7 research outputs found
Binary Star Disruption in Globular Clusters with Multiple Stellar Populations
The discovery of multiple stellar populations in globular clusters raises
fundamental questions concerning the formation and dynamical history of these
systems. In a previous study aimed at exploring the formation of
second-generation (SG) stars from the ejecta of first-generation (FG) AGB
stars, and the subsequent dynamical evolution of the cluster, we showed that SG
stars are expected to form in a dense subsystem concentrated in the inner
regions of the FG cluster. In this paper we explore the implications of the
structural properties of multiple-population clusters, and in particular the
presence of the inner SG subsystem, for the disruption of binary stars. We
quantify the enhancement of the binary disruption rate due to the presence of
the central SG subsystem for a number of different initial conditions. Our
calculations show that SG binaries, which are assumed to be more concentrated
in the cluster inner regions, are disrupted at a substantially larger rate than
FG binaries. Assuming a similar initial fraction of FG and SG binaries, our
dynamical study indicates that the SG population is now expected to contain a
significantly smaller binary fraction than the FG population.Comment: 6 pages, 6 figures. Accepted for publication in MNRA
Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen
Horizontal Branch stars belong to an advanced stage in the evolution of the
oldest stellar galactic population, occurring either as field halo stars or
grouped in globular clusters. The discovery of multiple populations in these
clusters, that were previously believed to have single populations gave rise to
the currently accepted theory that the hottest horizontal branch members (the
blue hook stars, which had late helium-core flash ignition, followed by deep
mixing) are the progeny of a helium-rich "second generation" of stars. It is
not known why such a supposedly rare event (a late flash followed by mixing) is
so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal
branch stars 10 , or why the blue hook luminosity range in this massive cluster
cannot be reproduced by models. Here we report that the presence of helium core
masses up to \sim 0.04 solar masses larger than the core mass resulting from
evolution is required to solve the luminosity range problem. We model this by
taking into account the dispersion in rotation rates achieved by the
progenitors, whose premain sequence accretion disc suffered an early disruption
in the dense environment of the cluster's central regions where
second-generation stars form. Rotation may also account for frequent
late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201
High-resolution three-dimensional simulations of gas removal from ultrafaint dwarf galaxies
Context. The faintest Local Group galaxies found lurking in and around the Milky Way halo provide a unique test bed for theories of structure formation and evolution on small scales. Deep Subaru and Hubble Space Telescope photometry demonstrates that the stellar populations of these galaxies are old and that the star formation activity did not last longer than 2 Gyr in these systems. A few mechanisms that may lead to such a rapid quenching have been investigated by means of hydrodynamic simulations, but these have not provided any final assessment so far.
Aims. This is the first in a series of papers aimed at analyzing the roles of stellar feedback, ram pressure stripping, host-satellite tidal interactions, and reionization in cleaning the lowest mass Milky Way companions of their cold gas using high-resolution, three-dimensional hydrodynamic simulations.
Methods. We simulated an isolated ultrafaint dwarf galaxy loosely modeled after Boötes I, and examined whether or not stellar feedback alone could drive a substantial fraction of the ambient gas out from the shallow potential well.
Results. In contrast to simple analytical estimates, but in agreement with previous hydrodynamical studies, we find that most of the cold gas reservoir is retained. Conversely, a significant amount of the metal-enriched stellar ejecta crosses the boundaries of the computational box with velocities exceeding the local escape velocity and is, thus, likely lost from the system.
Conclusions. Although the total energy output from multiple supernova explosions exceeds the binding energy of the gas, no galactic-scale outflow develops in our simulations and as such, most of the ambient medium remains trapped within the weak potential well of the model galaxy. It seems thus unavoidable that to explain the dearth of gas in ultrafaint dwarf galaxies, we will have to resort to environmental effects. This will be the subject of a forthcoming paper