1,237 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
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
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
The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters
Various galactic globular clusters display abundance anomalies that affect
the morphology of their colour-magnitude diagrams. In this paper we consider
the possibility of helium enhancement in the anomalous horizontal branch of NGC
2808. We examine the dynamics of a self-enrichment scenario in which an initial
generation of stars with a top-heavy initial mass function enriches the
interstellar medium with helium via the low-velocity ejecta of its asymptotic
giant branch stars. This enriched medium then produces a second generation of
stars which are themselves helium-enriched. We use a direct N-body approach to
perform five simulations and conclude that such two-generation clusters are
both possible and would not differ significantly from their single-generation
counterparts on the basis of dynamics. We find, however, that the stellar
populations of such clusters would differ from single-generation clusters with
a standard initial mass function and in particular would be enhanced in white
dwarf stars. We conclude, at least from the standpoint of dynamics, that
two-generation globular clusters are feasible.Comment: 24 pages, 7 figures, 3 tables. Accepted for publication in Ap
Monte Carlo Simulations of Globular Cluster Evolution. V. Binary Stellar Evolution
We study the dynamical evolution of globular clusters containing primordial
binaries, including full single and binary stellar evolution using our Monte
Carlo cluster evolution code updated with an adaptation of the single and
binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We
describe the modifications we have made to the code. We present several test
calculations and comparisons with existing studies to illustrate the validity
of the code. We show that our code finds very good agreement with direct N-body
simulations including primordial binaries and stellar evolution. We find
significant differences in the evolution of the global properties of the
simulated clusters using stellar evolution compared to simulations without any
stellar evolution. In particular, we find that the mass loss from stellar
evolution acts as a significant energy production channel simply by reducing
the total gravitational binding energy and can significantly prolong the
initial core contraction phase before reaching the binary-burning quasi steady
state of the cluster evolution as noticed in Paper IV. We simulate a large grid
of clusters varying the initial cluster mass, binary fraction, and
concentration and compare properties of the simulated clusters with those of
the observed Galactic globular clusters (GGCs). We find that our simulated
cluster properties agree well with the observed GGC properties. We explore in
some detail qualitatively different clusters in different phases of their
evolution, and construct synthetic Hertzprung-Russell diagrams for these
clusters.Comment: 46 preprint pages, 18 figures, 3 tables, submitted to Ap
Stellar Collisions and the Interior Structure of Blue Stragglers
Collisions of main sequence stars occur frequently in dense star clusters. In
open and globular clusters, these collisions produce merger remnants that may
be observed as blue stragglers. Detailed theoretical models of this process
require lengthy hydrodynamic computations in three dimensions. However, a less
computationally expensive approach, which we present here, is to approximate
the merger process (including shock heating, hydrodynamic mixing, mass
ejection, and angular momentum transfer) with simple algorithms based on
conservation laws and a basic qualitative understanding of the hydrodynamics.
These algorithms have been fine tuned through comparisons with the results of
our previous hydrodynamic simulations. We find that the thermodynamic and
chemical composition profiles of our simple models agree very well with those
from recent SPH (smoothed particle hydrodynamics) calculations of stellar
collisions, and the subsequent stellar evolution of our simple models also
matches closely that of the more accurate hydrodynamic models. Our algorithms
have been implemented in an easy to use software package, which we are making
publicly available (see http://vassun.vassar.edu/~lombardi/mmas/). This
software could be used in combination with realistic dynamical simulations of
star clusters that must take into account stellar collisions.Comment: This revised version has 37 pages, 13 figures, 4 tables; submitted to
ApJ; for associated software package, see
http://vassun.vassar.edu/~lombardi/mmas/ This revised version presents
additional comparisons with SPH results and slightly improved merger recipe
Dynamical age differences among coeval star clusters as revealed by blue stragglers
Globular star clusters that formed at the same cosmic time may have evolved
rather differently from a dynamical point of view (because that evolution
depends on the internal environment) through a variety of processes that tend
progressively to segregate stars more massive than the average towards the
cluster centre. Therefore clusters with the same chronological age may have
reached quite different stages of their dynamical history (that is, they may
have different dynamical ages). Blue straggler stars have masses greater than
those at the turn-off point on the main sequence and therefore must be the
result of either a collision or a mass-transfer event. Because they are among
the most massive and luminous objects in old clusters, they can be used as test
particles with which to probe dynamical evolution. Here we report that globular
clusters can be grouped into a few distinct families on the basis of the radial
distribution of blue stragglers. This grouping corresponds well to an effective
ranking of the dynamical stage reached by stellar systems, thereby permitting a
direct measure of the cluster dynamical age purely from observed properties.Comment: Published on the 20 December 2012 issue of Natur
Modest-2: A Summary
This is a summary paper of MODEST-2, a workshop held at the Astronomical
Institute ``Anton Pannekoek'' in Amsterdam, 16-17 December 2002. MODEST is a
loose collaboration of people interested in MOdelling DEnse STellar systems,
particularly those interested in modelling these systems using all the
available physics (stellar dynamics, stellar evolution, hydrodynamics and the
interplay between the three) by defining interfaces between different codes. In
this paper, we summarize 1) the main advances in this endeavour since MODEST-1;
2) the main science goals which can be and should be addressed by these types
of simulations; and 3) the most pressing theoretical and modelling advances
that we identified.Comment: Accepted by New Astronom
The Angular Momentum Evolution of Very Low Mass Stars
We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)Comment: To appear in the May 10, 2000 Ap
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