total of 42 radio detections were made including 27 new radio sources, four water masers, and eight compact HII regions. The lobes of a radio galaxy were resolved for the first time, and the host galaxy identified. Seven sources were associated with known massive young stellar objects (YSOs). A multi-wavelength analysis using both the infrared and radio spectrum was used to characterize the sources. Mid-infrared color-magnitude selection criteria for ultracompact HII (UCHII) regions in the LMC are presented, yielding 136 UCHII region candidates throughout that galaxy. New maser detections identified two previously unknown massive YSOs. No methanol masers were detected, consistent with previous studies and supporting the hypothesis that the LMC may be deficient in these molecules. These discoveries contribute to the history of star formation in the LMC, which will lead to a better understanding of star formation in the Milky Way and throughout the universe.

This thesis presents the first population study of supernova remnants (SNRs) in one whole galaxy – the Large Magellanic Cloud (LMC) on submm and FIR wavelengths. The first part is about the dust production in supernovae (SNe) and SNRs, based on several observations of SN1987A on mm and submm wavelengths that I made using the ATCA and APEX telescopes. SN1987A is found to produce ∼0.7 M⊙ of dust, which is 2 orders of magnitude higher than the masses found in most of other SN/SNR observations. I constrained the spectral energy distribution (SED) of SN1987A, confirming Herschel data using better resolution, but did not manage to resolve the object. These data were used in the preparation of the ALMA observations (Kamenetzky et al. 2013; Indebetouw et al. 2014). The second part of the thesis is the population study of all LMC SNRs using Herschel and Spitzer data, which resulted in the conclusion that SNRs are significant dust destroyers. This conclusion is based on dust mass maps of SNRs and their surroundings which have shown that there is less dust within SNRs than outside. My study shows that a SNR in the LMC removes on average 4–6 M⊙. I conclude that SNRs might not be the main suppliers of dust in galaxies, and that it is possible that other sources of dust production are needed to explain the origin of the dust at high red-shifts. I estimate the mass of sputtered dust from all SNRs in LMC to be ∼373+746 249 M⊙, a dust destruction rate in the LMC of ∼0.037+0.074 −0.025 M⊙ yr−1 due to SNRs and an average lifetime for interstellar dust in the regions close to SNRs of ∼2+4 −1.3 × 107 yr.

The study of supernovae and the expanding remnants that form after the explosion is important to understanding star formation and the distribution of elements in the interstellar medium (ISM). In supernova explosions all elements beyond iron are synthesized and then distributed into the ISM, where new star formation is begun in the wake of the explosion. Examining the galactic sample of supernova remnants (SNRs) is limited by absorption and distance uncertainties. The Large Magellanic Cloud (LMC) contains over thirty SNRs at a known distance with little absorption. By examining the X-ray spectrum of a large sample of supernova remnants we can learn how they evolve and how the elements are distributed. This work examines the X-ray spectrum of eleven archival ASCA observations and three new Chandra observations of LMC remnants. The plasma that is responsible for the X-ray emission is heated by a shock wave created by the supernova explosion that expands into the ISM. The picture is further complicated by a reverse shock that is created at the interface between the ISM and the material ejected from the star by the explosion. The ejecta is comprised of elements made by stellar fusion, while the ISM is mostly hydrogen and helium. After the shock passes, the ions contain most of the thermal energy while the electrons are still cold. Coulomb interaction between the ions and electrons behind the shock heat the electrons and continue to ionize the heavy elements, until equilibrium is reached. Non-equilibrium ionization (NEI) models are used to define the thermal contributions to the spectrum. Existing models assume that hydrogen and helium are the sole source of electrons in the plasma. A new NEI model is introduced which accounts for the overabundance of heavy elements in the remnant ejecta and their contribution to the electron density. At each step in the ionization history of the plasma, the ionization fractions of ten elements are determined. This is a plane-parallel shock model that can account for non-Coulomb heating at the shock front and uses shock velocity as a parameter rather than shock temperature. The heavy element model is tested against the current models and used to fit the Chandra observations. Hard, non-thermal tails that are well described by a power law have been observed in the X-ray spectrum of several remnants. Synchrotron radiation observed in the radio regime for low energy electrons can be extrapolated to X-ray energies to account for high energy(> 1 TeV) electrons. By combining X-ray data with radio observation parameters we can constrain the maximum energies of the shock-accelerated electrons in the supernovae above which the electron spectrum must steepen. None of the remnants in this study have a maximum energy above 100 TeV. The heavy element NEI model has proven a valuable tool that will be released to the community. It determined the forward shock velocities of the Chandra observations to be 1018 km/s (0534-69.9), 1080 km/s (0548-70.4) and 1170 km/s (0453-69.5). SNR 0534-69.9 and SNR 0548-70.4 spectra show middle-aged remnants, type II and type Ia explosions respectively, with significant contributions from the ejecta inside the outer shell. The SNR 0453-68.5 observation indicates the presence of a pulsar wind nebula.

We present N-body and hydrodynamical simulations of the response of the Milky Way's baryonic disc to the presence of the Large Magellanic Cloud during a first infall scenario. For a fiducial Galactic model reproducing the gross properties of the Galaxy, we explore a set of six initial conditions for the Large Magellanic Cloud (LMC) of varying mass which all evolve to fit the measured constraints on its current position and velocity with respect to the Galactic Centre. We find that the LMC can produce strong disturbances - warping of the stellar and gaseous discs - in the Galaxy, without violating constraints from the phase-space distribution of stars in the Solar Neighbourhood. All models correctly reproduce the phases of the warp and its antisymmetrical shape about the disc's mid-plane. If the warp is due to the LMC alone, then the largest mass model is favoured (2.5 x 10(11) M-circle dot). Still, some quantitative discrepancies remain, including deficits in height of Delta Z = 0.7 kpc at R = 22 kpc and Delta Z = 0.7 kpc at R = 16 kpc. This suggests that even higher infall masses for the LMC's halo are allowed by the data. A comparison with the vertical perturbations induced by a heavy Sagittarius dSph model (10(11) M-circle dot) suggest that positive interference with the LMC warp is expected at R = 16 kpc. We conclude that the vertical structure of the Galactic disc beyond the Solar Neighbourhood may jointly be shaped by its most massive satellites. As such, the current structure of the Milky Way suggests we are seeing the process of disc heating by satellite interactions in action.

This study seeks to enhance our understanding of the relationship between massive stars and their environment. New William Herschel Telescope (WHT) imaging and spectroscopy of 13 outer Milky Way HII regions is presented in Chapter 2. Properties of the nebulae and stars were separately determined and compared for consistency. Results showed that the ionising photons from the star were not accounted for by some nebulae, by as much as ~85% in some cases. This fraction depended on the age of the region and indicated that photons were either escaping the region or being absorbed by dust. The VLT-FLAMES Tarantula Survey (VFTS) is introduced in Chapter 3 with analyses of the emission line stars. Spectroscopic variabilities were investigated to improve understanding of their binary nature. In comparison to O2 stars, their spectral lines were found to be less reliable for determining radial velocities. Further derivations of their stellar properties were made via a series of template model atmospheres. A census of the hot luminous stars within 30 Doradus of the LMC is compiled in Chapter 4, based on literature results, largely drawn from the VFTS. They were selected through specific photometric criteria of which 500 stars had spectroscopic classification while a further 222 were classified via their photometry. The spectroscopic completeness of hot luminous stars was estimated at ~85% although this fell towards the central cluster due to crowding. In Chapter 5, the integrated ionising output and wind luminosity of the stellar population was determined with the most massive stars (>100Msun) found to make the highest contributions, and just ten stars providing 28\% of the integrated value in both cases. Comparisons to the population synthesis code Starburst99 showed significant underestimates in the integrated feedback if such massive stars were omitted. The stellar ionising output of 30 Doradus exceeded that derived from the nebula, indicating that ~6% of ionising photons were escaping the region. A brief summary and potential avenues for future work are presented in Chapter 6.

The process of mass-loss from evolved stars is the single largest contributor of matter back into the ISM. Intense mass-loss during the AGB phase of low-intermediate mass stars via a radiatively-driven wind can lead the stars to become enshrouded in an optically-thick layer of dust which condenses out of an extended molecular atmosphere. This thesis attempts to gain further insights into the mass-loss process that is presently poorly understood. We used the Parkes radio telescope to observe dust-enshrouded AGB stars and supergiants in the LMC and SMC, deriving the speed of the superwind from the doublepeaked OH maser profiles. Out of 8 targets in the LMC we detected 5, of which 3 are new detections. Our results confirm the simple theory for radiatively driven winds, this verifies the scaling relations we use in determining mass-loss rates and allows us to speculate on the chemical enrichment at different metallicities. From investigating mass-loss from clusters in the Magellanic Clouds we find that the mass-loss rate increases with larger progenitor mass, possibly due to a dependence on the initial metallicity or the stellar luminosity. We investigate the dust-enshrouded carbon star LI-LMC 1813 in more depth and derive an accurate mass-loss rate and the stellar parameters, mass and metallicity. It is now one of the few AGB stars currently undergoing the superwind phase for which values for the fundamental astrophysical parameters are known. With the ESO Very Large Telescope we obtained 3-4/-Lm spectra of IR stars in the LMC. 28 of 30 targets are identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We find evidence for a high abundance of C2H2, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity which would explain the large population of carbon stars.

I have identified 51 young stellar object candidates in N206, an H II complex in the nearby Large Magellanic Cloud galaxy. Using archival images from the Spitzer Space Telescope, supplemented with other infrared and optical images, I located point sources in this region. I distinguished possible young stellar objects based on their spectral energy distributions, morphologies, and locations in color-magnitude space. I classified the young stellar object candidates based on their likelihood of being young stellar objects and based on their apparent evolutionary stages. The spatial distribution of these candidates in N206 indicates that star formation is being triggered in a giant molecular cloud in the region.

We present our new, fully automated method to detect and measure the ages of star clusters in nearby galaxies, where individual stars can be resolved. The method relies purely on statistical analysis of observations and Monte-Carlo simulations to define stellar overdensities in the data. It decontaminates the cluster color-magnitude diagrams and, using a revised version of the Bayesian isochrone fitting code of Ramirez-Siordia et al., estimates the ages of the clusters. Comparisons of our estimates with those from other surveys show the superiority of our method to extract and measure the ages of star clusters, even in the most crowded fields. An application of our method is shown for the high-resolution, multiband imaging of the Large Magellanic Cloud. We detect 4850 clusters in the 7 deg(2) we surveyed, 3451 of which have not been reported before. Our findings suggest multiple epochs of star cluster formation, with the most probable occurring similar to 310 Myr ago. Several of these events are consistent with the epochs of the interactions among the Large and Small Magellanic Clouds, and the Galaxy, as predicted by N-body numerical simulations. Finally, the spatially resolved star cluster formation history may suggest an inside-out cluster formation scenario throughout the LMC, for the past 1 Gyr.

The Milky Way (MW) and M31 both harbour massive satellite galaxies, the Large Magellanic Cloud (LMC) and M33, which may comprise up to 10 per cent of their host's total mass. Massive satellites can change the orbital barycentre of the host-satellite system by tens of kiloparsec and are cosmologically expected to harbour dwarf satellite galaxies of their own. Assessing the impact of these effects crucially depends on the orbital histories of the LMC and M33. Here, we revisit the dynamics of theMW-LMC system and present the first detailed analysis of the M31-M33 system utilizing high-precision proper motions and statistics from the dark-matter-only Illustris cosmological simulation. With the latest Hubble Space Telescope proper motion measurements of M31, we reliably constrain M33' s interaction history with its host. In particular, like the LMC, M33 is either on its first passage (t(inf) < 2 Gyr ago) or if M31 is massive (>= 2 x 10(12) M-circle dot), it is on a long-period orbit of about 6 Gyr. Cosmological analogues of the LMC and M33 identified in Illustris support this picture and provide further insight about their host masses. We conclude that, cosmologically, massive satellites such as the LMC and M33 are likely completing their first orbits about their hosts. We also find that the orbital energies of such analogues prefer an MW halo mass similar to 1.5 x 10(12) M-circle dot and an M31 halo mass >= 1.5 x 10(12)M(circle dot). Despite conventional wisdom, we conclude it is highly improbable that M33 made a close (< 100 kpc) approach to M31 recently (t(peri) < 3 Gyr ago). Such orbits are rare (< 1 per cent) within the 4s error space allowed by observations. This conclusion cannot be explained by perturbative effects through four-body encounters amongst the MW, M31, M33, and the LMC. This surprising result implies that we must search for a new explanation for M33' s strongly warped gas and stellar discs.

Red Supergiant (RSG) stars are the most luminous stars in the infrared sky. Their intrinsic luminosities combined with the low dust extinction observed in this regime makes these objects very attractive to study in the near-infrared (IR). In addition, RSGs are necessarily young objects, as they are tracers of recent star formation in extra-galactic systems. As the next generation of telescopes will be optimised for study in the near-IR, it is clear that, in the coming years, RSGs will play a prominent role in the way that astronomers probe the local Universe and out to larger distances with space-based observations. Therefore, it is vital to better our understanding of these objects now and develop the tools that will allow us to take full advantage of the suite of instrumentation that will become available in the near future. This thesis aims to further the understanding of RSGs by focusing on quantitative studies of near-IR spectroscopic observations. To this end, I develop an analysis technique that uses spectroscopic and photometric observations to estimate stellar parameters of RSGs. The observations are compared with synthetic spectra extracted from stellar model atmospheres, where departures from local thermodynamic equilibrium have been calculated for the diagnostic spectral lines. This technique is tested thoroughly on synthetic and real observations and is shown to reliably estimate stellar parameters in both regimes when compared with input parameters and previous studies respectively. Using the analysis routines developed in Chapter 3, in Chapter 4 I measure the chemistry and kinematics of NGC2100, a young massive cluster (YMC) of stars in the Large Magellanic Cloud, using near-IR spectroscopic observations of 14 RSGs taken with the new K-band multi-object spectrograph (KMOS). I estimate the average metallicity to be -0.43±0.10 dex, which is in good agreement with previous studies. I compare the observed location of the target RSGs on the Hertzsprung{Russell diagram with that of a Solar-like metallicity YMC and show that there appears to be no significant difference in the appearance of the RSGs in these two clusters. By combining the individual RSG spectra, I create an integrated-light cluster spectrum and show that the stellar parameters estimated, using the same technique as for individual RSGs, are in good agreement with the average properties of the cluster. In addition, I measure - for the first time - an upper limit of the dynamical mass of NGC2100 to be 15.2 X 10⁴Mʘ, which is consistent with the literature measurement of the photometric mass of the cluster. In Chapter 5, I present observations of RSGs in NGC6822, a dwarf irregular with a turbulent history, observed with KMOS. The data reduction process with KMOS is described in detail, in particular where the reduction has been optimised for the data. Stellar parameters are estimated using the technique presented in Chapter 3 and an average metallicity in NGC6822 of -0.55±0.13 dex is found, consistent with previous measurements of young stars in this galaxy. The spatial distribution of metallicity is estimated and weak evidence is found for a radial metallicity gradient, which will require follow-up observations. In addition, I show that the metallicities of the young and old populations of NGC6822 are well explained using a simple closed-box chemical evolution model, an interesting result, as NGC6822 is expected to have undergone significant recent interactions. In Chapter 6, I present multi-epoch KMOS observations of 22 RSGs in the Sculptor Group galaxy NGC55. Radial velocities are measured for the sample and are shown to be in good agreement with previous studies. Using the multi-epoch data, I find no evidence for radial velocity variables within the sample. Stellar parameters are estimated for 10 targets and are shown to be in good agreement with previous estimates. I conclude this thesis by summarising the main results and present a first-look calibration of the relationship between galaxy mass and metallicity using RSGs. By comparing the RSG metallicity estimates to metallicities estimated from ~ 50 000 Sloan digital sky survey galaxies, I show that the absolute metallicities of the two samples disagree. A more quantitative analysis requires additional RSG observations. In addition, using ~ 80 RSGs, with stellar parameters estimated in a consistent way, I show that there appears to be no dependence of the temperature of RSGs upon metallicity. This is in disagreement with current evolutionary models, which display a temperature change of ~ 450K over the studied range in metallicity. Finally, I outline potential areas for future work, focusing on follow-up studies that have been identified as a result of the work done in this thesis.

I present a comprehensive investigation of fast stars in the Milky Way, from brisk disc stars to stars escaping the Galaxy. My thesis is that fast stars are the smoking guns of extreme stellar collisions and explosions, and so can act as an intermediary to studying these theoretically-unconquered astrophysical processes. In Chapter 1 I give a history of fast stars, address what it means for a star to be fast, and describe the processes that accelerate stars. I concisely summarise the Gaia mission, whose recent data releases heavily influenced this thesis. Supernovae in binary systems can fling away the companion; if a runaway companion can be associated with a supernova remnant, then together they reveal the evolution that led to the supernova. However, these associations are difficult to establish. In Ch. 2, I develop a sophisticated Bayesian methodology to search the nearest ten remnants for a companion, by combining data from Gaia DR1 with a 3D dust-map and binary population synthesis. With Gaia DR2, I will identify companions of tens of supernova remnants and thus open a new window to studying late-stage stellar evolution. It is unknown why 17% of B stars are spinning near break-up; these stars are termed Be stars because of emission lines from their ejected material. Their rapid spin could be due to mass transfer, but in Ch. 3 I show this would create runaway Be stars. I demonstrate using a hierarchical Bayesian model that these exist in sufficient numbers, and thus that all Be stars may arise from mass transfer. The stars escaping the Milky Way are termed hypervelocity stars. In Ch. 4, I overturn the consensus that the hypervelocity stars originated in the Galactic centre by showing that a Large Magellanic Cloud (LMC) origin better explains their distribution on the sky. In Ch. 5 I present three ground-breaking hypervelocity results with Gaia DR2: 1) only 41 of the 524 hypervelocity star candidates are truly escaping, 2) at least one of the hypervelocity stars originates in the LMC, and 3) the discovery of three hypervelocity white dwarf runaways from thermonuclear supernovae.

Eclipsing binary stars allow the physics of the component stars to be studied. These systems can provide a wealth of knowledge ranging from fundamental parameters such as mass, radius, and luminosity through to tests of stellar evolution and distances to the systems. A search for eclipsing binaries in the SMC using the MOA photometric time series database yielded 169 detections. These detections were cross-referenced with the OGLE catalogue of eclipsing binaries in the SMC. A total of 35 systems were new detections with most of these lying in the outer, less dense regions of the SMC, and some outside the OGLE fields. The remainder were within the overlap region of the fields of the two surveys. In the overlap region of the two MOA survey fields, 21 systems were detected but only 2 detections were common to both fields. This implies the MOA survey is far from complete. Additionally, 14 systems (~10%) were not detected by OGLE in the MOA-OGLE overlap region implying the OGLE survey is also not complete, though to a lesser extent. The unequivocal determination of whether all systems were real eclipsing binaries was not able to be made based on the photometric light curves alone. Objects for which there was some uncertainty concerning their true nature were indicated as such. Two of the new detections in the MOA SMC catalogue were selected for follow-up photometry, MOA J005018.4-723855 and MOA J005623.5-722123. An LMC target was selected from the MACHO LMC catalogue, MACHO*05:36:48.7-69:17:00. Observations were made from MJUO in Strömgren u, Johnson VJ and Cousins IC filters between 1999 April and 2002 July. The McLellan 1-m telescope and CCD photometer head were used. Differential fluxes for the target stars were extracted using the ISIS-2.1 difference imaging package. To obtain flux changes with respect to a level of zero flux, DAOPHOT II was used to establish the targets' flux levels in the reference images relative to which ISIS-2.1 works. The light curves were analysed using the Wilson synthetic light-curve code. It was not possible to make a definitive analysis of the physical nature of the three targets based on differential photometry alone. Nevertheless, it was possible to make rough estimates of the systems' apparent magnitudes and hence, via the known distance moduli and reddenings to the Magellanic Clouds, their absolute magnitudes. Coupled with additional constraints derived from Geneva models of stellar evolution for stars of reduced metallicity, it was possible to obtain astrophysically plausible parameters for the component stars comprising each target. The LMC target MACHO*05:36:48.7-69:17:00 is located near the Tarantula nebula and very close to SN1987A. It is a well-detached, eccentric system, (e = 0.2) with a sidereal period of 3.853529 ± 0.000005d and an apsidal period of 100 ± 5 years. The difference in mean epochs for the VJ and u/IC observations resulted in an averaging of the solution parameters. The most astrophysically plausible solution was obtained by matching the derived components to a pair of Geneva LMC models of identical age. This solution indicates a primary of ~20 Mסּ and secondary of ~14 Mסּ with a common age of 5 x 106 yr. Other parameters are mean effective temperatures Teff,1=33500K, Teff,2=29500K, semi-major axis a=33.3Rסּ , mean radii R1 /a = 0.21, R2/a = 0.15 and inclination i = 85.8º. MOA J005018.4-723855 is a semi-detached system, period 1.839870 ± 0.000005d, with the secondary filling its Roche lobe and with the O'Connell effect evident in the light curve. Of the three systems, this is the most affected by blending. Third light was necessary to obtain satisfactory light-curve fits in all bandpasses. Light-curve solutions fix the mass ratio and suggest the system is undergoing case A mass transfer from the present secondary. Since the mass receptor in such a system may mimic a normal main-sequence star to within a factor of two in luminosity for a given mass and temperature, an estimate of the physical nature of this system was found by requiring the parameters to match those of a Geneva evolutionary model. This yielded M1 =13.5Mסּ), Teff,1=29000K, R1=5.9Rסּ), M2 =20.5 Mסּ), Teff,2=20000K, R2=8.5Rסּ), a=20.5 Rסּ) and i=77.6°. MOA J005623.5-722123 is a detached system, period 2.32005 ± 0.00002d. Third light was necessary in the analysis of this star. Analysis of the derived solutions and comparison to the Geneva evolutionary models indicates a system 10 x 106 yr old. The most astrophysically plausible parameters for this system are M1 = 13.5 Mסּ), Teff,1 = 29 000K, R1 = 5.8 Rסּ), M2 = 15.5 Mסּ), Teff,2=29350K, R1 =7.8 Rסּ), a=22.65 Rסּ) and i=87.8°. The preferred light curve solution gives a secondary marginally hotter than the primary, yet with shallower eclipse depths. The high inclination as well as the gravity brightening in B-stars are the cause of this. During the secondary eclipse, the primary's path passes across the cooler central region the secondary, thereby leaving the hotter poles exposed. This accounts for the higher hemisphere-averaged temperature in the solution.

Double lined, eclipsing binaries are an important probe of stellar structure and evolution. Their study provides the most accurate data on the defining fundamental properties of stars, namely stellar masses, radii and luminosities. Observations made at the Mount John University Observatory (MJUO) have yielded high precision, six colour CCD light curves and calibrated standard system Strömgren uvby and Cousins VI photometry for three Magellanic Cloud eclipsing binaries; HV982 and HV2241 in the Large Magellanic Cloud (LMC) and HV 1620 in the Small Magellanic Cloud (SMC). The masses, radii and luminosities for HV 1620 and HV 2241 have been determined by analysis of the light curves and standard system photometry in conjunction with IUE ultraviolet spectrophotometry and spectroscopic radial velocity curves obtained by other investigators. The masses, radii and luminosities for HV 982 have been estimated in conjunction with IUE ultraviolet spectrophotometry by adopting a distance for the system. No spectroscopic radial-velocity curves are available for HV 982 at this time. Analysis of the calibrated standard system photometry, light curves and IUE spectrophotometry for the well-detached, 5.55335 d period HV 982 permits the determination of the effective temperatures of the two components of HV 982; Teff,₁= 24 000 ± 5 000 K and 23 400 ± 5 000 K. The large uncertainties result from the poorly defined reddening which in turn is primarily due to large uncertainty in the (b-y) photometry resulting from observations obtained on only two nights, both of questionable quality. The fact that no spectroscopic radial-velocity curves are available means that neither the mass ratio nor the physical size of the system can be determined directly. The derived temperatures are nonetheless insensitive to reasonable ranges of the mass ratio (as deduced from the the form of the light curve) and of the physical size of the system (in order to be consistent with the distance to the LMC). Adopting a value of unity for the mass ratio and a distance modulus of (m- M)₀ = 18.35 ± 0.2mag allows the estimation of masses, radii and thus luminosities of the individual components of HV982. They are M₁ = M₂ = 9.1 ± 3.2M⊙, R₁ = R₂ = 7.0 ± 1.1 R⊙ and log[L₂/ L⊙] = 4.2±0.5, log[L₂/ L⊙ = 4.1±0.5. These properties correspond well to the expected properties of normal single stars as predicted by modern theoretical stellar evolution models. Moreover the position of the components of HV 982 in the Hertzsprung-Russell Diagram (HRD) implies the components are main sequence stars, although not far from the Terminal Age Main Sequence (TAMS), i.e. HV 982 is still relatively young, which is consistent with the fact that HV 982 is an eccentric orbit system. Apsidal motion with a period of 206±6 yr has also been discovered and an improved ephemeris, including the effects of the apsidal motion has been derived. Combined analysis of the MJUO photometry and light curves, the IUE spectrophotometry and the published spectroscopic radial-velocity curves permits a complete analysis and direct determination of the properties of the 3.62642d period HV 1620. The analysis yields the following properties for the components of HV 1620: Teff,₁ = 33 000±4 500 K, M₁ = 20.9±0.4 M⊙, R₁ = 6.27±0.10 R⊙ and log[L₁/ L⊙] 4.62±0.25; Teff,₂ = 24 400±3 500 M₂ = 14.3±0. 7 ⊙, R₂ = 11.3±0.2 R₂ and log[L₂/ L⊙] 4.53±0.25. The system is found to be in a semi-detached configuration, with the cooler component filling its Roche lobe, while the hotter component is well-detached from its Roche lobe. Despite this evolved binary state, the properties of both components correspond well to those of normal single stars, implying that any mass transfer currently in progress is occuring at a rate slow enough that both components can retain at least the appearance of normal single stars. Comparison of the the observed spectral flux distribution with model flux distributions also yields the distance modulus, (m M)₀ = 18.6±0.3 mag, which is in agreement with other measurements of the distance modulus for the SMC. An improved ephemeris has been determined. Similarly, the combined analysis of the MJUO photometry and light curves, the IUE spectrophotometry and the published spectroscopic radial-velocity curves permits a complete analysis and direct determination of the properties of the 4.3426241 d period HV 2241. The effective temperatures, masses, radii and luminosities of the components of HV 2241 are Teff,₁ = 27 000±3 000 K, M₁ = 36.82±0.10 M⊙, R₁ 16.1±0.2 R⊙ and log[L₁/L⊙] = 5.10±0.20 and Teff,₂ = 20200±1500K, M₂ = 19.4±0.6M⊙, :R₂ 13.9±0.1 R⊙ and log[L₂/ L⊙] = 4.49±0.13. The distance modulus is 18.50±0.16 mag, likewise in good agreement with other measurements of the distance modulus for the LMC. Like HV 1620, this system is found to be in a semi-detached configuration with the cooler component filling its Roche lobe. However the hotter component of HV 2241 is very close to filling its Roche lobe also. The above properties give stars that are somewhat under-luminous in comparison to normal single-star models and are not consistent with binary-star evolution models for very massive, short period systems. The derived properties can however be brought into agreement with current theory if there is a large (~12 percent) systematic error in the radial velocity data for this star. (If there is a systematic error in one of the data sets, it seems most probable that it would be in the radial velocities since this data was obtained photographically on a 1-m telescope and a Mv = 13.5mag star like HV2241 must surely be close to the practical limit of such a system.) If on the other hand the properties derived here are accurate then it would appear that HV 2241 has been caught in an unprecedented evolutionary phase. A new analysis procedure has been employed in order to investigate the nature of a selection of eight stars from the recently published EROS catalogue of eclipsing binary stars in the bar of the Large Magellanic Cloud. All eight stars are well-detached systems with obviously eccentric orbits. The results of the analysis show seven of the eight to be composed of young (i.e. main sequence), coeval components, in accord with theoretical models for binary star formation and evolution. Consideration of the position of the components of eighth system, EROS 1061, in the HRD leads to the suggestion that this system is composed of two pre-main sequence stars in the final stages of contraction down onto the zero-age main sequence.

The next generation of ground-based optical/infrared (IR) telescopes will have primary mirrors of up to 42 m. To take advantage of the large potential increase in angular resolution, adaptive optics will be essential to overcome the resolution limits set by atmospheric turbulence. Novel techniques such as Multi-Conjugate Adaptive Optics (MCAO) and Multi-Object Adaptive Optics (MOAO) are being developed to achieve near diffraction-limited images over large fields-of-view. This thesis concerns the development of MCAO and MOAO pathfinders. Specifically, the construction of CANARY, aMOAO demonstrator, and the on-sky performance and scientific exploitation of the Multi-conjugate Adaptive optics Demonstrator (MAD). CANARY is under construction for the William Herschel Telescope (WHT) in La Palma and contains a telescope simulator to allow testing of the set-up in the laboratory. The simulator contains a natural guide star emulator, turbulence phase screens, and telescope relay optics. The work presented here concerns the integration of the various components in relation to numerical models and the CANARY specifications. MAD was a near-IR imager on the Very Large Telescope (VLT) in Chile. Science demonstration observations were taken of R136, the young, massive cluster situated in the 30 Doradus star-forming region in the Large Magellanic Cloud. These data were used here to determine the MCAO performance across the ~1’x1’ field-of-view, for different pointings with respect to the guide stars, finding high Strehl ratios and relatively uniform corrections across the fields. The MAD data are then used to construct radial surface brightness profiles for R136, providing new insights into intriguing past results from the Hubble Space Telescope. The MAD data reveal that the profile is strongly asymmetric, removing the need for dramatic dynamical evolution of the cluster in the recent past, and highlighting the importance of considering asymmetries when analysing clusters further afield. The MAD data, combined with other near-IR imaging from the VLT, are then used to investigate the nature of candidate young stellar objects from recent observations with the Spitzer Space Telescope.

碩士
國立中央大學
天文研究所
107
We study runaway stars in the superbubble N160, supernova remnants (SNRs) N9 and N63A in the Large Magellanic Cloud (LMC). A runaway star in the superbubble N160 was diagnosed from its long-slit spectrum. A bow-shock-like structure was observed near this runaway. Assuming ram pressure equilibrium, the Hα surface brightness and the [S II] λ6716/λ6731 ratio gave the mass-loss rate not larger than 1.1±0.5×10−5M⊙/yr. Gaia DR2 data were used to examine proper motions of massive stars in SNRs N9 and N63A to search for runaway stars as candidates for companions of their supernova progenitors. There is one star in N9 whose proper motion is large considered as the runaway star. However, the environment is so complex that it needs better observational data to confirm its natures. In the case of N63A, an eclipsing binary showed high proper motion. More observations and models are needed to explain the high proper motions of this binary and whether they are runaway companions or not in the future. We also noticed that some stars with large proper motion are actually unresolved multiple stars. The large proper motions of these stars are most likely spurious. Therefore, we note the importance of checking high-resolution images In summary, we diagnosed three runaway stars/binary that could be candidates for companions of SN progenitors in the LMC.

The Magellanic Clouds (MCs) consist of a pair of galaxies, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), which are located at a distance of 50 kpc and 60 kpc, with stellar masses of 1010 M and 109 M , respectively. Morphologically they are categorized as irregular type galaxies. The MCs are gas rich and metal poor (Z=0.008 for LMC, and 0.004 for SMC) as compared to the Milky Way (MW), and have active star-forming regions. Their proximity and location at high galactic latitude enable us to resolve their individual populations as well as detect faint stellar populations. It is well known that the MCs are interacting with each other, as well as with the MW. The interaction is supported by the presence of the Magellanic Bridge and the Magellanic Stream. The evolved stellar populations in the MCs help us to understand their evolution and interaction process. The MCs host both Population I as well as Population II stars. This extended range of star formation is a valuable source of information to understand the formation and evolution of galaxies in general, and the MCs in particular. Evolved stellar popu-lation means the stars that have evolved o the main sequence and the giants, such as red giants (RGs), red clump stars, and asymptotic giant branch stars. There is a dominant population of evolved stars present in the MCs, in star clusters as well as in the eld. The aim of the thesis is to study the evolved stellar populations for one of the component of the MCs, the LMC. The study is primarily divided into two parts. (1) Study of sparse star clusters in the LMC: To increase our understanding of sparse star clusters in the LMC, with well estimated parameters, using deep Washington photometric data for 45 LMC clusters. (2) To estimate a metallicity map of LMC: In order to understand the metallicity variation across the galaxy. This is done by creating a high spatial resolution metallicity map of the LMC, using red giant branch (RGB) stars, with the help of photometric data and calibrated using spectroscopic studies of RGs in eld and star clusters. The introduction to the thesis study along with the aim are described in Chapter 1 of the thesis. The three sets of photometric data used for this study are described in Chapter 2. The data sets are: CT1 Washington photometric data for 45 star clusters within the LMC, the VI photometric data from the Optical Gravitational Lensing Experiment Phase-III survey (OGLE III), and the Magellanic Cloud Photometric Survey (MCPS). Study of sparse star clusters in the LMC: A systematic study is per-formed to analyse the 45 cluster candidates, to estimate their parameters (radius, reddening, and age) using the main-sequence turn-o (MSTO), as well as the evolved portion of the colour{magnitude diagram (CMD). The basic parameters were estimated for 33 genuine clusters, whereas the other 12 cluster candidates have been classi ed as possible clusters/asterisms. The study of 33 star clusters are presented in Chapter 3. These clus-ters are categorized as genuine star clusters based on their strong density enhancement and cluster features with respect to their surrounding eld regions. Out of the 33 clusters, 23 are identi ed as single clusters and 10 are found to be members of double clusters. Detailed discussions of all the individual clusters are presented. The estimated parameters for the single and double clusters are listed in two di erent tables. About 50% of the clusters are in the age range 100{300 Myr, the rest of them being older or younger. Comparison with previous age estimates shows some agreement as well as some deviation. The remaining 12 clusters which could not be categorized as genuine star clusters are studied in Chapter 4. These clusters have poor (/suspi-cious) density enhancement and cluster features when compared to their surrounding elds. It is important to study such cluster candidates, as these objects probe the lower limit of the cluster mass function. Detailed discussion on these individual objects are presented and their estimated parameters are tabulated in this chapter. A detailed discussion based on the study of all the 45 inconspicuous clusters is presented in this chapter, including the estimated sizes (radii 2{10 pc), reddening with respect to eld, and location in the LMC. The mass limit estimated for genuine clusters is found to be 1000 M , whereas for possible clusters/asterisms it is few 100 M , using synthetic CMDs. The study of sparse clusters enlarged the number of objects con rmed as genuine star clusters (33) and estimated their fundamental parameters. The study emphasizes that the sizes and masses of the studied sample are found to be similar to that of open clusters in the MW. Thus, this study adds to the lower end of cluster mass distribution in the LMC, suggesting that the LMC, apart from hosting rich clusters, also has formed small, less massive open clusters in the 100{300 Myr age range. The 12 cases of possible clusters/asterisms are worthy of attention, in the sense that they can throw light on the survival time of such objects in the LMC. Photometric metallicity map of the LMC using RGB stars: A metallic-ity map of the LMC is estimated using OGLE III and MCPS photometric data. This is a rst of its kind map of metallicity up to a radius of 4{5 de-grees, derived using photometric data and calibrated using spectroscopic data of RGB stars. The RGB is identi ed in the V, (V I) CMDs of small areal subregions of varying sizes in both data sets. The slope of the RGB is used as an indicator of the average metallicity of a subregion, and this RGB slope is calibrated to metallicity using spectroscopic data for eld and cluster RGs in selected subregions. The metallicity map estimated using OGLE III photometric data is presented in Chapter 5. A method to identify the RGB of small subre-gions within the LMC and estimate its slope by using a consistent and automated method was developed. The technique is robust and indepen-dent of reddening and extinction. The details of calibrating the RGB slopes to metallicities, using previous spectroscopic results of RGs in eld and star clusters are presented. The OGLE III metallicity maps are pre sented, based on four cut-o criteria to separate regions with good ts. The OGLE III map has substantial coverage of the bar, the eastern and western LMC, but does not cover the northern and southern regions. The OGLE III metallicity map shows the bar region to be metal rich whereas the eastern and western regions to be relatively metal poor. The mean metallicity is estimated for three di erent regions within the LMC. For the complete LMC the mean [Fe/H] is = 0.39 dex ( [Fe/H] = 0.10); for the bar region it is = 0.35 dex ( [Fe/H] = 0.9); and for the outer LMC it is = 0.46 dex ( [Fe/H] = 0.11). The metallicity histogram for these di erent regions are also estimated. A radial metallicity gradient is estimated in the de-projected plane of the LMC. The metallicity gradient is seen to remain almost constant in the bar region (till a radius of 2.5 kpc) and has a shallow gradient of 0.066 0.006 dex kpc 1 beyond that till 4 kpc. In Chapter 6 the metallicity map based on MCPS photometric data is estimated. The MCPS data covers more of the northern and south-ern LMC (less of eastern and western regions) and is important to be analysed in order to reveal the metallicity trend of the overall disk. The systematic di erences between the lter systems of MCPS and OGLE III are corrected, and the MCPS slopes are then calibrated using the OGLE III slope{metallicity relation. The MCPS metallicity maps are presented, based on four cut-o criteria to separate regions with good ts. The bar region is found to be metal rich as was found using OGLE III data, whereas the northern and southern regions are marginally metal poor. The mean metallicity estimated for the complete LMC is = 0.37 dex ( [Fe/H] = 0.12); and for the outer LMC it is = 0.41 dex ( [Fe/H] = 0.11). The metallicity histogram for these di erent regions are estimated and compared with the OGLE III distribution. The metallicity range of the complete LMC is found to be almost similar for both data sets. The metallicity distribution within the bar has a narrow range as found using both data sets. The slight di erence between mean metallicity of outer LMC for the two data sets is attributed to their coverage. We suggest that the northern and southern regions of the LMC could be marginally more metal rich than the eastern and western regions. The metallicity gradient of the LMC disk, estimated from MCPS data is found to be shallow 0.049 0.002 dex kpc 1 till about 4 kpc. We also constructed a metallicity map of outliers using both OGLE III and MCPS data, and identi ed subregions where the mean metallic-ity di ers from the surrounding areas. We suggest further spectroscopic studies in order to assess their physical significance. The detailed conclusion of the thesis and future work are presented in Chapter 7. From the study of sparse star clusters in the LMC, it is concluded that LMC has open cluster like star cluster systems. It is important to include them to understand the cluster formation history (CFH) and their survival time scale. Presently, our understanding of the CFH is dominated by rich clusters. The bar of the LMC is found to be the most metal rich region, and the LMC metallicity gradient though shallow, resembles the gradient seen in spiral galaxies. The gradient is also similar to that found in our Galaxy. The higher metallicity in the bar region might indicate an active bar in the past.

碩士
國立臺灣大學
天文物理研究所
102
Unresolved distant galaxies can typically only be studied through their spectral energy distributions (SEDs). An SED model of a galaxy usually con- sists of contributions due to starlight (in the UV/optical and near-infrared) and thermal emission of interstellar dust (in the far-infrared). However, present-day SED models typically do not accounted for dusty envelopes of AGB stars. This might be acceptable for starburst galaxies with a relatively large population of young stars, but it might not accurately repre- sent galaxies with a significant population of dusty Asymptotic Giant Branch (AGB) stars. In order to estimate the contribution from circumstellar dust, we first gen- erate model stellar SEDs with the population synthesis code Pegase and the star formation history of the LMC from Harris &; Zaritsky (2009), which we use to fit the observed SED from previous works at wavelengths less than 1 μm. Then we subtract the best-fit model SED and the interstellar medium (ISM) SED constructed by Bernard et al. (2008) from the observed SED. We found that there is a significant discrepancy (20 &;#8722; 40%) between our best-fit SED and the observed SED minus the ISM SED. This result indicates the importance of the contribution from circumstellar dust in the near-infrared SED. Thus, we suggest that an improved stellar spectral library that includes dusty AGB spectra and an improved set of stellar evolutionary tracks that considers dust formalism in stellar winds is needed.