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1
Jackson, R. A., S. Kaviraj, G. Martin, J. E. G. Devriendt, A. Slyz, J. Silk, Y. Dubois, et al. "Dark matter-deficient dwarf galaxies form via tidal stripping of dark matter in interactions with massive companions." Monthly Notices of the Royal Astronomical Society 502, no. 2 (January 15, 2021): 1785–96. http://dx.doi.org/10.1093/mnras/stab093.
Повний текст джерелаАнотація:
ABSTRACT In the standard ΛCDM (Lambda cold dark matter) paradigm, dwarf galaxies are expected to be dark matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter content appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark matter-deficient, even though their initial dark matter fractions are normal. The process of dark matter stripping is responsible for the large scatter in the halo-to-stellar mass relation in the dwarf regime. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs with halo-to-stellar mass ratios as low as unity, consistent with the findings of recent observational studies. ∼30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M⋆ < 10). Given their close orbits, a significant fraction of dark matter-deficient dwarfs merge with their massive companions (e.g. ∼70 per cent merge over time-scales of ∼3.5 Gyr), with the dark matter-deficient population being constantly replenished by new interactions between dwarfs and massive companions. The creation of these galaxies is therefore a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm.
2
Chandra, Vedant, Hsiang-Chih Hwang, Nadia L. Zakamska, and Tamás Budavári. "Computational tools for the spectroscopic analysis of white dwarfs." Monthly Notices of the Royal Astronomical Society 497, no. 3 (July 25, 2020): 2688–98. http://dx.doi.org/10.1093/mnras/staa2165.
Повний текст джерелаАнотація:
ABSTRACT The spectroscopic features of white dwarfs are formed in the thin upper layer of their stellar photosphere. These features carry information about the white dwarf’s surface temperature, surface gravity, and chemical composition (hereafter ‘labels’). Existing methods to determine these labels rely on complex ab-initio theoretical models, which are not always publicly available. Here, we present two techniques to determine atmospheric labels from white dwarf spectra: a generative fitting pipeline that interpolates theoretical spectra with artificial neural networks and a random forest regression model using parameters derived from absorption line features. We test and compare our methods using a large catalogue of white dwarfs from the Sloan Digital Sky Survey (SDSS), achieving the same accuracy and negligible bias as compared to previous studies. We package our techniques into an open-source python module ‘wdtools’ that provides a computationally inexpensive way to determine stellar labels from white dwarf spectra observed from any facility. We will actively develop and update our tool as more theoretical models become publicly available. We discuss applications of our tool in its present form to identify interesting outlier white dwarf systems including those with magnetic fields, helium-rich atmospheres, and double-degenerate binaries.
3
Martín, Eduardo L., Mike Connelley, Dan Potter, and Hervé Bouy. "Brown Dwarf Companions to Solar-Type Stars and other Brown Dwarfs." Symposium - International Astronomical Union 213 (2004): 111–14. http://dx.doi.org/10.1017/s007418090019309x.
Повний текст джерелаАнотація:
We report on our ongoing imaging efforts to detect brown dwarfs orbiting solar-type stars and other brown dwarfs. We study the properties of brown dwarf companions as a function of primary mass. Our results indicate that the frequency of brown dwarf companions around solar-type stars for separations larger than about 40 AU is non-negligible. The frequency of brown dwarf binaries is about 20%. There is a dearth of brown dwarf binaries with separations larger than ∼20 AU. We propose that brown dwarf systems are a scaled down version of stellar systems, which probably form via triggered collapse of small molecular cores.
4
Allard, France, Tristan Guillot, Hans-Günter Ludwig, Peter H. Hauschildt, Andreas Schweitzer, David R. Alexander, and Jason W. Ferguson. "Model Atmospheres and Spectra: The Role of Dust." Symposium - International Astronomical Union 211 (2003): 325–32. http://dx.doi.org/10.1017/s0074180900210875.
Повний текст джерелаАнотація:
Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties.
5
Nomoto, K., and M. Hashimoto. "White Dwarf Models of Supernovae and Cataclysmic Variables." International Astronomical Union Colloquium 93 (1987): 395–411. http://dx.doi.org/10.1017/s0252921100105135.
Повний текст джерелаАнотація:
AbstractIf the accreting white dwarf increases its mass to the Chandrasekhar mass, it will either explode as a Type I supernova or collapse to form a neutron star. In fact, there is a good agreement between the exploding white dwarf model for Type I supernovae and observations. We describe various types of evolution of accreting white dwarfs as a function of binary parameters (i.e, composition, mass, and age of the white dwarf, its companion star, and mass accretion rate), and discuss the conditions for the precursors of exploding or collapsing white dwarfs, and their relevance to cataclysmic variables. Particular attention is given to helium star cataclysmics which might be the precursors of some Type I supernovae or ultrashort period X-ray binaries. Finally we present new evolutionary calculations using the updated nuclear reaction rates for the formation of O+Ne+Mg white dwarfs, and discuss the composition structure and their relevance to the model for neon novae.
6
Damasco, OP, ID Godwin, MK Smith, and SW Adkins. "Gibberellic acid detection of dwarf offtypes in micropropagated Cavendish bananas." Australian Journal of Experimental Agriculture 36, no. 2 (1996): 237. http://dx.doi.org/10.1071/ea9960237.
Повний текст джерелаАнотація:
Detection of dwarf offtypes produced by micropropagation of Cavendish bananas (Musa spp.) cultivars New Guinea Cavendish and Williams was achieved by spraying gibberellic acid (GA3) solution (289 pmol/L) onto deflasked plants and measuring various plantlet responses. The most useful identification criterion was elongation of the sheath of the first leaf to form after GA3 application. Elongation of this structure was about 2-fold greater in normal plants than observed in dwarfs. Similar measurements taken earlier during in vitro culture or later during plant establishment in soil were not as useful in discriminating between normals and dwarfs as the measurements made at deflasking. The similar GA3-induced elongation response of the dwarf offtype and that of the naturally occurring dwarf cultivar Dwarf Parfitt suggests that the mechanism for dwarfism could be the same in the 2 cases.
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Nelson, A. H., and P. R. Williams. "Numerical Simulation of the Dwarf Companions of Giant Galaxies." Proceedings of the International Astronomical Union 3, S244 (June 2007): 226–30. http://dx.doi.org/10.1017/s1743921307014020.
Повний текст джерелаАнотація:
AbstractWe report simulations of the formation of a giant disc galaxy from cosmological initial conditions. Two sets of initial conditions are used, initially smooth density for both gas and stars, representing the Warm dark Matter scenario, and an initially fluctuating density representing the standard spectrum for the Cold dark Matter scenario. For the WDM initial conditions, the galaxy has a population of long lived dwarf satellites at z = 0, with orbits close to a plane coincident with that of the giant galaxy disc. The detailed properties of these dwarfs mimic closely the observed properties of Local Group dwarfs with respect to mass, and kinematics. However they do not have individual dark matter halos, but orbit in the nearly spherical dark matter halo of the giant galaxy. The reason for this is that the initial population of dwarf dark matter haloes, which form during the initial collapse phase, all merge into the halo of the giant galaxy within a few to several Gyears, while the long lived dwarfs form as a secondary population by gravitational collapse of high angular momentum gas in the outer reaches of the giants proto-galactic disc. Due to their late formation and their more distant orbits, they survive until the present epoch as individual dwarf galaxies at radii 20-50kpc from the giants centre. For CDM initial conditions there are many more dwarf satellites at z = 0, some of which form early on as gas condensations in DM sub-halos, and survive with these individual DM halos till z = 0 due to their being sufficiently well bound to avoid merging with the main galaxy. However even in this case some second generation satellites form as initially gas only objects, just as for the smooth initial conditions of WDM.
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Tremmel, M., A. C. Wright, A. M. Brooks, F. Munshi, D. Nagai, and T. R. Quinn. "The formation of ultradiffuse galaxies in the RomulusC galaxy cluster simulation." Monthly Notices of the Royal Astronomical Society 497, no. 3 (July 13, 2020): 2786–810. http://dx.doi.org/10.1093/mnras/staa2015.
Повний текст джерелаАнотація:
ABSTRACT We study the origins of 122 ultradiffuse galaxies (UDGs) in the Romulus c zoom-in cosmological simulation of a galaxy cluster (M200 = 1.15 × 1014 M⊙), one of the only such simulations capable of resolving the evolution and structure of dwarf galaxies (M⋆ < 109 M⊙). We find broad agreement with observed cluster UDGs and predict that they are not separate from the overall cluster dwarf population. UDGs in cluster environments form primarily from dwarf galaxies that experienced early cluster in-fall and subsequent quenching due to ram pressure. The ensuing dimming of these dwarf galaxies due to passive stellar evolution results in a population of very low surface brightness galaxies that are otherwise typical dwarfs. UDGs and non-UDGs alike are affected by tidal interactions with the cluster potential. Tidal stripping of dark matter, as well as mass-loss from stellar evolution, results in the adiabatic expansion of stars, particularly in the lowest mass dwarfs. High-mass dwarf galaxies show signatures of tidal heating while low-mass dwarfs that survive until z = 0 typically have not experienced such impulsive interactions. There is little difference between UDGs and non-UDGs in terms of their dark matter haloes, stellar morphology, colours, and location within the cluster. In most respects cluster UDG and non-UDGs alike are similar to isolated dwarf galaxies, except for the fact that they are typically quenched.
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Kilic, Mukremin, P. Bergeron, Simon Blouin, and A. Bédard. "The most massive white dwarfs in the solar neighbourhood." Monthly Notices of the Royal Astronomical Society 503, no. 4 (March 16, 2021): 5397–408. http://dx.doi.org/10.1093/mnras/stab767.
Повний текст джерелаАнотація:
ABSTRACT We present an analysis of the most massive white dwarf candidates in the Montreal White Dwarf Database 100 pc sample. We identify 25 objects that would be more massive than $1.3\, {\rm M}_{\odot }$ if they had pure H atmospheres and CO cores, including two outliers with unusually high photometric mass estimates near the Chandrasekhar limit. We provide follow-up spectroscopy of these two white dwarfs and show that they are indeed significantly below this limit. We expand our model calculations for CO core white dwarfs up to M = 1.334 M⊙, which corresponds to the high-density limit of our equation-of-state tables, ρ = 109 g cm−3. We find many objects close to this maximum mass of our CO core models. A significant fraction of ultramassive white dwarfs are predicted to form through binary mergers. Merger populations can reveal themselves through their kinematics, magnetism, or rapid rotation rates. We identify four outliers in transverse velocity, four likely magnetic white dwarfs (one of which is also an outlier in transverse velocity), and one with rapid rotation, indicating that at least 8 of the 25 ultramassive white dwarfs in our sample are likely merger products.
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Whelan, Emma T., Thomas P. Ray, Francesca Bacciotti, Sofia Randich, Ray Jayawardhana, Antonella Natta, Leonardo Testi, and Subu Mohanty. "Outflow activity in brown dwarfs." Proceedings of the International Astronomical Union 3, S243 (May 2007): 357–64. http://dx.doi.org/10.1017/s1743921307009714.
Повний текст джерелаАнотація:
AbstractWhile numerous studies have been aimed at understanding the properties of young brown dwarfs relatively little exploration of their potential as drivers of outflows has occurred. Forbidden emission lines are important probes of outflows from young stellar objects, as they trace the shocks which form as an outflow interacts with the ambient medium of its driving source. While forbidden emission was identified in the spectra of young brown dwarfs, indicating the presence of outflows, these lines were weak and confined to the brown dwarf continuum position. Hence their origin in an outflow could not be confirmed. Our approach to this problem, is to analyse the forbidden line regions of brown dwarfs using spectro-astrometry. Spectro-astrometry is a novel technique which allows the user to recover spatial information from a spectrum beyond the limitations of the seeing of the observation. Using this technique we have found two brown dwarf outflows to date. In this chapter we outline this technique, describe our results for the brown dwarfs ρ-Oph 102 and 2MASS1207-3932 and discuss our future plans.
11
Ricotti, Massimo. "The First Galaxies and the Likely Discovery of Their Fossils in the Local Group." Advances in Astronomy 2010 (2010): 1–21. http://dx.doi.org/10.1155/2010/271592.
Повний текст джерелаАнотація:
The lower bound for the mass of a galaxy is unknown, as are the typical luminosity of the smallest galaxies and their numbers. The answers depend on the extent to which star formation in the first population of small mass halos may be suppressed by radiative feedback loops. If early populations of dwarf galaxies did form in significant number before reionization, their “fossils” should be found today in the Local Group. This paper reviews our ongoing efforts to simulate and identify fossil dwarfs in the Local Group. It is widely believed that reionization stopped star formation in fossil dwarfs. However, here we dispute this idea and discuss a physical mechanism whereby recent episodes of star formation would be produced in some fossil dwarfs that, hence, may characterized by a bimodal star formation history. The same mechanism could turn dark halos that failed to form stars before reionization into gas-rich “dark galaxies”. We believe that current observational data supports the thesis that a fraction of the new ultra-faint dwarfs discovered in the Local Group are fossil dwarfs and we predict the existence of a population of ultra-faint dwarfs with lower surface brightness than currently observed.
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Fenner, Yeshe, and Brad K. Gibson. "Deriving the Metallicity Distribution Function of Galactic Systems." Publications of the Astronomical Society of Australia 20, no. 2 (2003): 189–95. http://dx.doi.org/10.1071/as02047.
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AbstractThe chemical evolution of the Milky Way is investigated using a dual-phase metal-enriched infall model in which primordial gas fuels the earliest epoch of star formation, followed by the ongoing formation of stars from newly accreted gas. The latest metallicity distribution of local K-dwarfs is reproduced by this model, which allows the Galactic thin disk to form from slightly metal-enriched gas with α-element enhancement. Our model predicts ages for the stellar halo and thin disk of 12.5 and 7.4 Gyr respectively, in agreement with empirically determined values. The model presented in this paper is compared with a similar dual-phase infall model from Chiappini et al. (2001). We discuss a degeneracy that enables both models to recover the K-dwarf metallicity distribution while yielding different star formation histories.The metallicity distribution function (MDF) of K-dwarfs is proposed to be more directly comparable to chemical evolution model results than the G-dwarf distribution because lower mass K-dwarfs are less susceptible to stellar evolutionary effects. The K-dwarf MDF should consequently be a better probe of star formation history and provide a stronger constraint to chemical evolution models than the widely used G-dwarf MDF. The corrections that should be applied to a G-dwarf MDF are quantified for the case of the outer halo of NGC 5128.
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Conselice, Christopher J. "Are Cluster Dwarfs Recycled Galaxies?" Symposium - International Astronomical Union 217 (2004): 556–61. http://dx.doi.org/10.1017/s0074180900198377.
Повний текст джерелаАнотація:
Although cluster dwarf galaxies are often neglected due to their faintness, recent observations demonstrate they may be critical for understanding the physical processes behind galaxy formation. Dwarfs are the most common galaxy type and are particularly abundant in clusters. Recent observational results suggest that dwarfs in dense environments do not all form early in the universe, as expected in hierarchical structure formation models. Many of these systems appear to be younger and more metal rich than dwarfs in lower density areas, suggesting they are possibly created by a tidal process. Several general galaxy cluster observations, including steep luminosity functions and the origin of intracluster light, are natural outcomes of these processes.
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Łokas, Ewa L. "Tidal evolution of dwarf galaxies with shallow dark matter density profiles." Proceedings of the International Astronomical Union 10, H16 (August 2012): 346. http://dx.doi.org/10.1017/s1743921314011181.
Повний текст джерелаАнотація:
AbstractOne of the scenarios for the formation of dwarf spheroidal galaxies in the Local Group proposes that the objects formed from late type dwarfs via tidal interaction with bigger galaxies such as the Milky Way and Andromeda. The scenario naturally explains the morphology-density relation observed for dwarf galaxies in the Local Group. Using N-body simulations we study the long-term tidal evolution of dwarf galaxies in the vicinity of the Milky Way. The dwarf galaxies were initially composed of stellar disks embedded in dark matter haloes of different inner density slopes including shallow ones recently obtained in N-body+hydro simulations of dwarf galaxy formation in isolation. Such progenitors were placed on five different orbits around the Milky Way and their evolution was followed for 10 Gyr. The outcome of the evolution, in terms of the mass loss, morphological transformation and randomization of stellar orbits depends very sensitively on the inner density slope of dark matter. The effects of tides are stronger for dwarfs with shallower slopes; they are more heavily stripped, in some cases down to the scale of ultra-faint satellites of the Milky Way or even dissolved completely with obvious implications for the missing satellites problem. The morphological evolution of the stellar component, from rotationally supported disks to spheroids dominated by random motions, also proceeds faster. In addition, bars which usually form at the first pericenter passage are created more easily and live longer in dwarfs with shallow dark matter density profiles on extended orbits.
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Stamatellos, Dimitris, and Anthony Whitworth. "The formation of brown dwarfs in discs: Physics, numerics, and observations." Proceedings of the International Astronomical Union 6, S270 (May 2010): 223–26. http://dx.doi.org/10.1017/s174392131100041x.
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AbstractA large fraction of brown dwarfs and low-mass stars may form by gravitational fragmentation of relatively massive (a few 0.1 M⊙) and extended (a few hundred AU) discs around Sun-like stars. We present an ensemble of radiative hydrodynamic simulations that examine the conditions for disc fragmentation. We demonstrate that this model can explain the low-mass IMF, the brown dwarf desert, and the binary properties of low-mass stars and brown dwarfs. Observing discs that are undergoing fragmentation is possible but very improbable, as the process of disc fragmentation is short lived (discs fragment within a few thousand years).
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Parsons, S. G., B. T. Gänsicke, M. R. Schreiber, T. R. Marsh, R. P. Ashley, E. Breedt, S. P. Littlefair, and H. Meusinger. "Magnetic white dwarfs in post-common-envelope binaries." Monthly Notices of the Royal Astronomical Society 502, no. 3 (February 4, 2021): 4305–27. http://dx.doi.org/10.1093/mnras/stab284.
Повний текст джерелаАнотація:
ABSTRACT Magnitude-limited samples have shown that 20–25 per cent of cataclysmic variables contain white dwarfs with magnetic fields of Mega Gauss strength, in stark contrast to the approximately 5 per cent of single white dwarfs with similar magnetic field strengths. Moreover, the lack of identifiable progenitor systems for magnetic cataclysmic variables leads to considerable challenges when trying to understand how these systems form and evolve. Here, we present a sample of six magnetic white dwarfs in detached binaries with low-mass stellar companions where we have constrained the stellar and binary parameters including, for the first time, reliable mass estimates for these magnetic white dwarfs. We find that they are systematically more massive than non-magnetic white dwarfs in detached binaries. These magnetic white dwarfs generally have cooling ages of more than 1 Gyr and reside in systems that are very close to Roche lobe filling. Our findings are more consistent with these systems being temporarily detached cataclysmic variables, rather than pre-cataclysmic binaries, but we cannot rule out the latter possibility. We find that these systems can display unusual asymmetric light curves that may offer a way to identify them in larger numbers in future. Seven new candidate magnetic white dwarf systems are also presented, three of which have asymmetric light curves. Finally, we note that several newly identified magnetic systems have archival spectra where there is no clear evidence of magnetism, meaning that these binaries have been previously missed. Nevertheless, there remains a clear lack of younger detached magnetic white dwarf systems.
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Rodríguez-Barrera, M. I., Ch Helling, and K. Wood. "Environmental effects on the ionisation of brown dwarf atmospheres." Astronomy & Astrophysics 618 (October 2018): A107. http://dx.doi.org/10.1051/0004-6361/201832685.
Повний текст джерелаАнотація:
Context. Brown dwarfs emit bursts of Hα, white-light flares, and show radio flares and quiescent radio emission. They are suggested to form aurorae, similar to planets in the solar system, but much more energetic. All these processes require a source gas with an appropriate degree of ionisation, which, so far, is mostly postulated to be sufficient. Aims. We aim to demonstrate that the Galactic environment influences atmospheric ionisation, and that it hence amplifies or enables the magnetic coupling of the atmospheres of ultra-cool objects, like brown dwarfs and free-floating planets. Methods. We build on our previous work on thermal ionisation of ultra-cool atmospheres and explore the effect of environmental high-energy radiation on the degree of ionisation in the atmosphere. We consider the effect of photoionisation by Lyman-continuum radiation in three different environments: in the interstellar radiation field (ISRF), O and B stars in star-forming regions, and in white dwarf companions in binary systems. We apply our Monte Carlo radiation transfer code to investigate the effect of Lyman-continuum photoionisation for prescribed atmosphere structures for very low-mass objects. Results. The external radiation environment plays an important role for the atmospheric ionisation of very low-mass, ultra-cool objects. Lyman-continuum irradiation greatly increases the level of ionisation in the uppermost atmospheric regions. Our results suggest that a shell of an almost fully ionised atmospheric gas emerges for brown dwarfs in star-forming regions and brown dwarfs in white dwarf binary systems. As a consequence, brown dwarf atmospheres can be magnetically coupled, which is the presumption for chromospheric heating to occur and for aurorae to emerge. First tests for assumed chromosphere-like temperature values suggest that the resulting free-free X-ray luminosities are comparable with those observed from non-accreting brown dwarfs in star-forming regions.
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Pritchard, Erin. "“She Finds People Like You Hilarious!”." Journal of Literary & Cultural Disability Studies: Volume 15, Issue 4 15, no. 4 (November 1, 2021): 455–70. http://dx.doi.org/10.3828/jlcds.2021.35.
Повний текст джерелаАнотація:
For centuries, people with dwarfism have been sought after for humorous entertainment purposes. Even today, dwarfs are employed within various forms of lowbrow entertainment that is unique to them. This begs the question, why do we laugh at people with dwarfism?1 Using superiority and inferiority theories, the article aims to demonstrate why we laugh at dwarfs by exploring both historical and present forms of dwarf entertainment. Laughing at dwarfs is a form of disablism that permits dwarfism to be deemed inferior within society due to their non-normative embodiment. The article demonstrates some of the implications this sort of humour has upon how people with dwarfism are perceived and subsequently treated within society. The article calls for a more ethical consideration of the humour used in relation to dwarfism with the entertainment industry.
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Mercer, Anthony, and Dimitris Stamatellos. "Planet formation around M dwarfs via disc instability." Astronomy & Astrophysics 633 (January 2020): A116. http://dx.doi.org/10.1051/0004-6361/201936954.
Повний текст джерелаАнотація:
Context. Around 30 per cent of the observed exoplanets that orbit M dwarf stars are gas giants that are more massive than Jupiter. These planets are prime candidates for formation by disc instability. Aims. We want to determine the conditions for disc fragmentation around M dwarfs and the properties of the planets that are formed by disc instability. Methods. We performed hydrodynamic simulations of M dwarf protostellar discs in order to determine the minimum disc mass required for gravitational fragmentation to occur. Different stellar masses, disc radii, and metallicities were considered. The mass of each protostellar disc was steadily increased until the disc fragmented and a protoplanet was formed. Results. We find that a disc-to-star mass ratio between ~0.3 and ~0.6 is required for fragmentation to happen. The minimum mass at which a disc fragment increases with the stellar mass and the disc size. Metallicity does not significantly affect the minimum disc fragmentation mass but high metallicity may suppress fragmentation. Protoplanets form quickly (within a few thousand years) at distances around ~50 AU from the host star, and they are initially very hot; their centres have temperatures similar to the ones expected at the accretion shocks around planets formed by core accretion (up to 12 000 K). The final properties of these planets (e.g. mass and orbital radius) are determined through long-term disc-planet or planet–planet interactions. Conclusions. Disc instability is a plausible way to form gas giant planets around M dwarfs provided that discs have at least 30% the mass of their host stars during the initial stages of their formation. Future observations of massive M dwarf discs or planets around very young M dwarfs are required to establish the importance of disc instability for planet formation around low-mass stars.
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Vavrukh, M., D. Dzikovskyi, and S. Smerechynskyi. "White dwarfs with rapid rotation." Mathematical Modeling and Computing 9, no. 2 (2022): 278–302. http://dx.doi.org/10.23939/mmc2022.02.278.
Повний текст джерелаАнотація:
A new analytical approach for calculation of white dwarfs characteristics that accounts for two important competing factors — axial rotation and Coulomb interparticle interactions, is proposed. The feature of our approach is simultaneous usage of differential and integral forms of equilibrium equation. In dimensionless form the differential equilibrium equation is strongly nonlinear inhomogeneous equation of the second order in partial derivatives with two dimensionless parameters — the relativistic parameter in stellar center $x_0$ and dimensionless angular velocity $\Omega$. In inner stellar region, rotation is taken into account as perturbation in the linear approximation for $\Omega^2$. In stellar periphery rotation is considered as the main factor. Usage of the integral equation provides correct calculations of integration constants. Dwarf's mass, moment of inertia relative to the axis of rotation, equatorial and polar radii, equatorial gravity in the following parameter space $1\leq x_0\leq24$, $0\leq\Omega<\Omega_{\rm max}(x_0)$ have been calculated based on the solutions of equilibrium equation. For the first time it was calculated the total energy of dwarf as function of these parameters. By the extrapolation, it was calculated the maximal values $\Omega_{\rm max}(x_0)$, as well as the observed angular velocity $\omega_{\rm max}(x_0)$. The considered model is generalized by taking into account Coulomb interparticle interactions. Also, we provide the examples of application of obtained results. It was shown that the characteristics of observed massive dwarfs do not contradict the calculated values for the model with consideration of solid body rotation and Coulomb interparticle interactions.
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Wheeler, Coral, Philip F. Hopkins, Andrew B. Pace, Shea Garrison-Kimmel, Michael Boylan-Kolchin, Andrew Wetzel, James S. Bullock, Dušan Kereš, Claude-André Faucher-Giguère, and Eliot Quataert. "Be it therefore resolved: cosmological simulations of dwarf galaxies with 30 solar mass resolution." Monthly Notices of the Royal Astronomical Society 490, no. 3 (October 16, 2019): 4447–63. http://dx.doi.org/10.1093/mnras/stz2887.
Повний текст джерелаАнотація:
ABSTRACT We study a suite of extremely high-resolution cosmological Feedback in Realistic Environments simulations of dwarf galaxies ($M_{\rm halo} \lesssim 10^{10}\rm \, M_{\odot }$), run to z = 0 with $30\, \mathrm{M}_{\odot }$ resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with $M_{\rm halo} \gtrsim 10^{8.6}\, \mathrm{M}_{\odot }$ is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field. Our ultra-faint dwarfs (UFDs; $M_{\ast }\lt 10^{5}\, \mathrm{M}_{\odot }$) have their star formation (SF) truncated early (z ≳ 2), likely by reionization, while classical dwarfs ($M_{\ast }\gt 10^{5}\, \mathrm{M}_{\odot }$) continue forming stars to z < 0.5. The systems have bursty star formation histories, forming most of their stars in periods of elevated SF strongly clustered in both space and time. This allows our dwarf with M*/Mhalo > 10−4 to form a dark matter core ${\gt}200\rm \, pc$, while lower mass UFDs exhibit cusps down to ${\lesssim}100\rm \, pc$, as expected from energetic arguments. Our dwarfs with $M_{\ast }\gt 10^{4}\, \mathrm{M}_{\odot }$ have half-mass radii (R1/2) in agreement with Local Group (LG) dwarfs (dynamical mass versus R1/2 and stellar rotation also resemble observations). The lowest mass UFDs are below surface brightness limits of current surveys but are potentially visible in next-generation surveys (e.g. LSST). The stellar metallicities are lower than in LG dwarfs; this may reflect pre-enrichment of the LG by the massive hosts or Pop-III stars. Consistency with lower resolution studies implies that our simulations are numerically robust (for a given physical model).
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Afram, N., and S. V. Berdyugina. "Complexity of magnetic fields on red dwarfs." Astronomy & Astrophysics 629 (September 2019): A83. http://dx.doi.org/10.1051/0004-6361/201935793.
Повний текст джерелаАнотація:
Context. Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature- and Zeeman-sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Aims. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how “surface” inhomogeneities are distributed in height – the dimension that is usually neglected in stellar magnetic studies. Methods. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1–M7). Our χ2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Results. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3–6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma β is less than one, implying highly magnetized stars. Conclusions. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.
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Bekki, Kenji, Warrick J. Couch, Michael J. Drinkwater, and Yasuhiro Shioya. "Galaxy Threshing and the Origin of Intracluster Stellar Objects." Symposium - International Astronomical Union 217 (2004): 77–82. http://dx.doi.org/10.1017/s0074180900197165.
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We numerically investigate the dynamical evolution of non-nucleated dwarf elliptical/spiral galaxies (dE) and nucleated ones (dE,Ns) in clusters of galaxies in order to understand the origin of intracluster stellar objects, such as intracluster stars (ICSs), GCs (ICGCs), and “ultra-compact dwarfs” (UCDs) recently discovered by all-object spectroscopic survey centred on the Fornax cluster of galaxies. We find that the outer stellar components of a nucleated dwarf are removed by the strong tidal field of the cluster, whereas the nucleus manages to survive as a result of its initially compact nature. The developed naked nucleus is found to have physical properties (e.g., size and mass) similar to those observed for UCDs. We also find that the UCD formation process does depend on the radial density profile of the dark halo in the sense that UCDs are less likely to be formed from dwarfs embedded in dark matter halos with central ‘cuspy’ density profiles. Our simulations also suggest that very massive and compact stellar systems can be rapidly and efficiently formed in the central regions of dwarfs through the merging of smaller GCs. GCs initially in the outer part of dE and dE,Ns are found to be stripped to form ICGCs.
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Fattahi, Azadeh, Alis J. Deason, Carlos S. Frenk, Christine M. Simpson, Facundo A. Gómez, Robert J. J. Grand, Antonela Monachesi, Federico Marinacci, and Rüdiger Pakmor. "A tale of two populations: surviving and destroyed dwarf galaxies and the build-up of the Milky Way’s stellar halo." Monthly Notices of the Royal Astronomical Society 497, no. 4 (August 6, 2020): 4459–71. http://dx.doi.org/10.1093/mnras/staa2221.
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ABSTRACT We use magnetohydrodynamical simulations of Milky Way-mass haloes from the Auriga project to investigate the properties of surviving and destroyed dwarf galaxies that are accreted by these haloes over cosmic time. We show that the combined luminosity function of surviving and destroyed dwarfs at infall is similar in the various Auriga haloes, and is dominated by the destroyed dwarfs. There is, however, a strong dependence on infall time: destroyed dwarfs typically have early infall times of less than 6 Gyr (since the big bang), whereas the majority of dwarfs accreted after 10 Gyr have survived to the present day. Because of their late infall, the surviving satellites have higher metallicities at infall than their destroyed counterparts of similar mass at infall; the difference is even more pronounced for the present-day metallicities of satellites, many of which continue to form stars after infall, in particular for $M_{\rm star}\gt 10^7 \, {\rm M}_\odot$. In agreement with previous work, we find that a small number of relatively massive destroyed dwarf galaxies dominate the mass of stellar haloes. However, there is a significant radial dependence: while 90 per cent of the mass in the inner regions (${\lt}20\,$ kpc) is contributed, on average, by only three massive progenitors, the outer regions (${\gt}100\,$ kpc) typically have ∼8 main progenitors of relatively lower mass. Finally, we show that a few massive progenitors dominate the metallicity distribution of accreted stars, even at the metal-poor end. Contrary to common assumptions in the literature, stars from dwarf galaxies of mass $M_{\rm star}\lt 10^7 \, {\rm M}_\odot$ make up less than 10 per cent of the accreted, metal poor stars ([Fe/H] ${\lt}-3$) in the inner $50\,$ kpc.
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Mikołajewski, M., J. Mikołajewska, and T. Tomov. "Propellers — A New Class of Interacting Binaries." Symposium - International Astronomical Union 165 (1996): 451–56. http://dx.doi.org/10.1017/s0074180900055911.
Повний текст джерелаАнотація:
CH Cyg and MWC 560 are very peculiar symbiotic binaries consisting of an M giant and a white-dwarf companion. The systems have many features in common. In particular, both show occasional eruptions with sub-Eddington luminosity accompanied by flickering activity, and appearance of high-velocity jets. We present arguments that objects like CH Cyg and MWC 560 form a new subclass of interacting binaries distinguished by the presence of wind accreting magnetic white dwarfs.
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Barstow, Martin A., Ivan Hubeny, Thierry Lanz, Jay B. Holberg, and Edward M. Sion. "The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy." International Astronomical Union Colloquium 152 (1996): 203–10. http://dx.doi.org/10.1017/s0252921100035983.
Повний текст джерелаАнотація:
The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settling of He from either DAO or He-rich central stars of planetary nebulae. In this case, the youngest (hottest) DA white dwarfs may still contain visible traces of He. Spectroscopic observations now available with EUVE provide a crucial test of these ideas. Analysis of data from the EUVE Guest Observer programme and EUVE public archive allows quantitative consideration of the sources of EUV opacity and places limits on the abundance of He which may be present.
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Deason, Alis J., Sownak Bose, Azadeh Fattahi, Nicola C. Amorisco, Wojciech Hellwing, and Carlos S. Frenk. "Dwarf stellar haloes: a powerful probe of small-scale galaxy formation and the nature of dark matter." Monthly Notices of the Royal Astronomical Society 511, no. 3 (December 3, 2021): 4044–59. http://dx.doi.org/10.1093/mnras/stab3524.
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ABSTRACT We use N-body cosmological simulations and empirical galaxy models to study the merger history of dwarf-mass galaxies (with $M_{\rm halo}\sim 10^{10}\, \mathrm{M}_\odot$). Our input galaxy models describe the stellar mass–halo mass relation, and the galaxy occupation fraction. The number of major and minor mergers depends on the type of dark matter; in particular, minor mergers are greatly suppressed in warm dark matter models. In addition, the number of mergers that bring in stars is strongly dependent on the galaxy occupation model. For example, minor mergers are negligible for stellar halo growth in models with a high mass threshold for galaxy formation (i.e. $10^{9.3}\, \mathrm{M}_\odot$ at z = 0). Moreover, this threshold for galaxy formation can also determine the relative difference (if any) between the stellar haloes of satellite and field dwarfs. Using isolated simulations of dwarf–dwarf mergers, we show that the relative frequency of major and minor mergers predict very different stellar haloes: Typically, ‘intermediate’ dark matter merger ratios (∼1:5) maximize the growth of distant stellar haloes. We discuss the observability of dwarf stellar haloes and find that the surface brightness of these features are incredibly faint. However, when several dwarfs are stacked together, models that form particularly rich stellar haloes could be detectable. Finally, we show that stellar streams in the Galactic halo overlapping in phase space with known dwarf satellites are likely remnants of their stripped stellar haloes. The mere existence of dwarf stellar haloes can already put constraints on some small-scale models, and thus observational probes should be a high priority.
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Pandya, Viraj, Drummond B. Fielding, Daniel Anglés-Alcázar, Rachel S. Somerville, Greg L. Bryan, Christopher C. Hayward, Jonathan Stern, et al. "Characterizing mass, momentum, energy, and metal outflow rates of multiphase galactic winds in the FIRE-2 cosmological simulations." Monthly Notices of the Royal Astronomical Society 508, no. 2 (October 2, 2021): 2979–3008. http://dx.doi.org/10.1093/mnras/stab2714.
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ABSTRACT We characterize mass, momentum, energy, and metal outflow rates of multiphase galactic winds in a suite of FIRE-2 cosmological ‘zoom-in’ simulations from the Feedback in Realistic Environments (FIRE) project. We analyse simulations of low-mass dwarfs, intermediate-mass dwarfs, Milky Way-mass haloes, and high-redshift massive haloes. Consistent with previous work, we find that dwarfs eject about 100 times more gas from their interstellar medium (ISM) than they form in stars, while this mass ‘loading factor’ drops below one in massive galaxies. Most of the mass is carried by the hot phase (>105 K) in massive haloes and the warm phase (103−105 K) in dwarfs; cold outflows (<103 K) are negligible except in high-redshift dwarfs. Energy, momentum, and metal loading factors from the ISM are of order unity in dwarfs and significantly lower in more massive haloes. Hot outflows have 2−5 × higher specific energy than needed to escape from the gravitational potential of dwarf haloes; indeed, in dwarfs, the mass, momentum, and metal outflow rates increase with radius whereas energy is roughly conserved, indicating swept up halo gas. Burst-averaged mass loading factors tend to be larger during more powerful star formation episodes and when the inner halo is not virialized, but we see effectively no trend with the dense ISM gas fraction. We discuss how our results can guide future controlled numerical experiments that aim to elucidate the key parameters governing galactic winds and the resulting associated preventative feedback.
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Canal, Ramon, Domingo Garcia, Jordi Isern, and Javier Labay. "Can C + O white dwarfs form neutron stars?" Astrophysical Journal 356 (June 1990): L51. http://dx.doi.org/10.1086/185748.
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Fensch, Jérémy, Pierre-Alain Duc, Médéric Boquien, Debra M. Elmegreen, Bruce G. Elmegreen, Frédéric Bournaud, Elias Brinks, et al. "Massive star cluster formation and evolution in tidal dwarf galaxies." Astronomy & Astrophysics 628 (August 2019): A60. http://dx.doi.org/10.1051/0004-6361/201834403.
Повний текст джерелаАнотація:
Context. The formation of globular clusters remains an open debate. Dwarf starburst galaxies are efficient at forming young massive clusters with similar masses as globular clusters and may hold the key to understanding their formation. Aims. We study star cluster formation in a tidal debris, including the vicinity of three tidal dwarf galaxies, in a massive gas-dominated collisional ring around NGC 5291. These dwarfs have physical parameters that differ significantly from local starbursting dwarfs. They are gas rich, highly turbulent, their gas metallicity is already enriched up to half solar values, and they are expected to be free of dark matter. The aim is to study massive star cluster formation in this as yet unexplored type of environment. Methods. We used imaging from the Hubble Space Telescope using broadband filters that cover the wavelength range from the near-ultraviolet to the near-infrared. We determined the masses and ages of the cluster candidates by using the spectral energy distribution-fitting code CIGALE. We considered age-extinction degeneracy effects on the estimation of the physical parameters. Results. We find that the tidal dwarf galaxies in the ring of NGC 5291 are forming star clusters with an average efficiency of ∼40%, which is similar to blue compact dwarf galaxies. We also find massive star clusters for which the photometry suggests that they were formed at the very birth of the tidal dwarf galaxies. These clusters have survived for several hundred million years. Therefore our study shows that extended tidal dwarf galaxies and compact clusters may be formed simultaneously. In the specific case observed here, the young star clusters are not massive enough to survive for a Hubble time. However, it may be speculated that similar objects at higher redshift, with a higher star formation rate, might form some of the long-lived globular clusters.
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Kouwenhoven, M. B. N., Yun Li, D. Stamatellos, and S. P. Goodwin. "Circumstellar disk fragmentation and the origin of massive planetary companions, brown dwarfs, and very low-mass stars." Proceedings of the International Astronomical Union 14, S345 (August 2018): 239–40. http://dx.doi.org/10.1017/s174392131800827x.
Повний текст джерелаАнотація:
AbstractThe low-mass end of the initial mass function remains poorly understood. In this mass range, very low-mass stars, brown dwarfs, and massive planets are able to form through a variety of physical processes. Here, we study the long-term evolution of disk-fragmented systems around low-mass stars, for the epoch up to 10 Myr (the typical lifetime of an embedded cluster) and up to 10 Gyr (the age of the Milky Way). We carry out N-body simulations to study the decay of disk-fragmented systems and the resulting end products. Our simulations indicate rapid decay and frequent physical collisions during the first 10 Myr. We find that disk fragmentation provides a viable mechanism for explaining hierarchical triple systems, the brown dwarf desert, single and binary brown dwarfs, and very low-mass binary systems in the solar neighbourhood.
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Haynes, Martha P. "The Future of Dwarf Galaxy Research: What Telescopes Will Discover." Proceedings of the International Astronomical Union 14, S344 (August 2018): 3–16. http://dx.doi.org/10.1017/s1743921319000073.
Повний текст джерелаАнотація:
AbstractDwarf galaxies make ideal laboratories to test galaxy evolution paradigms and cosmological models. Detailed studies of dwarfs across the spectrum allow us to gauge the efficacy of astrophysical processes at play in the lowest mass halos such as gas accretion, feedback, turbulence and chemical enrichment. Future observational studies will deliver unprecedented insights on the orbits of dwarf companions around the Milky Way, on their star formation histories and on the 3-D internal motions of their stars. Over large volumes, we will assess the impact of local environment on baryon cycling and star formation laws, leading to a full picture of the evolution of dwarfs across cosmic time. In combination, future discoveries promise to trace the history of assembly within the Local Group and beyond, probe how stars form under pristine conditions, and test models of structure formation on small scales.
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Helling, Christiane, and Paul B. Rimmer. "Lightning and charge processes in brown dwarf and exoplanet atmospheres." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 377, no. 2154 (August 5, 2019): 20180398. http://dx.doi.org/10.1098/rsta.2018.0398.
Повний текст джерелаАнотація:
The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from three-dimensional simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionization processes that will affect their atmosphere chemistry: the dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H 2 O. Such planetary atmospheres exhibit a substantial degree of thermal ionization and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionized, H 3 + -forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather system-driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H 3 + in the upper atmosphere of a brown dwarf (e.g. LSR-J1835), which may react with it to form hydronium, H 3 O + , as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H 3 O + as an H 3 + product. This article is part of a discussion meeting issue ‘Advances in hydrogen molecular ions: H 3 + , H 5 + and beyond’.
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Bekki, Kenji. "Physical processes for the origin of globular clusters with multiple stellar populations." Proceedings of the International Astronomical Union 10, H16 (August 2012): 253–54. http://dx.doi.org/10.1017/s1743921314005651.
Повний текст джерелаАнотація:
AbstractWe numerically investigate the formation processes of globular clusters (GCs) in gas-rich dwarf galaxies at high redshifts. Our particular focus is on how the first and second generations of stars can be formed from high-density gas clouds in dwarf galaxies. We find that massive stellar clumps first form from massive gas clumps that are developed from local gravitational instability in gas-rich dwarfs. These stellar clumps with masses larger than ~ 2 × 106 M⊙ can finally become the first generation of stars in GCs. After supernova explosion expels the remaining gas in the clumps, stars can form from eject of AGB stars that is accreted onto the central regions of the clumps (i.e., first generation of stars). The compact clusters of these stars have much higher densities and a significant amount of internal rotation (~ 5 km s−1) in comparison with the first generation and thus correspond to the second generation.
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Meier, David S., Jean L. Turner, and Sara C. Beck. "High-resolution radio data of molecular gas in dwarf starburst galaxies." Symposium - International Astronomical Union 193 (1999): 747–48. http://dx.doi.org/10.1017/s0074180900206712.
Повний текст джерелаАнотація:
Star-formation in dwarf starburst galaxies can be quite intense for their sizes, especially in the very young ones characterized by Wolf-Rayet emission features. Since stars form out of molecular gas (H2), it is important to investigate in detail, where the molecular gas is relative to the star formation in such young starbursts. The molecular gas may also indicate what might have triggered the starburst. The ISM in dwarfs can be quite warm, is the molecular gas also warm? In an attempt to investigate the properties of molecular gas in WR galaxies we have obtained high resolution images of the J = 2–1 transitions of CO for two dwarf starbursts, NGC5253 and He2–10. Both galaxies are nearby dwarf starbursts with WR emission features (e.g., Conti 1991). NGC 5253 may be one of the youngest starbursts known (e.g., Rieke, Lebofsky & Walker 1988).
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Natarajan, Priyamvada. "A new channel to form IMBHs throughout cosmic time." Monthly Notices of the Royal Astronomical Society 501, no. 1 (December 3, 2020): 1413–25. http://dx.doi.org/10.1093/mnras/staa3724.
Повний текст джерелаАнотація:
ABSTRACT While the formation of the first black holes (BHs) at high redshift is reasonably well understood though debated, massive BH formation at later cosmic epochs has not been adequately explored. We present a gas accretion driven mechanism that can build-up BH masses rapidly in dense, gas-rich nuclear star clusters (NSCs). Wind-fed supraexponential accretion in these environments under the assumption of net zero angular momentum for the gas, can lead to extremely rapid growth, scaling stellar mass remnant seed BHs up to the intermediate mass black hole (IMBH) range. This new long-lived channel for IMBH formation permits growth to final masses ranging from 50 to 105 M⊙. Growth is modulated by the gas supply, and premature termination can result in the formation of BHs with masses between 50 and a few 100 M⊙ filling in the so-called mass gap. Typically, growth is unimpeded and will result in the formation of IMBHs with masses ranging from ∼100 to 105 M⊙. New detections from the LIGO–VIRGO source GW190521 to the emerging population of ∼105 M⊙ BHs harboured in low-mass dwarf galaxies are revealing this elusive population. Naturally accounting for the presence of off-centre BHs in low-mass dwarfs, this new pathway also predicts the existence of a population of wandering non-central BHs in more massive galaxies detectable via tidal disruption events and as gravitational wave coalescences. Gas-rich NSCs could therefore serve as incubators for the continual formation of BHs over a wide range in mass throughout cosmic time.
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Guevara-Aguirre, Jaime, Carolina Guevara, Alexandra Guevara, and Antonio AWD Gavilanes. "Branding of subjects affected with genetic syndromes of severe short stature in developing countries." BMJ Case Reports 13, no. 2 (February 2020): e231737. http://dx.doi.org/10.1136/bcr-2019-231737.
Повний текст джерелаАнотація:
In Ecuador, a developing South American country, subjects affected with genetic syndromes of severe short stature are commonly referred to as dwarfs or midgets. Furthermore, and because in earlier studies some patients had evidenced mental retardation, such abnormality is assumed to exist in all affected subjects. Herein, we present two discrete instances in which this type of branding occurs. The first is that of individuals with Laron syndrome who are still called ‘dwarfs’ and considered as having a degree of mental retardation despite evidence showing otherwise. A similar problem, that of a girl affected with a genetic syndrome of short stature, which might include mental retardation, is also discussed. Considering that stigmatising is a form of discrimination, it concerns us all. Hence, the use of derogatory terms such as midget, dwarf or cretin, that might unintentionally occur even when delivering the best and most devoted medical care, must be eliminated.
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Day, Charles. "How did dwarf galaxies form?" Physics Today 68, no. 3 (March 2015): 16–17. http://dx.doi.org/10.1063/pt.3.2710.
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Guidarelli, G., J. Nordhaus, J. Carroll-Nellenback, L. Chamanady, A. Frank, and E. G. Blackman. "The formation of discs in the interior of AGB stars from the tidal disruption of planets and brown dwarfs." Monthly Notices of the Royal Astronomical Society 511, no. 4 (March 8, 2022): 5994–6000. http://dx.doi.org/10.1093/mnras/stac463.
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ABSTRACT A significant fraction of isolated white dwarfs host magnetic fields in excess of a MegaGauss. Observations suggest that these fields originate in interacting binary systems where the companion is destroyed thus leaving a singular, highly magnetized white dwarf. In post-main-sequence evolution, radial expansion of the parent star may cause orbiting companions to become engulfed. During the common envelope phase, as the orbital separation rapidly decreases, low-mass companions will tidally disrupt as they approach the giant’s core. We hydrodynamically simulate the tidal disruption of planets and brown dwarfs, and the subsequent accretion disc formation, in the interior of an asymptotic giant branch star. Compared to previous steady-state simulations, the resultant discs form with approximately the same mass fraction as estimated but have not yet reached steady state and are morphologically more extended in height and radius. The long-term evolution of the disc and the magnetic fields generated therein require future study.
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Trevascus, David, Daniel J. Price, Rebecca Nealon, David Liptai, Christopher J. Manser, and Dimitri Veras. "Formation of eccentric gas discs from sublimating or partially disrupted asteroids orbiting white dwarfs." Monthly Notices of the Royal Astronomical Society: Letters 505, no. 1 (May 5, 2021): L21—L25. http://dx.doi.org/10.1093/mnrasl/slab043.
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ABSTRACT Of the 21 known gaseous debris discs around white dwarfs, a large fraction of them display observational features that are well described by an eccentric distribution of gas. In the absence of embedded objects or additional forces, these discs should not remain eccentric for long time-scales, and should instead circularize due to viscous spreading. The metal pollution and infrared excess we observe from these stars is consistent with the presence of tidally disrupted sub-stellar bodies. We demonstrate, using smoothed particle hydrodynamics, that a sublimating or partially disrupting planet on an eccentric orbit around a white dwarf will form and maintain a gas disc with an eccentricity within 0.1 of, and lower than, that of the orbiting body. We also demonstrate that the eccentric gas disc observed around the white dwarf SDSS J1228 + 1040 can be explained by the same hypothesis.
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Dalle Mese, G., O. López-Cruz, W. J. Schuster, C. Chavarría-K та H. J. Ibarra-Medel. "The average physical properties of A-G stars derived from uvby-H β Strömgren–Crawford photometry as the basis for a spectral-classification synthetical approach". Monthly Notices of the Royal Astronomical Society 494, № 2 (30 березня 2020): 2995–3013. http://dx.doi.org/10.1093/mnras/staa816.
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ABSTRACT We have revisited and updated the uvby Strömgren colour and colour–index distributions of A-, F-, and early G-type main-sequence stars. For this aim, we selected 7054 dwarf stars along with 65 Morgan–Keeman (MK) standard stars within the same spectral range but covering all luminosity classes. The standard stars were selected following the MK mandate strictly, using spectra taken at classification resolution recorded on photographic plates. We used the colours of these stars to determine the effective temperature and surface gravity. After correcting for systematic offsets using fundamental parameters and considering a few exceptions, we find a one-to-one correspondence, among MK spectral types, Strömgren photometry, and their associated physical properties. We have applied a principal component analysis to the mean Strömgren indices for dwarf stars complemented by MK standards for higher luminosity classes. We have used the projections to introduce three new photometric metaindices, namely SM1, SM2, and SM3. We have defined a 3D-box, which allowed us to segregate dwarf stars from bright giants and supergiant stars, with the aid of the metaindices. Two of the planes show that the projections of dwarfs and supergiants are ordered by temperature; however, the temperature dependence for the supergiants is not as well defined as for the dwarfs. Following the MK Process, we were able to form an automatic classifier. We present some applications and assigned synthetical spectral types. We suggest that our metaindices formalism allows the extension of Strömgren photometric outside its original mandate (i.e. later types), without requiring the introduction of additional photometric filters.
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McGehee, Peregrine M., Suzanne L. Hawley, and Kevin R. Covey. "Looking for Very Low-Mass Pre-Main Sequence Objects with SDSS." Symposium - International Astronomical Union 211 (2003): 525–26. http://dx.doi.org/10.1017/s0074180900211285.
Повний текст джерелаАнотація:
Determining the process(es) by which brown dwarfs form is key to understanding their intrinsic nature. If their origins are within circumstellar disks they are akin to giant planets. If, on the other hand, they coalesce from molecular cloud cores, then they share a common lineage with low mass stars. These two mechanisms can be distinguished by investigation of young (< 10 Myr) substellar objects. If brown dwarfs are small failed stars, we expect to find very low mass analogs of the Classical T Tauris, with primordial magnetic fields from the molecular cloud collapse. Accretion onto these objects leads to characteristic magnetic activity signatures such as chromospheric and coronal emission resulting in an ultraviolet excess continuum.The Orion OB1b association (m-M = 7.9, 2 Myr) provides a laboratory for following the strength and occurrence of the accretion process as a function of mass. Studies of the substellar mass function within the sigma Orionis cluster at the southern end of the association indicate that brown dwarfs are common. Based on model isochrones and the SDSS M dwarf sequence we expect the 95% completeness limit of the “Orion” scans to correspond to 0.1 and 0.03 solar masses for the u and g bands.
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Veras, Dimitri, Nikolaos Georgakarakos, Alexander J. Mustill, Uri Malamud, Tim Cunningham, and Ian Dobbs-Dixon. "The entry geometry and velocity of planetary debris into the Roche sphere of a white dwarf." Monthly Notices of the Royal Astronomical Society 506, no. 1 (June 11, 2021): 1148–64. http://dx.doi.org/10.1093/mnras/stab1667.
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ABSTRACT Our knowledge of white dwarf planetary systems predominately arises from the region within a few Solar radii of the white dwarfs, where minor planets breakup, form rings and discs, and accrete on to the star. The entry location, angle, and speed into this Roche sphere has rarely been explored but crucially determines the initial geometry of the debris, accretion rates on to the photosphere, and ultimately the composition of the minor planet. Here, we evolve a total of over 105 asteroids with single-planet N-body simulations across the giant branch and white dwarf stellar evolution phases to quantify the geometry of asteroid injection into the white dwarf Roche sphere as a function of planetary mass and eccentricity. We find that lower planetary masses increase the extent of anisotropic injection and decrease the probability of head-on (normal to the Roche sphere) encounters. Our results suggest that one can use dynamical activity within the Roche sphere to make inferences about the hidden architectures of these planetary systems.
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Pustilnik, Simon A., Dmitri I. Makarov, and Arina L. Tepliakova. "Nearby void dwarf galaxies: recent results, the ongoing project and prospects." Proceedings of the International Astronomical Union 14, S344 (August 2018): 396–99. http://dx.doi.org/10.1017/s1743921318006713.
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AbstractProperties of dwarf galaxies formed and evolved in the lowest density environment remain largely unexplored and poorly understood. Especially this concerns the low-mass end (Mbar < 109M⊙). We overview the results of systematic study of a hundred void dwarfs from the nearby Lynx-Cancer void. We describe the ongoing project aiming to form Nearby Void galaxy sample (R < 25 Mpc) over the whole sky. 1354 objects with distances less than 25 Mpc fall within 25 voids delineated by 460 luminous galaxies/groups. The void major sizes range from 13 to 37 Mpc. 1088 of 1354 void galaxies reside deeply in voids, having distances to the nearest luminous neighbour of 2–11 Mpc. 195 nearest void galaxies reside in the Local Volume. We summarize the main statistical properties of the new sample and outline the prospects of study of both, the void dwarf properties and the fine structure of voids.
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De Luca, Andrea, Beate Stelzer, Adam J. Burgasser, Daniele Pizzocaro, Piero Ranalli, Stefanie Raetz, Martino Marelli, et al. "EXTraS discovery of an X-ray superflare from an L dwarf." Astronomy & Astrophysics 634 (February 2020): L13. http://dx.doi.org/10.1051/0004-6361/201937163.
Повний текст джерелаАнотація:
We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331−27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331−27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331−27 to be L1. The X-ray flare has an energy of EX, F ∼ 2 × 1033 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Ms of XMM-Newton data we derive an upper limit of LX, qui < 1027 erg s−1. The flare peak luminosity (LX, peak = 6.3 × 1029 erg s−1), flare duration (τdecay ≈ 2400 s), and plasma temperature (≈16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ∼2100 K. However, the absence of any other flares above the detection threshold of EX, F ∼ 2.5 × 1032 erg in a total of ∼2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ∼ E−2, suggesting that magnetic energy release in J0331−27 – and possibly in all L dwarfs – takes place predominantly in the form of giant flares.
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Bradley, P. A., and M. A. Wood. "An Example Demonstrating the Potential for Asteroseismology of DB White Dwarf Stars." International Astronomical Union Colloquium 139 (1993): 116. http://dx.doi.org/10.1017/s025292110011704x.
Повний текст джерелаАнотація:
AbstractWe present the results of a parametric survey of evolutionary models of compositionally stratified white dwarfs with helium surface layers (DB white dwarfs). Because white dwarfs are the most common final end state of stellar evolution, determining their internal structure will offer us many clues about stellar evolution, the physics of matter under extreme conditions, plus the history of star formation and age of the local Galactic disk. As a first step towards determining the internal structure of DB white dwarf stars, we provide a comprehensive set of theoretical g-mode pulsation periods for comparison to observations.Because DB white dwarfs have a layered structure consisting of a helium layer overlying the carbon/oxygen core, some modes will have the same wavelength as the thickness of the helium layer, allowing a resonance to form. This resonance is called mode trapping (see Brassard et al. 1992 and references therein) and has directly observable consequences, because modes at or near the resonance have eigenfunctions and pulsation periods that are similar to each other. This results in much smaller period spacings between consecutive overtone modes of the same spherical harmonic index than the uniform period spacings seen between non-trapped modes. We demonstrate with an example how one can use the distribution of pulsation periods to determine the total stellar mass, the mass of the helium surface layer, and the extent of the helium/carbon and carbon/oxygen transition zones. With these tools, we have the prospect of being able to determine the structure of the observed DBV white dwarfs, once the requisite observations become available.We are grateful to C.J. Hansen, S.D. Kawaler, R.E. Nather, and D.E. Winget for their encouragement and many discussions. This research was supported by the National Science Foundation under grants 85-52457 and 90-14655 through the University of Texas and McDonald Observatory.
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Meng, Xiangcun, and Philipp Podsiadlowski. "The common-envelope wind model for type Ia supernovae." Proceedings of the International Astronomical Union 14, S343 (August 2018): 470–71. http://dx.doi.org/10.1017/s1743921318006543.
Повний текст джерелаАнотація:
AbstractWe have developed a new version of the SD model for type Ia supernovae (SNe Ia) in which a common envelope (CE) is assumed to form if the mass-transfer rate between a carbon/oxygen white dwarf (CO WD) and its companion exceeds a critical accretion rate. Based on this model, we found that both SN 2002cx-like and SN Ia-CSM objects may share a similar origin, i.e. these peculiar objects may originate from the explosion of hybrid carbon/oxygen/neon white dwarfs (CONe WDs) in SD systems, where SNe Ia-CSM explode in systems with a massive CE of ∼1 M⊙, while SN 2002cx-like events correspond to events without a massive CE.
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Tsuji, Takashi. "Dust Formation in Stellar Photospheres." Symposium - International Astronomical Union 177 (2000): 313–24. http://dx.doi.org/10.1017/s0074180900002552.
Повний текст джерелаАнотація:
We examine whether dust forms in the photospheres of carbon-rich stars by referring to the case of red and brown dwarfs for which some observational clues on dust formation are now known. Dust may form in the photospheres of dwarf carbon stars and produce significant effects on both their structure and spectra. In carbon-rich asymptotic giant branch stars, dust probably forms in the photosphere, if not in the circumstellar envelope, and radiation pressure on dust is sufficient to expel the matter directly from the photosphere. This fact may play some role in mass-loss from cool luminous stars in general, including non-pulsating stars for which no successful mechanism of mass-loss was known.
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Makarova, Viktorija, and Justyna Petrovska. "The Narrative Form in Leonid Yuzefovich's Novel “Cranes and Dwarfs”." Verbum 11 (January 20, 2021): 5. http://dx.doi.org/10.15388/verb.16.
Повний текст джерелаАнотація:
Исследование показало, что свободный косвенный дискурс использовался в романе Леонида Юзефовича «Журавли и карлики»для передачи точки зрения речи и повествования от лица всех трех главных героев романа (Жохова, Шубина и Анкудинова), доминировала при этом точка зрения Жохова. В статье обсуждаются обнаруженные нами в тексте романа «Журавли и карлики» виды конструирования свободного косвенного дискурса. А именно:несовпадение субъекта речи и субъекта сознания в указании пространственной или временной локализации персонажа; вплетение в голос субъекта речи голоса персонажа посредством указания на восприятие (физическое или интеллектуальное) ситуации, или объекта, или природных явлений персонажем; передача в рамках речевого акта повествователя мыслей и субъективного восприятия действительности персонажем; передача в рамках речевого акта повествователя внутреннего состояния и эмоциональных переживаний персонажа; соединение голосов повествователя и персонажа способом называния родственных связей и частей тела персонажа.В статье также приводится список речевых маркеров, свидетельствующих о совмещении точек зрения автора и персонажа в рамках одного речевого акта.
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Jahn, Ethan D., Laura V. Sales, Andrew Wetzel, Michael Boylan-Kolchin, T. K. Chan, Kareem El-Badry, Alexandres Lazar, and James S. Bullock. "Dark and luminous satellites of LMC-mass galaxies in the FIRE simulations." Monthly Notices of the Royal Astronomical Society 489, no. 4 (September 5, 2019): 5348–64. http://dx.doi.org/10.1093/mnras/stz2457.
Повний текст джерелаАнотація:
ABSTRACT Within lambda cold dark matter ($\Lambda$CDM), dwarf galaxies like the Large Magellanic Cloud (LMC) are expected to host numerous dark matter subhaloes, several of which should host faint dwarf companions. Recent Gaia proper motions confirm new members of the LMC system in addition to the previously known SMC, including two classical dwarf galaxies ($M_\ast$$\gt 10^5$ M$_{\odot }$; Carina and Fornax) as well as several ultrafaint dwarfs (Car2, Car3, Hor1, and Hyd1). We use the Feedback In Realistic Environments (FIRE) simulations to study the dark and luminous (down to ultrafaint masses, $M_\ast$$\sim$6$\times 10^ {3}$ M$_{\odot }$) substructure population of isolated LMC-mass hosts ($M_{\text{200m}}$ = 1–3$\times 10^ {11}$ M$_{\odot }$) and place the Gaia + DES results in a cosmological context. By comparing number counts of subhaloes in simulations with and without baryons, we find that, within 0.2 $r_{\text{200m}}$, LMC-mass hosts deplete $\sim$30 per cent of their substructure, significantly lower than the $\sim$70 per cent of substructure depleted by Milky Way (MW) mass hosts. For our highest resolution runs ($m_\text{bary}$ = 880 M$_{\odot }$), $\sim 5\!-\!10$ subhaloes form galaxies with $M_\ast$$\ge 10^{4}$ M$_{\odot }$ , in agreement with the seven observationally inferred pre-infall LMC companions. However, we find steeper simulated luminosity functions than observed, hinting at observation incompleteness at the faint end. The predicted DM content for classical satellites in FIRE agrees with observed estimates for Carina and Fornax, supporting the case for an LMC association. We predict that tidal stripping within the LMC potential lowers the inner dark matter density of ultrafaint companions of the LMC. Thus, in addition to their orbital consistency, the low densities of dwarfs Car2, Hyd1, and Hyd2 reinforce their likelihood of Magellanic association.