Alison Sills

Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog

We derive detailed theoretical models for 1074 nearby stars from the SPOCS (Spectroscopic Properties of Cool Stars) Catalog. The California and Carnegie Planet Search has obtained high-quality (R 70,000-90,000, S/N 300-500) echelle spectra of over 1000 nearby stars taken with the Hamilton spectrograph at Lick Observatory, the HIRES spectrograph at Keck, and UCLES at the Anglo Australian Observatory. A uniform analysis of the high-resolution spectra has yielded precise stellar parameters (Teff, log g, v sin i, [M/H], and individual elemental abundances for Fe, Ni, Si, Na, and Ti), enabling systematic error analyses and accurate theoretical stellar modeling. We have created a large database of theoretical stellar evolution tracks using the Yale Stellar Evolution Code (YREC) to match the observed parameters of the SPOCS stars. Our very dense grids of evolutionary tracks eliminate the need for interpolation between stellar evolutionary tracks and allow precise determinations of physical stellar parameters (mass, age, radius, size and mass of the convective zone, surface gravity, etc.). Combining our stellar models with the observed stellar atmospheric parameters and uncertainties, we compute the likelihood for each set of stellar model parameters separated by uniform time steps along the stellar evolutionary tracks. The computed likelihoods are used for a Bayesian analysis to derive posterior probability distribution functions for the physical stellar parameters of interest. We provide a catalog of physical parameters for 1074 stars that are based on a uniform set of high-quality spectral observations, a uniform spectral reduction procedure, and a uniform set of stellar evolutionary models. We explore this catalog for various possible correlations between stellar and planetary properties, which may help constrain the formation and dynamical histories of other planetary systems.

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☉). We also present models of solar analogs (0.6-1.1 M☉) for comparison with previous work. We investigate the effect of rotation on the effective temperature and luminosity of these stars. Rotation lowers the effective temperature and luminosity of the models relative to standard models of the same mass and composition. We find that the decrease in Teff and L can be significant at the higher end of our mass range but becomes small below 0.4 M☉. The effects of different assumptions about internal angular momentum transport are discussed. Formulae for relating Teff to mass and vrot are presented. We demonstrate that the kinetic energy of rotation is not a significant contribution to the luminosity of low-mass stars. Previous studies of the angular momentum evolution of low-mass stars concentrated on solar analogs and were complicated by uncertainties related to the internal transport of angular momentum. In this paper we extend our theoretical models for the angular momentum evolution of stars down to 0.1 M☉. 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☉ do not depend on the assumptions about internal angular momentum transport with the exception of a zero-point shift in the angular momentum loss saturation threshold. We argue that this makes these low-mass stars ideal candidates for the study of the angular momentum loss law and distribution of initial conditions. For stars with masses between 0.6 and 1.1 M☉, scaling the saturation threshold by the Rossby number can reproduce the observed mass dependence of the stellar angular momentum evolution. We find that neither models with solid-body rotation nor differentially rotating models can simultaneously reproduce the observed stellar spin-down in the 0.6-1.1 M☉ range and for stars between 0.1 and 0.6 M☉. We argue that 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☉). This suggests either inefficient angular momentum loss or mass-dependent initial conditions for stars near the fully convective boundary.

Cataclysmic Variables: An Empirical Angular Momentum Loss Prescription from Open Cluster Data

We apply the angular momentum loss rates inferred from open cluster stars to the evolution of cataclysmic variables (CVs). We show that the angular momentum prescriptions used in earlier CV studies are inconsistent with the measured rotation data in open clusters. The timescale for angular momentum loss () above the fully convective boundary is ~2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in behavior at the fully convective boundary. This provides evidence against the hypothesis that the period gap is caused by an abrupt change in the angular momentum loss law when the secondary becomes fully convective. Furthermore, the empirical angular momentum loss law implies a timescale for CV evolution that is comparable to a Hubble time; for the same reason, it will be more difficult to produce CVs from the products of common envelope evolution, and it implies a lower space density of CVs. The predicted loss rate for short-period CVs is consistent with the observed period minimum (1.3 hr). We infer the time-averaged mass accretion rate and derive the mass-period relation for different evolutionary states of the secondary. The steady-state accretion rates are significantly lower than the claimed observational rates; we discuss some possible explanations. The mass-period relationship is more consistent with evolved secondaries than with unevolved secondaries above the period gap. Implications for the CV period gap are discussed, including the possibility that two populations of secondaries could produce the gap.

Two distinct sequences of blue straggler stars in the globular cluster M 30

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy’s history. ‘Blue stragglers’ are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone ‘core collapse’, during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.

Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae*

We have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamics.

Blue Straggler Stars: A Direct Comparison of Star Counts and Population Ratios in Six Galactic Globular Clusters

The central regions of six Galactic globular clusters (GGCs) (M3, M80, M10, M13, M92, and NGC 288) have been imaged using HST-WFPC2 and the ultraviolet filters (F255W and F336W). The selected sample covers a large range in both central density (log ρ0) and metallicity ([Fe/H]). In this paper, we present a direct cluster-to-cluster comparison of the blue straggler star (BSS) population as selected from (m255, m255-m336) color magnitude diagrams. We have found (1) that BSSs in three of the clusters (M3, M80, M92) are much more concentrated toward the center of the cluster than the red giants; because of the smaller BSS samples for the other clusters, we can only note that the BSS radial distributions are consistent with central concentration; and (2) that the specific frequency of BSSs varies greatly from cluster to cluster. The most interesting result is that the two clusters with largest BSS specific frequency are at the central density extremes of our sample: NGC 288 (lowest central density) and M80 (highest). This evidence, together with the comparison with theoretical collisional models, suggests that both stellar interactions in high-density cluster cores and at least one other alternate channel operating in low-density GGCs play an important role in the production of BSSs. We also note a possible connection between horizontal-branch morphology and blue straggler luminosity functions in these six clusters.

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 the 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.

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. This software could be used in combination with realistic dynamical simulations of star clusters that must take into account stellar collisions.

Evolution of Stellar Collision Products in Globular Clusters. II. Off-Axis Collisions

We continue our exploration of collisionally merged stars in the blue straggler region of the color-magnitude diagram. We report the results of new smoothed particle hydrodynamics (SPH) calculations of parabolic collisions between two main-sequence stars, with the initial structure and composition profiles of the parent stars having been determined from stellar evolution calculations. Parallelization of the SPH code has permitted much higher numerical resolution of the hydrodynamics. We also present evolutionary tracks for the resulting collision products, which emerge as rapidly rotating blue stragglers. The rotating collision products are brighter, bluer, and remain on the main sequence longer than their nonrotating counterparts. In addition, they retain their rapid rotation rates throughout their main-sequence lifetime. Rotationally induced mixing strongly affects the evolution of the collision products, although it is not sufficient to mix the entire star. We discuss the implications of these results for studies of blue straggler populations in clusters. This work shows that off-axis collision products cannot become blue stragglers unless they lose a large fraction of their initial angular momentum. The mechanism for this loss is not apparent, although some possibilities are discussed.

The Contribution of Primordial Binaries to the Blue Straggler Population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue str aggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution i n the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise bey ond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be expl ained within a purely collisional scenario in which blue stragglers are generated exclusively in the core throu gh direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generate d in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-trans fer. Subject headings: stars: blue stragglers - binaries: general - globular clust ers: individual (47 Tuc)