Peter A. Bergbusch

The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting*

Seventy-two grids of stellar evolutionary tracks, along with the means to generate isochrones and luminosity/color functions from them, are presented in this investigation. Sixty of them extend (and encompass) the sets of models reported by VandenBerg et al. for 17 [Fe/H] values from -2.31 to -0.30 and ?-element abundances corresponding to [?/Fe] = 0.0, 0.3, and 0.6 (at each iron abundance) to the solar metallicity and to sufficiently high masses (up to ~2.2 M?) that isochrones may be computed for ages as low as 1 Gyr. The remaining grids contain tracks for masses from 0.4 to 4.0 M? and 12 [Fe/H] values between -0.60 and +0.49 (assuming solar metal-to-hydrogen number abundance ratios): in this case, isochrones may be calculated down to ~0.2 Gyr. The extent of convective core overshooting has been modeled using a parameterized version of the Roxburgh criterion, in which the value of the free parameter at a given mass and its dependence on mass have been determined from analyses of binary star data and the observed color-magnitude diagrams for several open clusters. Because the calculations reported herein satisfy many empirical constraints, they should provide useful probes into the properties of both simple and complex stellar populations.

Oxygen-enhanced models for globular cluster stars. II: Isochrones and luminosity functions

Isochrones and luminosity functions (LFs) on the BV photometric system have been constructed for ages from 10 to 18 Gyr for each of the seven values of [Fe/H] = -2.26, -2.03, -1.78, - 1.66, -1.48, -1.26, and -1.03, and for ages from 5 to 18 Gyr for [Fe/H] = -0.78, -0.65, and -0.47. The loci for ages of 10 Gyr or more have been provided mainly for application to photometry of the Galactic globular clusters (GCs); the rest should be useful for interpreting data for old, metal-poor open clusters. Oxygen-to-iron ratios have been assumed that closely follow the relation [O/Fe] = -0.5 [Fe/H], for [Fe/H] ≥ -1.0, and [O.Fe] = -0.2 [Fe/H] + 0.3, for lower iron abundances

Models for Old, Metal-poor Stars with Enhanced α-Element Abundances. III. Isochrones and Isochrone Population Functions

An isochrone population function (IPF) gives the relative distribution of stars along an isochrone. IPFs contain the information needed to calculate both luminosity functions and color functions, and they provide a straightforward way of generating synthetic stellar populations. An improved algorithm for interpolating isochrones and IPFs, based on the scheme introduced by Bergbusch & VandenBerg, is described. Software has been developed to permit such interpolations for any age encompassed by an input grid of stellar evolutionary tracks. Our first application of this software is to the models presented in this series of papers for 17 [Fe/H] values between -2.31 and -0.3, with three choices of [α/Fe] at each iron abundance (specifically, 0.0, 0.3, and 0.6). (These models do not treat gravitational settling or radiative acceleration processes, but otherwise they are based on up-to-date physics. Additional grids will be added to this database as they are completed.) The computer programs (written in FORTRAN 77) and the grids of evolutionary tracks that are presently available for processing by these codes into isochrones and IPFs are freely available to interested users. In addition, we add to the evidence presented in previous papers in this series in support of the Teff and color scales of our models. In particular, the temperatures derived by Gratton et al. for local Population II subdwarfs with accurate (Hipparcos) parallaxes are shown to be in excellent agreement with those predicted for them, when the Gratton et al. [Fe/H] scale is also assumed. Interestingly, the locus defined by local subdwarfs and subgiants on the (MV, log Teff)-plane and the morphologies of globular cluster (GC) color-magnitude diagrams are well matched by the present models, despite the neglect of diffusion, which suggests that some other process(es) must be at play to limit the expected effects of gravitational settling on predicted temperatures. The three field halo subgiants in our sample all appear to have ages 15 Gyr, which is favored for the Galaxys most metal-poor GCs as well. (The settling of helium and heavy elements in the central regions of stars is expected to cause about a 10% reduction in these age estimates: this effect should persist even if some process, such as turbulence at the base of the convective envelope, counteracts diffusion in the surface layers.) Furthermore, our isochrones accurately reproduce the Da Costa & Armandroff red giant branch fiducials for M15, NGC 6752, NGC 1851, and 47 Tuc on the [MI, (V-I)0]-diagram. However, our models fail to predict the observed luminosities of the red giant bump by ≈0.25 mag: this could be an indication that there is some amount of inward overshooting of convective envelopes in red giants. For consistency reasons, the Zinn & West metallicities for intermediate metal-poor GCs (-1.8 [Fe/H] -1.1) seem to be preferred over recent spectroscopic results (based on the brightest cluster giants), suggesting that there is an inconsistency between current subdwarf and GC [Fe/H] scales.

Isochrones for old (>5 Gyr) stars and stellar populations. I. Models for –2.4 ≤ [Fe/H] ≤+0.6, 0.25 ≤ Y ≤ 0.33, and –0.4 ≤ [α/Fe] ≤+0.4

Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances

BV CCD photometry of the old open cluster NGC 2243

Y = 0.25

Isochrones for Old (g 5 Gyr) Stars and Stelllar Populations. I. Models for-2.4 l= [Fe/H] l=+0.6, 0.25 l= Y l= 0.33, and-0.4 l= [alpha/Fe] l=+0.4

,

Isochrones for old (> 5 GYR) stars and stellar populations. I. models for-2.4 <= [Fe/H] <=+0.6, 0.25 <= Y <= 0.33, and-0.4 <= [alpha/Fe] <=+0.4

0.29

Luminosity functions for post-turnoff stars in globular clusters. I: NGC 288

, and

Distance-Dependent Absolute Globular Cluster Ages From the Color Distribution of Subgiant-Branch Stars

0.33

Subgiant-Branch Color Function Ages for Globular Clusters

, and for iron abundances that vary from