1,745 research outputs found
Rotation of Horizontal Branch Stars in Globular Clusters
The rotation of horizontal branch stars places important constraints on
angular momentum evolution in evolved stars and therefore rotational mixing on
the giant branch. Prompted by new observations of rotation rates of horizontal
branch stars, we calculate simple models for the angular momentum evolution of
a globular cluster giant star from the base of the giant branch to the star's
appearance on the horizontal branch. We include mass loss, and infer the
accompanied loss of angular momentum for each of four assumptions about the
internal angular momentum profile. These models are compared to observations of
horizontal branch rotation rates in M13. We find that rapid rotation on the
horizontal branch can be reconciled with slow solid body main sequence rotation
if giant branch stars have differential rotation in their convective envelopes
and a rapidly rotating core, which is then followed by a redistribution of
angular momentum on the horizontal branch. We discuss the physical reasons why
these very different properties relative to the solar case may exist in giants.
Rapid rotation in the core of the main sequence precursors of the rapidly
rotating horizontal branch star, or an angular momentum source on the giant
branch is required for all cases if the rotational velocity of turnoff stars is
less than 4 km s. We suggest that the observed range in rotation rates
on the horizontal branch is caused by internal angular momentum redistribution
which occurs on a timescale comparable to the evolution of the stars on the
horizontal branch. The apparent lack of rapid horizontal branch rotators hotter
than 12 000 K in M13 could be a consequence of gravitational settling, which
inhibits internal angular momentum transport. Alternative explanations and
observational tests are discussed.Comment: 32 pages, 7 figures, submitted to the Astrophysical Journa
Kinematic fingerprint of core-collapsed globular clusters
Dynamical evolution drives globular clusters toward core collapse, which
strongly shapes their internal properties. Diagnostics of core collapse have so
far been based on photometry only, namely on the study of the concentration of
the density profiles. Here we present a new method to robustly identify
core-collapsed clusters based on the study of their stellar kinematics. We
introduce the \textit{kinematic concentration} parameter, , the ratio
between the global and local degree of energy equipartition reached by a
cluster, and show through extensive direct -body simulations that clusters
approaching core collapse and in the post-core collapse phase are strictly
characterized by . The kinematic concentration provides a suitable
diagnostic to identify core-collapsed clusters, independent from any other
previous methods based on photometry. We also explore the effects of incomplete
radial and stellar mass coverage on the calculation of and find that our
method can be applied to state-of-art kinematic datasets.Comment: Accepted for publication in MNRAS Lette
Models of Individual Blue Stragglers
This chapter describes the current state of models of individual blue
stragglers. Stellar collisions, binary mergers (or coalescence), and partial or
ongoing mass transfer have all been studied in some detail. The products of
stellar collisions retain memory of their parent stars and are not fully mixed.
Very high initial rotation rates must be reduced by an unknown process to allow
the stars to collapse to the main sequence. The more massive collision products
have shorter lifetimes than normal stars of the same mass, while products
between low mass stars are long-lived and look very much like normal stars of
their mass. Mass transfer can result in a merger, or can produce another binary
system with a blue straggler and the remnant of the original primary. The
products of binary mass transfer cover a larger portion of the colour-magnitude
diagram than collision products for two reasons: there are more possible
configurations which produce blue stragglers, and there are differing
contributions to the blended light of the system. The effects of rotation may
be substantial in both collision and merger products, and could result in
significant mixing unless angular momentum is lost shortly after the formation
event. Surface abundances may provide ways to distinguish between the formation
mechanisms, but care must be taking to model the various mixing mechanisms
properly before drawing strong conclusions. Avenues for future work are
outlined.Comment: Chapter 12, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
The Spatial Structure of Young Stellar Clusters. III. Physical Properties and Evolutionary States
We analyze the physical properties of stellar clusters that are detected in
massive star-forming regions in the MYStIX project--a comparative,
multiwavelength study of young stellar clusters within 3.6 kpc that contain at
least one O-type star. Tabulated properties of subclusters in these regions
include physical sizes and shapes, intrinsic numbers of stars, absorptions by
the molecular clouds, and median subcluster ages. Physical signs of dynamical
evolution are present in the relations of these properties, including
statistically significant correlations between subcluster size, central
density, and age, which are likely the result of cluster expansion after gas
removal. We argue that many of the subclusters identified in Paper I are
gravitationally bound because their radii are significantly less than what
would be expected from freely expanding clumps of stars with a typical initial
stellar velocity dispersion of ~3 km/s for star-forming regions. We explore a
model for cluster formation in which structurally simpler clusters are built up
hierarchically through the mergers of subclusters--subcluster mergers are
indicated by an inverse relation between the numbers of stars in a subcluster
and their central densities (also seen as a density vs. radius relation that is
less steep than would be expected from pure expansion). We discuss implications
of these effects for the dynamical relaxation of young stellar clusters.Comment: Accepted for publication in The Astrophysical Journal ; 48 pages, 13
figures, and 6 table
Constraining global properties of the Draco dwarf spheroidal galaxy
By fitting a flexible stellar anisotropy model to the observed surface
brightness and line-of-sight velocity dispersion profiles of Draco we derive a
sequence of cosmologically plausible two-component (stars + dark matter) models
for this galaxy. The models are consistent with all the available observations
and can have either cuspy Navarro-Frenk-White or flat-cored dark matter density
profiles. The dark matter halos either formed relatively recently (at z~2...7)
and are massive (up to ~5x10^9 M_Sun), or formed before the end of the
reionization of the universe (z~7...11) and are less massive (down to ~7x10^7
M_Sun). Our results thus support either of the two popular solutions of the
"missing satellites" problem of Lambda cold dark matter cosmology - that dwarf
spheroidals are either very massive, or very old. We carry out high-resolution
simulations of the tidal evolution of our two-component Draco models in the
potential of the Milky Way. The results of our simulations suggest that the
observable properties of Draco have not been appreciably affected by the
Galactic tides after 10 Gyr of evolution. We rule out Draco being a "tidal
dwarf" - a tidally disrupted dwarf galaxy. Almost radial Draco orbits (with the
pericentric distance <15 kpc) are also ruled out by our analysis. The case of a
harmonic dark matter core can be consistent with observations only for a very
limited choice of Draco orbits (with the apocentric-to-pericentric distances
ratio of <2.5).Comment: 18 pages, 14 figures; accepted by Ap
Globular Cluster Scale Sizes in Giant Galaxies: Orbital Anisotropy and Tidally Under-filling Clusters in M87, NGC 1399, and NGC 5128
We investigate the shallow increase in globular cluster half-light radii with
projected galactocentric distance observed in the giant galaxies M87,
NGC 1399, and NGC 5128. To model the trend in each galaxy, we explore the
effects of orbital anisotropy and tidally under-filling clusters. While a
strong degeneracy exists between the two parameters, we use kinematic studies
to help constrain the distance beyond which cluster orbits become
anisotropic, as well as the distance beyond which clusters are
tidally under-filling. For M87 we find kpc and kpc and kpc.
The connection of with each galaxy's mass profile indicates the
relationship between size and may be imposed at formation, with only
inner clusters being tidally affected. The best fitted models suggest the
dynamical histories of brightest cluster galaxies yield similar present-day
distributions of cluster properties. For NGC 5128, the central giant in a small
galaxy group, we find kpc and kpc. While we
cannot rule out a dependence on , NGC 5128 is well fitted by a tidally
filling cluster population with an isotropic distribution of orbits, suggesting
it may have formed via an initial fast accretion phase. Perturbations from the
surrounding environment may also affect a galaxy's orbital anisotropy profile,
as outer clusters in M87 and NGC 1399 have primarily radial orbits while outer
NGC 5128 clusters remain isotropic.Comment: 16 pages, 7 figures, 4 tables, Accepted for publication in MNRA
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