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Stellar evolutionary tracks for $0.12 \le M/M_\odot \le 1.0$ have been computed for each of several variations in the abundances of C, N, and O, assuming mass-fraction helium abundances $Y = 0.25$ and $0.29$, and 11 metallicities in the range $-1.5 \le$ [Fe/H] $\le -0.5$, in 0.2 dex increments. Such computations are provided for mixtures with [O/Fe] between $+0.4$ and $+0.8$, for different C:N:O ratios at a fixed value of [CNO/Fe], and for enhanced C. Computer codes are provided to interpolate within these grids to produce isochrones for ages $> 7$ Gyr and to generate magnitudes and colours for many broad-band filters using bolometric corrections based on MARCS model atmospheres and synthetic spectra. The models are compared with (i) similar computations produced by other workers, (ii) observed UV, optical, and IR colour-magnitude diagrams (CMDs), (iii) the effective temperatures, $(V-I_C)_0$ and $(V-K_S)_0$ colours of Pop. II stars in the solar neighbourhood, and (iv) empirical data for the absolute magnitude of the tip of the giant branch (TRGB). The isochrones are especially successful in reproducing the observed morphologies of observed CMDs and in satisfying the TRGB constraints. They also fare quite well in explaining the IR colours of low mass stars in globular clusters, indicating that they have [O/Fe] $\approx +0.6$, though some challenges remain., Accepted for publication in the MNRAS; the paper consists of 20 pages, incuding 13 figures

$��$ Cassiopeiae is a nearby, high-velocity, metal-poor ($\rm[Fe/H]=-0.81$) visual binary. We have used high-resolution imaging with the Hubble Space Telescope (HST), obtained over nearly two decades, to determine the period (21.568 yr) and precise orbital elements. Combining these with published ground- and space-based astrometry, we determined dynamical masses for both components of $��$ Cas: $0.7440\pm0.0122\,M_\odot$ for the G5 V primary, and $0.1728\pm0.0035\,M_\odot$ for its faint dM companion. We detect no significant perturbations in the HST astrometry due to a third body in the system. The primary aim of our program was to determine, with the aid of stellar models, the helium content and age of the metal-deficient primary star, $��$ Cas A. Although we now have a precise mass, there remain uncertainties about other parameters, including its effective temperature. Moreover, a re-examination of archival interferometric observations leads to a suspicion that the angular diameter was overestimated by a few percent. In the absolute magnitude versus color plane, $��$ Cas A lies slightly cooler and more luminous than the main sequence of the globular cluster 47 Tucanae; this may imply that the star has a lower helium content, and/or is older, and/or has a higher metallicity, than the cluster. Our best estimates for the helium content and age of $��$ Cas A are $Y=0.255\pm0.014$ and $12.7\pm2.7$ Gyr--making $��$ Cas possibly the oldest star in the sky visible to the naked eye. Improved measurements of the absolute parallax of the system, the effective temperature of $��$ Cas A, and its angular diameter would provide tighter constraints., Accepted by Astrophysical Journal

We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51 arcmin (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at V = 30.7 mag. The CMD, constructed from approximately 300,000 stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as chi-squared fitting to a finely-spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (approximately 30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population, as well as a significant globular-cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo., 4 pages, Latex. 1 color figure. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication in ApJ Letters. Images available at http://hubblesite.org/newscenter/archive/2003/15/

Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances $Y = 0.25$, $0.29$, and $0.33$, and for iron abundances that vary from $-2.4$ to $+0.4$ (in 0.2 dex increments) when [$\alpha$/Fe] $= +0.4$, or for the ranges $-2.0 \le$ [Fe/H] $\le +0.6$, $-1.8 \le$ [Fe/H] $\le +0.6$ when [$\alpha$/Fe] $= 0.0$ and $-0.4$, respectively. The grids, which consist of tracks for masses from $0.12 {{\cal M}_\odot}$ to $1.1$-$1.5 {{\cal M}_\odot}$ (depending on the metallicity) are based on up-to-date physics, including the gravitational settling of helium (but not metals diffusion). Interpolation software is provided to generate isochrones for arbitrary ages between $\approx 5$ and $15$ Gyr and any values of $Y$, [$\alpha$/Fe], and [Fe/H] within the aforementioned ranges. Comparisons of isochrones with published color-magnitude diagrams (CMDs) for the open clusters M67 ([Fe/H] $\approx 0.0$) and NGC 6791 ([Fe/H] $\approx 0.3$) and for four of the metal-poor globular clusters (47 Tuc, M3, M5, and M92) indicate that the models for the observed metallicities do a reasonably good job of reproducing the locations and slopes of the cluster main sequences and giant branches. The same conclusion is reached from a consideration of plots of nearby subdwarfs that have accurate Hipparcos parallaxes and metallicities in the range $-2.0 \le$ [Fe/H] $\le -1.0$ on various CMDs and on the ($\log T_{\rm eff},\,M_V$)-diagram. A relatively hot temperature scale similar to that derived in recent calibrations of the infrared flux method is favored by both the isochrones and the adopted color transformations, which are based on the latest MARCS model atmospheres., Comment: 55 pages, including 1 table and 18 figures; accepted for publication in the Astrophysical Journal

Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances $Y = 0.25$, $0.29$, and $0.33$, and for iron abundances that vary from $-2.4$ to $+0.4$ (in 0.2 dex increments) when [$��$/Fe] $= +0.4$, or for the ranges $-2.0 \le$ [Fe/H] $\le +0.6$, $-1.8 \le$ [Fe/H] $\le +0.6$ when [$��$/Fe] $= 0.0$ and $-0.4$, respectively. The grids, which consist of tracks for masses from $0.12 {{\cal M}_\odot}$ to $1.1$-$1.5 {{\cal M}_\odot}$ (depending on the metallicity) are based on up-to-date physics, including the gravitational settling of helium (but not metals diffusion). Interpolation software is provided to generate isochrones for arbitrary ages between $\approx 5$ and $15$ Gyr and any values of $Y$, [$��$/Fe], and [Fe/H] within the aforementioned ranges. Comparisons of isochrones with published color-magnitude diagrams (CMDs) for the open clusters M67 ([Fe/H] $\approx 0.0$) and NGC 6791 ([Fe/H] $\approx 0.3$) and for four of the metal-poor globular clusters (47 Tuc, M3, M5, and M92) indicate that the models for the observed metallicities do a reasonably good job of reproducing the locations and slopes of the cluster main sequences and giant branches. The same conclusion is reached from a consideration of plots of nearby subdwarfs that have accurate Hipparcos parallaxes and metallicities in the range $-2.0 \le$ [Fe/H] $\le -1.0$ on various CMDs and on the ($\log T_{\rm eff},\,M_V$)-diagram. A relatively hot temperature scale similar to that derived in recent calibrations of the infrared flux method is favored by both the isochrones and the adopted color transformations, which are based on the latest MARCS model atmospheres., 55 pages, including 1 table and 18 figures; accepted for publication in the Astrophysical Journal

We present early results from a Hubble Space Telescope survey of the ultra-faint dwarf galaxies. These Milky Way satellites were discovered in the Sloan Digital Sky Survey, and appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. The primary goal of the survey is to measure the star-formation histories of these galaxies and discern any synchronization due to the reionization of the universe. We find that the six galaxies of our survey have very similar star-formation histories, and that each is dominated by stars older than 12 Gyr., Comment: 5 pages, 2 color figures. To appear in the proceedings of "EWASS 2013 Symposium 5: Local Group, Local Cosmology," held 8-9 July 2013 in Turku, Finland. Eds. S. Salvadori and M. Monelli. To be published in Memorie della Societa' Astronomica Italiana

We have used a combination of broad-band near-infrared and optical Johnson-Cousins photometry to study the dust properties in the line of sight to the Galactic globular cluster M4. We have investigated the reddening effects in terms of absolute strength and variation across the cluster field, as well as the form of the reddening law defined by the type of dust. Here, we determine the ratio of absolute to selective extinction (R_V) in the line of sight towards M4, which is known to be a useful indicator for the type of dust and therefore characterizes the applicable reddening law. Our method is independent of age assumptions and appears to be significantly more precise and accurate than previous approaches. We obtain A_V/E(B-V)=3.76\pm0.07 (random error) for the dust in the line of sight to M4 for our set of filters. With this value, the distance to M4 is found to be 1.80\pm0.05 kpc (random error). A reddening map for M4 has been created, which reveals a spatial differential reddening of delta E(B-V)>0.2 mag across a field within 10' around the cluster centre and a total mean reddening of E(B-V)=0.37\pm0.01. In order to provide accurate zero points for the extinction coefficients of our photometric filters, we investigated the impact of stellar parameters such as temperature, surface gravity and metallicity on the extinction properties in different bandpasses. Using both synthetic ATLAS9 spectra and observed spectral energy distributions, we found similar sized effects for the range of temperature, surface gravity, and metallicity typical of globular cluster stars: each causes a change of about 3% in the necessary correction factor for each filter combination. From our calculations, we provide extinction zero points for Johnson-Cousins and 2MASS filters, spanning a wide range of stellar parameters and dust types, suited for accurate, object-specific extinction corrections., 24 pages, 19 figures, 13 tables

Recent work has shown that most globular clusters have at least two chemically distinct components, as well as cluster-to-cluster differences in the mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order to investigate the implications of variations in the abundances of these and other metals for H-R diagrams and predicted ages, grids of evolutionary sequences have been computed for scaled solar and enhanced alpha-element mixtures, and for mixtures in which the assumed [m/Fe] value for each of the metals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by 0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages from 6 to 14 Gyr, have been computed for -3.0 < [Fe/H] < -0.6, with helium abundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded versions of the Victoria stellar structure program and the Regina interpolation code, respectively. Turnoff luminosity versus age relations from isochrones are found to depend almost entirely on the importance of the CNO-cycle, and thereby mainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do not contribute significantly to the opacities at low temperatures and densities, variations in their abundances do not impact the Teff scale of red giants. The latter is a strong function of the abundances of only Mg and Si (and Fe, possibly to a lesser extent), because they are so abundant and because they are strong sources of opacity at low temperatures. For these reasons, Mg and Si also have important effects on the temperatures of main-sequence stars. Due to their low abundances, Na, Ca, and Ti are of little consequence for stellar models. The effects of varying the adopted solar metals mix and the helium abundance at a fixed [Fe/H] are also briefly discussed., 27 pages, 18 figures; accepted and tentatively scheduled for publication in ApJ, volume 755 (Aug 10, 2012 issue)

The temperatures and colors of stellar models are much less secure than predicted luminosities; consequently, it is important to ensure that the models which are used to interpret data for stellar populations satisfy existing observational constraints on these properties. At the present time, the available Teff constraints do not appear to pose a problem for evolutionary calculations. However, the evidence seems compelling that adjustments of the color transformations from model atmospheres are needed to achieve consistency with observations of cool stars. Stellar models that are normalized to the Sun and transformed to the observed plane using recent semi-empirical color-Teff relations are able to provide superb fits to the M67 and Hyades color-magnitude diagrams. As highlighted in the following examination of Population II subdwarfs and several well-observed globular clusters (M3, M5, M68, M92, and 47 Tuc), it is much harder to constrain the Teff and color scales at low metallicities than at [Fe/H] near 0.0. It is not clear that significant improvements to current semi-empirical color-Teff relations for metal-deficient stars will be possible until much tigher observational constraints become available. Brief comparisons of the evolutionary tracks computed by different workers are also included in this investigation., Comment: To appear in Resolved Stellar Populations, ASP Conf. Ser., eds. D. Valls-Gabaud & M. Chavez, 2006, in press; 28 pages

Evolutionary models have been calculated for Pop II stars of 0.5 to 1.0$M_\odot$ from the pre-main-sequence to the lower part of the giant branch. Rosseland opacities and radiative accelerations were calculated taking into account the concentration variations of 28 chemical species, including all species contributing to Rosseland opacities in the OPAL tables. The effects of radiative accelerations, thermal diffusion and gravitational settling are included. While models were calculated both for Z=0.00017 and 0.0017, we concentrate on models with Z=0.00017 in this paper. These are the first Pop II models calculated taking radiative acceleration into account. It is shown that, at least in a 0.8$M_\odot$ star, it is a better approximation not to let Fe diffuse than to calculate its gravitational settling without including the effects of $g_{rad}(Fe)$. In the absence of any turbulence outside of convection zones, the effects of atomic diffusion are large mainly for stars more massive than 0.7$M_\odot$. Overabundances are expected in some stars with $\teff \ge 6000K$. Most chemical species heavier than CNO are affected. At 12 Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while others are limited to 3 (e.g. for Fe). The calculated surface abundances are compared to recent observations of abundances in globular clusters as well as to observations of Li in halo stars. It is shown that, as in the case of Pop I stars, additional turbulence appears to be present., 40 pages, 17 color figures, to appear in The Astrophysical Journal, April 2002 (paper with original high resolution figures can be found at http://www.cerca.umontreal.ca/~richer/Fichiersps/popII_1.ps)

With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass \sim 0.5 M_{\odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{\odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{\odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond \sim 3 \times 10^8 years will require deeper and more complete data., To appear in the Astrophysical Journal Letters, 451, September 20, 1995. The text is also available as a postscript file from http://www.astro.ubc.ca/people/richer/m4wd/m4wd.ps with postscript files of the figures available in http://www.astro.ubc.ca/people/richer/m4wd/fig1.ps http://www.astro.ubc.ca/people/richer/m4wd/fig2.ps http://www.astro.ubc.ca/people/richer/m4wd/fig3.ps