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Grids of zero-age horizontal branch (ZAHB) models are presented, along with a suitable interpolation code, for -2.5 <= [Fe/H] <= -0.5, in steps of 0.2 dex, assuming Y = 0.25 and 0.29, [O/Fe] = +0.4 and +0.6, and [m/Fe] = 0.4 for all of the other alpha elements. The HB populations of 37 globular clusters (GCs) are fitted to these ZAHBs to derive their apparent distance moduli, (m-M)_V. With few exceptions, the best estimates of their reddenings from dust maps are adopted. The distance moduli are constrained using the prediction that (M_F606W-M_F814W)_0 colours of metal-poor, main-sequence stars at M_F606W >~ 5.0 have very little sensitivity to [Fe/H]. Intrinsic (M_F336W-M_F606W)_0 colours of blue HB stars, which provide valuable connections between GCs with exclusively blue HBs and other clusters of similar metallicity that also have red HB components, limit the uncertainties of relative (m-M)_V values to within +/- 0.03-0.04 mag. The ZAHB-based distances agree quite well with the distances derived by Baumgardt & Vasiliev (2021, MNRAS, 505, 5957). Their implications for GC ages are briefly discussed. Stellar rotation and mass loss appear to be more important than helium abundance variations in explaining the colour-magnitude diagrams of second-parameter GCs (those with anomalously very blue HBs for their metallicities)., Comment: Accepted for publication in the MNRAS; 26 page article with 20 figures

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., Comment: Accepted for publication in the MNRAS; the paper consists of 20 pages, incuding 13 figures

Stellar models for -2.5 <= [Fe/H] <= -0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M5, M5, M55, and M92. The bolometric corrections used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other alpha elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour-magnitude diagrams (CMDs) to within ~ 0.03 mag. In particular, they appear to match the spreads in the observed (M_F336W - M_F438W)_0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] <~ -0.5). Both the absolute (M_F438W - M_F606W)_0 colours and the variations in these colours at a given M_F606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] >~ +0.5). Allowing for moderate He abundance variations (delta Y ~ 0.05) improves the fits to the observations., Comment: Accepted for publication in the MNRAS; the article consists of 21 pages, including 17 figures

New, high-resolution MARCS synthetic spectra have been calculated for more than a dozen mixtures of the metals allowing, in turn, for variations in C:N:O, [CNO/Fe], and enhanced abundances of C, O, Mg, and Si. Bolometric Corrections (BCs) for many of the broad-band filters currently in use have been generated from these spectra. Due to improved treatments of molecules that involve atoms of C, N, and O, the BCs for UV and blue passbands, in particular, differ substantially from those derived from previous MARCS models. These differences, and the effects on BCs of varying the abundances of the metals, are shown in a number of instructive plots. Stellar evolutionary grids for -2.5 <= [Fe/H] <= -0.5 have also been computed for the different mixtures. Isochrones based on these tracks are intercompared on the theoretical H-R diagram and on a few of the colour-magnitude diagrams that can be constructed from HST Wide Field Camera 3 (WFC3) F336W, F438W, F606W, F814W, F110W, and F160W observations. For the first application of these models, isochrones have been fitted to WFC3 photometry of the globular cluster NGC 6496 from the HST UV Legacy Survey, with very encouraging results., Comment: Accepted for publication in the MNRAS; the article consists of 20 pages, including 14 figures

$\mu$ 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 $\mu$ 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, $\mu$ 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, $\mu$ 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 $\mu$ Cas A are $Y=0.255\pm0.014$ and $12.7\pm2.7$ Gyr--making $\mu$ 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 $\mu$ Cas A, and its angular diameter would provide tighter constraints., Comment: Accepted by Astrophysical Journal

The HST/WFC3 multiband photometry spanning from the UV to the near-IR of four fields in the Galactic bulge, together with that for six template globular and open clusters, are used to photometrically tag the metallicity [Fe/H] of stars in these fields after proper-motion rejecting most foreground disk contaminants. Color-magnitude diagrams and luminosity functions are then constructed, in particular for the most metal rich and most metal poor stars in each field. We do not find any significant difference between the $I$-band and $H$-band luminosity functions, hence turnoff luminosity and age, of the metal rich and metal poor components which therefore appear essentially coeval. In particular, we find that no more than $\sim 3\%$ of the metal-rich component can be $\sim 5$ Gyr old, or younger. Conversely, theoretical luminosity functions give a good match to the observed ones for an age of ~10 Gyr. Assuming this age is representative for the bulk of bulge stars, we then recall the observed properties of star-forming galaxies at 10 Gyr lookback time, i.e., at z~2, and speculate about bulge formation in that context. We argue that bar formation and buckling instabilities leading to the observed boxy/peanut, X-shaped bulge may have arisen late in the history of the Milky Way galaxy, once its gas fraction had decreased compared to the high values typical of high-redshift galaxies. This paper follows the public release of the photometric and astrometric catalogs for the measured stars in the four fields., Comment: 11 pages, 9 figures, to appear on the Astrophysical Journal

M55 (NGC 6809) and NGC 6362 are among the few globular clusters for which masses and radii have been derived to high precision for member binary stars. They also contain RR Lyrae variables which, together with their non-variable horizontal-branch (HB) populations, provide tight constraints on the cluster reddenings and distance moduli through fits of stellar models to their pulsational and evolutionary properties. Reliable (m-M)_V estimates yield M_V and M_bol values of comparable accuracy for binary stars because the V-band bolometric corrections applicable to them have no more than a weak dependence on effective temperature (T_eff) and [Fe/H]. Chemical abundances derived from the binary mass-M_V relations are independent of determinations based on their spectra. The temperatures of the binaries, which are calculated directly from their luminosities and the measured radii, completely rule out the low T_eff scale that has been determined for metal-deficient stars in some recent spectroscopic and interferometric studies. If [alpha/Fe] = 0.4 and [O/Fe] = 0.5 +/- 0.1, we find that M55 has (m-M)_V = 13.95 +/- 0.05, [Fe/H] = -1.85 +/- 0.1, and an age of 12.9 +/- 0.8 Gyr, whereas NGC 6362 has (m-M)_V = 14.56 +/- 0.05, [Fe/H] = -0.90 +/- 0.1, and an age of 12.4 +/- 0.8 Gyr. The HB of NGC 6362 shows clear evidence for multiple stellar populations. Constraints from the RR Lyrae standard candle and from local subdwarfs (with Gaia DR2 parallaxes) are briefly discussed., Comment: Accepted for publication in the Astrophysical Journal; 25 figures and 7 tables

The availability of reliable bolometric corrections and reddening estimates, rather than the quality of parallaxes will be one of the main limiting factors in determining the luminosities of a large fraction of Gaia stars. With this goal in mind, we provide Gaia G, BP and RP synthetic photometry for the entire MARCS grid, and test the performance of our synthetic colours and bolometric corrections against space-borne absolute spectrophotometry. We find indication of a magnitude-dependent offset in Gaia DR2 G magnitudes, which must be taken into account in high accuracy investigations. Our interpolation routines are easily used to derive bolometric corrections at desired stellar parameters, and to explore the dependence of Gaia photometry on Teff, log(g), [Fe/H], alpha-enhancement and E(B-V). Gaia colours for the Sun and Vega, and Teff-dependent extinction coefficients, are also provided., Comment: MNRAS Letter. Solar colours: BP-G = 0.33, G-RP = 0.49, BP-RP = 0.82. Mean extinction coefficients at turn-off: R_G = 2.740 , R_BP = 3.374, R_RP = 2.035. Interpolation routines available at https://github.com/casaluca/bolometric-corrections

We use MARCS model atmosphere fluxes to compute synthetic colours, bolometric corrections and reddening coefficients for the Hipparcos/Tycho, Pan-STARRS1, SkyMapper and JWST systems. Tables and interpolation subroutines are provided to transform isochrones from the theoretical to various observational planes, to derive bolometric corrections, synthetic colours and colour-temperature relations at nearly any given point of the HR diagram for 2600 K < Teff < 8000 K, and different values of reddening in 85 photometric filters. We use absolute spectrophotometry from the CALSPEC library to show that bolometric fluxes can be recovered to ~2 percent from bolometric corrections in a single band, when input stellar parameters are well known for FG dwarfs at various metallicities. This sole source of uncertainty impacts interferometric effective temperatures to ~0.5 percent (or 30 K at the solar temperature). Uncertainties are halved when combining bolometric corrections in more bands, and limited by the fundamental uncertainty of the current absolute flux scale at 1 percent. Stars in the RAVE DR5 catalogue are used to validate the quality of our MARCS synthetic photometry in selected filters across the optical and infrared range. This investigation shows that extant MARCS synthetic fluxes are able to reproduce the main features observed in stellar populations across the Galactic disc., Comment: MNRAS, accepted. Tables and interpolation routines available at https://github.com/casaluca/bolometric-corrections

We present a new set of horizontal-branch (HB) models computed with the MESA stellar evolution code. The models adopt $\alpha$-enhanced \cite{ags09} metals mixtures and include the gravitational settling of He. They are used in our HB population synthesis tool to generate theoretical distributions of HB stars in order to describe the multiple stellar populations in the globular clusters 47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our simulations for [Fe/H] $= -0.70$ and [$\alpha$/Fe] $= +0.4$ if the initial helium mass fraction varies by $\Delta Y_0 \sim 0.03$ and approximately 21%, 37%, and 42% of the stars have $Y_0 = 0.257$, 0.270, and 0.287, respectively. These simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In the case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and [$\alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $\Delta Y_0 \sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13 has $\Delta Y_0 \sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr. Mass loss during giant-branch evolution and $\Delta Y_0$ appear to be the primary second parameters for M3 and M13. New observations for 7 of the 9 known RR Lyrae in M13 are also reported. Surprisingly, periods predicted for the $c$-type variables tend to be too high (by up to $\sim 0.1$~d)., Comment: 27 pages, 21 figures, 5 tables, accepted for publication in ApJ

Up-to-date isochrones, zero-age horizontal branch (ZAHB) loci, and evolutionary tracks for core He-burning stars are applied to the color-magnitude diagrams (CMDs) of M3, M15, and M92, focusing in particular on their RR Lyrae populations. Periods for ab- and c-type variables are calculated using the latest theoretical calibrations of log P_ab and log P_c as a function of luminosity, mass, effective temperature, and metallicity. Our models are generally able to reproduce the measured periods to well within the uncertainties implied by the stellar properties on which pulsation periods depend, as well as the mean periods and cluster-to-cluster differences in and , on the assumption of well-supported values of E(B-V), (m-M)_V, and [Fe/H]. While many of the RR Lyrae in M3 lie close to the same ZAHB that fits the faintest HB stars at bluer or redder colors, the M92 variables are all significantly evolved stars from ZAHB locations on the blue side of the instability strip. M15 appears to contain a similar population of HB stars as M92, along with additional helium-enhanced populations not present in the latter which comprise most of its RR Lyrae stars. The large number of variables in M15 and the similarity of the observed values of and in M15 and M92 can be explained by HB models that allow for variations in Y. Similar ages (~12.5 Gyr) are found for all three clusters, making them significantly younger than the field halo subgiant HD 140283. Our analysis suggests a preference for stellar models that take diffusive processes into account., Comment: 24 pages, 22 figures; accepted for publication in the Astrophysical Journal

Victoria-Regina isochrones for $-0.4 \le$ [alpha/Fe] $\le +0.4$ and a wide range in [Fe/H], along with complementary zero-age horizontal branch (ZAHB) loci, have been applied to the color-magnitude diagram (CMD) of Carina. The color transformations that we have used have been "calibrated" so that isochrones provide excellent fits to the $[(B-V)_0,\,M_V]$-diagrams of M3 and M92, when well supported estimates of the globular cluster (GC) reddenings and metallicities are assumed. The adopted distance moduli, for both the GCs and Carina, are based on our ZAHB models, which are able to reproduce the old HB component (as well as the luminosity of the HB clump) of the dwarf spheroidal galaxy quite well --- even if it spans a range in [Fe/H] of ~ 1.5 dex, provided that [alpha/Fe] varies with [Fe/H] in approximately the way that has been derived spectroscopically. Ages derived here agree reasonably well with those found previously for the old and intermediate-age turnoff stars, as well as for the period of negligible star formation (SF) activity (~ 6-10 Gyr ago). CMD simulations have been carried out for the faintest turnoff and subgiant stars. They indicate a clear preference for SF that lasted several Gyr instead of a short burst, with some indication that ages decrease with increasing [Fe/H]. In general, stellar models that assume spectroscopic metallicities provide satisfactory fits to the observations, including the thin giant branch of Carina, though higher oxygen abundances than those implied by the adopted values of [alpha/Fe] would have favorable consequences., Comment: 15 pages, including 12 figures; accepted for publication in the Astrophysical Journal

We present new constraints on the star formation histories of six ultra-faint dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV, and Ursa Major I. Our analysis employs a combination of high-precision photometry obtained with the Advanced Camera for Surveys on the Hubble Space Telescope, medium-resolution spectroscopy obtained with the DEep Imaging Multi-Object Spectrograph on the W.M. Keck Observatory, and updated Victoria-Regina isochrones tailored to the abundance patterns appropriate for these galaxies. The data for five of these Milky Way satellites are best fit by a star formation history where at least 75% of the stars formed by z~10 (13.3 Gyr ago). All of the galaxies are consistent with 80% of the stars forming by z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The similarly ancient populations of these galaxies support the hypothesis that star formation in the smallest dark matter sub-halos was suppressed by a global outside influence, such as the reionization of the universe., Comment: Accepted for publication in The Astrophysical Journal. 13 pages, 5 color and 3 grayscale figures, 2 tables

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

We demonstrate that among the potential sources of the primordial abundance variations of the proton-capture elements in globular-cluster stars proposed so far, such as the hot-bottom burning in massive AGB stars and H burning in the convective cores of supermassive and fast-rotating massive MS stars, only the supermassive MS stars with M > 10,000 Msun can explain all the observed abundance correlations without any fine-tuning of model parameters. We use our assumed chemical composition for the pristine gas in M13 (NGC6205) and its mixtures with 50% and 90% of the material partially processed in H burning in the 60,000 Msun MS model star as the initial compositions for the normal, intermediate and extreme populations of low-mass stars in this globular cluster, as suggested by its O-Na anti-correlation. We evolve these stars from the zero-age MS to the RGB tip with the thermohaline and parametric prescriptions for the RGB extra mixing. We find that the 3He-driven thermohaline convection cannot explain the evolutionary decline of [C/Fe] in M13 RGB stars, which, on the other hand, is well reproduced with the universal values for the mixing depth and rate calibrated using the observed decrease of [C/Fe] with MV in the globular cluster NGC5466 that does not have the primordial abundance variations., Comment: 11 pages, 1 table, 8 figures, accepted for publication in MNRAS

The most accurate ages for the oldest stars are those obtained for nearby halo subgiants, because they depend almost entirely on just the measured parallaxes and absolute oxygen abundances. In this study, we have used the Fine Guidance Sensors on the Hubble Space Telescope to determine trigonometric parallaxes, with precisions of 2.1% or better, for the Population II subgiants HD 84937, HD 132475, and HD 140283. High quality spectra have been used to derive their surface abundances of O, Fe, Mg, Si, and Ca, which are assumed to be 0.1-0.15 dex less than their initial abundances due to the effects of diffusion. Comparisons of isochrones with the three subgiants on the $(\log\,T_{\rm eff}, M_V)$-diagram yielded ages of $12.08 \pm 0.14, 12.56 \pm 0.46$, and $14.27 \pm 0.38$ Gyr for HD 84937, HD 132475, and HD 140283, in turn, where each error bar includes only the parallax uncertainty. The total uncertainty is estimated to be $\sim\pm 0.8$ Gyr (larger in the case of the near-turnoff star HD 84937). Although the age of HD 140283 is greater than the age of the universe as inferred from the cosmic microwave background by $\sim$ 0.4-0.5 Gyr, this discrepancy is at a level of $< 1\,\sigma$. Nevertheless, the first Population II stars apparently formed very soon after the Big Bang. (Stellar models that neglect diffusive processes seem to be ruled out as they would predict that HD 140283 is $\sim 1.5$ Gyr older than the universe.) The halo field subgiants appear to be older than globular clusters of similar metallicities: if distances close to those implied by the RR Lyrae standard candle are assumed, M92 and M5 are younger than HD 140283 and HD 132475 by $\sim 1.5$ and $\sim 1.0$ Gyr, respectively., Comment: 55 pages, including 10 figures and 5 tables - accepted for publication in the Astrophysical Journal

After a pedagogical introduction to the main concepts of synthetic photometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS, and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes for various [Fe/H] and [alpha/Fe] combinations, and virtually any value of reddening E(B-V) < 0.7. The successes and failures of model fluxes in reproducing the observed magnitudes are highlighted. Overall, extant synthetic fluxes predict quite realistic broad-band colours and bolometric corrections, especially at optical and longer wavelengths: further improvements of the predictions for the blue and ultraviolet spectral regions await the use of hydrodynamic models where the microturbulent velocity is not treated as a free parameter. We show how the morphology of the colour-magnitude diagram (CMD) changes for different values of [Fe/H] and [alpha/Fe]; in particular, how suitable colour combinations can easily discriminate between red giant branch and lower main sequence populations with different [alpha/Fe], due to the concomitant loops and swings in the CMD. We also provide computer programs to produce tables of synthetic bolometric corrections as well as routines to interpolate in them. These colour-Teff-metallicity relations may be used to convert isochrones for different chemical compositions to various bandpasses assuming observed reddening values, thus bypassing the standard assumption of a constant colour excess for stars of different spectral type. We also show how such an assumption can lead to significant systematic errors. The MARCS transformations presented in this study promise to provide important constraints on our understanding of the multiple stellar populations found in globular clusters (e.g., the colours of lower main sequence stars are predicted to depend strongly on [alpha/Fe]) and of those located towards/in the Galactic bulge., Comment: MNRAS, accepted. Tables and programs to generate synthetic colours and bolometric corrections in various photometric systems and for different combination of E(B-V), [Fe/H], [alpha/Fe], Teff and logg available via CDS at http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/MNRAS/444/392

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 use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to show that their age-metallicity relation (AMR) can be divided into two distinct, parallel sequences at [Fe/H] $\ga -1.8$. Approximately one-third of the clusters form an offset sequence that spans the full range in age ($\sim 10.5$--13 Gyr), but is more metal rich at a given age by $\sim 0.6$ dex in [Fe/H]. All but one of the clusters in the offset sequence show orbital properties that are consistent with membership in the MW disk. They are not simply the most metal-rich GCs, which have long been known to have disk-like kinematics, but they are the most metal-rich clusters at all ages. The slope of the mass-metallicity relation (MMR) for galaxies implies that the offset in metallicity of the two branches of the AMR corresponds to a mass decrement of 2 dex, suggesting host galaxy masses of $M_{*} \sim 10^{7-8} \msol$ for GCs that belong to the more metal-poor AMR. We suggest that the metal-rich branch of the AMR consists of clusters that formed in-situ in the disk, while the metal-poor GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf galaxies are shown to be consistent with this interpretation, and the relative distribution of implied progenitor masses for the halo GC clusters is in excellent agreement with the MW subhalo mass function predicted by simulations. A notable implication of the bifurcated AMR, is that the identical mean ages and spread in ages, for the metal rich and metal poor GCs are difficult to reconcile with an in-situ formation for the latter population., Comment: 16 pages, 9 figures, accepted for publication in MNRAS

Ages have been derived for 55 globular clusters (GCs) from overlays of isochrones onto the turnoff photometry, assuming distances based on fits of zero-age horizontal branch (ZAHB) models to the lower bound of the observed distributions of HB stars. The error bar arising just from the "fitting" of ZAHBs and isochrones is ~ +/- 0.25 Gyr, while that associated with distance and chemical abundance uncertainties is ~ +/- 1.5-2 Gyr. Ages vary from mean values of ~12.5 Gyr at [Fe/H] < -1.7 to ~11 Gyr at [Fe/H] > -1.0. At intermediate metallicities, the age-metallicity relation (AMR) appears to be bifurcated: one branch apparently contains clusters with disk-like kinematics, whereas the other branch is populated by clusters with halo-type orbits. There is no apparent dependence of age on Galactocentric distance (R_G) nor is there a clear correlation of HB type with age. Subtle variations in the subgiant branch (SGB) slopes of [Fe/H] < -1.5 GCs are tentatively attributed to helium abundance differences. Curiously, GCs with steep "M13-like" SGBs tend to be massive systems, located at small R_G, that show the strongest evidence for multiple stellar populations. The others are typically low-mass systems that, at the present time, should not be able to retain the matter lost by mass-losing stars. The apparent separation of the two groups in terms of their present-day gas retention properties is difficult to understand if all GCs were initial ~20 times their current masses. The lowest mass systems may have never been able to retain enough gas to produce a significant population of second-generation stars; in this case, the observed light element abundance variations were presumably present in the gas out of which the observed cluster stars formed., Comment: Accepted for publication in the Astrophysical Journal (tentatively for volume 775): 124 page preprint with 40 figures

HD 140283 is an extremely metal-deficient and high-velocity subgiant in the solar neighborhood, having a location in the HR diagram where absolute magnitude is most sensitive to stellar age. Because it is bright, nearby, unreddened, and has a well-determined chemical composition, this star avoids most of the issues involved in age determinations for globular clusters. Using the Fine Guidance Sensors on the Hubble Space Telescope, we have measured a trigonometric parallax of 17.15 +/- 0.14 mas for HD 140283, with an error one-fifth of that determined by the Hipparcos mission. Employing modern theoretical isochrones, which include effects of helium diffusion, revised nuclear reaction rates, and enhanced oxygen abundance, we use the precise distance to infer an age of 14.46 +/- 0.31 Gyr. The quoted error includes only the uncertainty in the parallax, and is for adopted surface oxygen and iron abundances of [O/H] = -1.67 and [Fe/H] = -2.40. Uncertainties in the stellar parameters and chemical composition, especially the oxygen content, now contribute more to the error budget for the age of HD 140283 than does its distance, increasing the total uncertainty to about +/-0.8 Gyr. Within the errors, the age of HD 140283 does not conflict with the age of the Universe, 13.77 +/- 0.06 Gyr, based on the microwave background and Hubble constant, but it must have formed soon after the big bang., Comment: 19 pages, 1 figure. Accepted by ApJ Letters

[Abbreviated] We have investigated the color-magnitude diagram of Omega Centauri and find that the blue main sequence (bMS) can be reproduced only by models that have a of helium abundance in the range Y=0.35-$0.40. To explain the faint subgiant branch of the reddest stars ("MS-a/RG-a" sequence), isochrones for the observed metallicity ([Fe/H]\approx0.7) appear to require both a high age (~13Gyr) and enhanced CNO abundances ([CNO/Fe]\approx0.9$). Y~0.35 must also be assumed in order to counteract the effects of high CNO on turnoff colors, and thereby to obtain a good fit to the relatively blue turnoff of this stellar population. This suggest a short chemical evolution period of time (<1Gyr) for Omega Cen. Our intermediate-mass (super-)AGB models are able to reproduce the high helium abundances, along with [N/Fe]~2 and substantial O depletions if uncertainties in the treatment of convection are fully taken into account. These abundance features distinguish the bMS stars from the dominant [Fe/H] $\approx1.7$ population. The most massive super-AGB stellar models (M_zams>=6.8M_sun, M_He,core>=1.245M_sun) predict too large N-enhancements, which limits their role in contributing to the extreme populations. We show quantitatively that highly He- and N-enriched AGB ejecta have particularly efficient cooling properties. Based on these results and on the reconstruction of the orbit of Omega Cen with respect to the Milky Way we propose the galactic plane passage gas purging scenario for the chemical evolution of this cluster. Our model addresses the formation and properties of the bMS population (including their central location in the cluster). We follow our model descriptively through four passage events, which could explain not only some key properties of the bMS, but also of the MS-a/RGB-a and the s-enriched stars., Comment: Accepted for publication in ApJ. 10 figures, 5 tables, 21 pages

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., Comment: 27 pages, 18 figures; accepted and tentatively scheduled for publication in ApJ, volume 755 (Aug 10, 2012 issue)

We present new constraints on the star formation histories of the ultra-faint dwarf (UFD) galaxies, using deep photometry obtained with the Hubble Space Telescope (HST). A galaxy class recently discovered in the Sloan Digital Sky Survey, the UFDs appear to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in efforts to understand the missing satellite problem. They are the least luminous, most dark-matter dominated, and least chemically-evolved galaxies known. Our HST survey of six UFDs seeks to determine if these galaxies are true fossils from the early universe. We present here the preliminary analysis of three UFD galaxies: Hercules, Leo IV, and Ursa Major I. Classical dwarf spheroidals of the Local Group exhibit extended star formation histories, but these three Milky Way satellites are at least as old as the ancient globular cluster M92, with no evidence for intermediate-age populations. Their ages also appear to be synchronized to within ~1 Gyr of each other, as might be expected if their star formation was truncated by a global event, such as reionization., Comment: Accepted for publication in The Astrophysical Journal Letters. Latex, 5 pages, 2 color figures, 1 table

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., Comment: 24 pages, 19 figures, 13 tables

We present new UV-to-IR stellar photometry of four low-extinction windows in the Galactic bulge, obtained with the Wide Field Camera 3 on the Hubble Space Telescope (HST). Using our five bandpasses, we have defined reddening-free photometric indices sensitive to stellar effective temperature and metallicity. We find that the bulge populations resemble those formed via classical dissipative collapse: each field is dominated by an old (~10 Gyr) population exhibiting a wide metallicity range (-1.5 < [Fe/H] < 0.5). We detect a metallicity gradient in the bulge population, with the fraction of stars at super-solar metallicities dropping from 41% to 35% over distances from the Galactic center ranging from 0.3 to 1.2 kpc. One field includes candidate exoplanet hosts discovered in the SWEEPS HST transit survey. Our measurements for 11 of these hosts demonstrate that exoplanets in the distinct bulge environment are preferentially found around high-metallicity stars, as in the solar neighborhood, supporting the view that planets form more readily in metal-rich environments., Comment: Accepted for publication in The Astrophysical Journal Letters. Latex, 5 pages, ApJ format

We have obtained broad-band near-infrared photometry for seven Galactic star clusters (M92, M15, M13, M5, NGC1851, M71 and NGC6791) using the WIRCam wide-field imager on the Canada-France-Hawaii Telescope, supplemented by images of NGC1851 taken with HAWK-I on the VLT. In addition, 2MASS observations of the [Fe/H] ~ 0.0 open cluster M67 were added to the cluster database. From the resultant (V-J)-V and (V-Ks)-V colour-magnitude diagrams (CMDs), fiducial sequences spanning the range in metallicity, -2.4 < [Fe/H] < +0.3, have been defined which extend (for most clusters) from the tip of the red-giant branch (RGB) to ~ 2.5 magnitudes below the main-sequence turnoff. These fiducials provide a valuable set of empirical isochrones for the interpretation of stellar population data in the 2MASS system. We also compare our newly derived CMDs to Victoria isochrones that have been transformed to the observed plane using recent empirical and theoretical colour-Teff relations. The models are able to reproduce the entire CMDs of clusters more metal rich than [Fe/H] ~ -1.4 quite well, on the assumption of the same reddenings and distance moduli that yield good fits of the same isochrones to Johnson-Cousins BV(RI)C photometry. However, the predicted giant branches become systematically redder than the observed RGBs as the cluster metallicity decreases. Possible explanations for these discrepancies are discussed., Comment: 18 pages, 20 figures, Accepted for publication in AJ

Isochrones for ages > 4 Gyr and metallicities in the range -2.5 < [Fe/H] < +0.3 that take the diffusion of helium and recent advances in stellar physics into account are compared with observations in the Johnson-Cousins BV(RI)_C photometric system for several open and globular star clusters. The adopted color-Teff relations include those which we have derived from the latest MARCS model atmospheres and empirical transformations for dwarf and subgiant stars given by Casagrande et al (2010, A&A, 512, 54; CRMBA). Those reported by VandenBerg & Clem (2003, AJ, 126, 778) have also been considered, mainly to resolve some outstanding questions concerning them. Remarkably, when the subdwarfs in the CRMBA data set that have sigma_pi/pi < 0.15 are superimposed on a set of 12 Gyr isochrones spanning a wide range in [Fe/H], the inferred metallicities and effective temperatures agree, in the mean, with those given by CRMBA to within +/- 0.05 dex and +/- 10 K, respectively. Thus the hot Teff scale derived by CRMBA is nearly identical with that predicted by stellar models and consequently, there is excellent consistency between theory and observations on the H-R diagram and the different color-magnitude diagrams considered in this investigation. To obtain similar consistency, the colors obtained from the MARCS and VandenBerg & Clem B-V vs. Teff relations for metal-poor dwarf stars should be adjusted to the red by 0.02-0.03 mag. In general, isochrones that employ the CRMBA transformations provide reasonably good fits to our BV(RI)_C photometry for main-sequence stars in the globular clusters 47 Tuc, M3, M5, M92 and NGC 1851 - but not the cluster giants (when adopting the synthetic MARCS colors). We speculate that differences between the actual heavy-element mixtures and those assumed in the theoretical models may be the primary cause of this difficulty., Comment: To appear in 2010, AJ, 140, 1020

The rich open cluster M67 is known to have a chemical composition close to solar, and an age around 4Gyr. It thus offers the opportunity to check our understanding of the physics and the evolution of solar-type stars in a cluster environment. We present the first spectroscopic study at high resolution, R~50,000, of the potentially best solar twin, M67-1194, identified among solar-like stars in M67. Based on a pre-selection of solar-twin candidates performed at medium resolution by Pasquini et al. (2008), we explore the chemical-abundance similarities and differences between M67-1194 and the Sun, using VLT/FLAMES-UVES. Working with a solar twin in the framework of a differential analysis, we minimize systematic model errors in the abundance analysis compared to previous studies which utilized more evolved stars to determine the metallicity of M67. We find M67-1194 to have stellar parameters indistinguishable from the solar values, with the exception of the overall metallicity which is slightly super-solar ([Fe/H]=0.023 +/- 0.015). An age determination based on evolutionary tracks yields 4.2 +/- 1.6Gyr. Most surprisingly, we find the chemical abundance pattern to closely resemble the solar one, in contrast to most known solar twins in the solar neighbourhood. We confirm the solar-twin nature of M67-1194, the first solar twin known to belong to a stellar association. This fact allows us to put some constraints on the physical reasons for the seemingly systematic departure of M67-1194 and the Sun from most known solar twins regarding chemical composition. We find that radiative dust cleansing by nearby luminous stars may be the explanation for the peculiar composition of both the Sun and M67-1194, but alternative explanations are also possible. The chemical similarity between the Sun and M67-1194 also suggests that the Sun once formed in a cluster like M67.

Using the Advanced Camera for Surveys on the Hubble Space Telescope, we have obtained deep optical images reaching well below the oldest main sequence turnoff in six fields of the Andromeda Galaxy. The fields fall at four positions on the southeast minor axis, one position in the giant stellar stream, and one position on the northeast major axis. These data were obtained as part of three large observing programs designed to probe the star formation history of the stellar population in various structures of the galaxy. In this paper, we present the images, catalogs, and artificial star tests for these observing programs as a supplement to the analyses published previously. These high-level science products are also archived at the Multimission Archive at the Space Telescope Science Institute., Comment: Accepted for publication in The Astrophysical Journal Supplement Series. 6 pages, latex. The full set of high-level science products can be found at http://archive.stsci.edu/prepds/andromeda/

[Abridged] When WFC3 is installed on HST, the community will have powerful new tools for investigating resolved stellar populations. The WFC3 Galactic Bulge Treasury program will obtain deep imaging on 4 low-extinction fields. These non-proprietary data will enable a variety of science investigations not possible with previous data sets. To aid in planning for the use of these data and for future proposals, we provide an introduction to the program, its photometric system, and the associated calibration effort. The observing strategy is based upon a new 5-band photometric system spanning the UV, optical, and near-infrared. With these broad bands, one can construct reddening-free indices of Teff and [Fe/H]. Besides the 4 bulge fields, the program will target 6 fields in well-studied star clusters, spanning a wide range of [Fe/H]. The cluster data serve to calibrate the indices, provide population templates, and correct the transformation of isochrones into the WFC3 photometric system. The bulge data will shed light on the bulge formation history, and will also serve as population templates for other studies. One of the fields includes 12 candidate hosts of extrasolar planets. CMDs are the most popular tool for analyzing resolved stellar populations. However, due to degeneracies among Teff, [Fe/H], and reddening in traditional CMDs, it can be difficult to draw robust conclusions from the data. The 5-band system used for the bulge Treasury observations will provide indices that are roughly orthogonal in Teff and [Fe/H], and we argue that model fitting in an index-index diagram will make better use of the information than fitting separate CMDs. We provide simulations to show the expected data quality and the potential for differentiating between different star-formation histories., Comment: Accepted for publication in The Astronomical Journal. 9 pages, 8 figures, latex, AJ format

We describe an extensive observational project that has obtained high-quality and homogeneous photometry for a number of different Galactic star clusters (including M 92, M 13, M 3, M 71, and NGC 6791) spanning a wide range in metallicity (-2.3<[Fe/H]<+0.4), as observed in the u'g'r'i'z' passbands with the MegaCam wide-field imager on the Canada-France-Hawaii Telescope. By employing these purest of stellar populations, fiducial sequences have been defined from color-magnitude diagrams that extend from the tip of the red-giant branch down to approximately 4 magnitudes below the turnoff: these sequences have been accurately calibrated to the standard u'g'r'i'z' system via a set of secondary photometric standards located within these same clusters. Consequently, they can serve as a valuable set of empirical fiducials for the interpretation of stellar populations data in the u'g'r'i'z' system., Comment: 16 pages, 7 tables, 13 figures; accepted for publication in AJ

Stellar models have been computed for stars having [Fe/H] = 0.0 and -2.0 to determine the effects of using boundary conditions derived from the latest MARCS model atmospheres. The latter were fitted to the interior models at both the photosphere and at tau = 100, and at least for the 0.8-1.0 solar mass stars considered here, the resultant evolutionary tracks were found to be nearly independent of the chosen fitting point. Particular care was taken to treat the entire star as consistently as possible; i.e., both the interior and atmosphere codes assumed the same abundances and the same treatment of convection. Tracks were also computed using either the classical gray T(tau,T_eff) relation or that derived by Krishna Swamy (1966) to derive the boundary pressure. The latter predict warmer giant branches (by ~150 K) at solar abundances than those based on gray or MARCS atmospheres, which happens to be in good agreement with the inferred temperatures of giants in the open cluster M67 from the latest (V-K)-T_eff relations. Most of the calculations assumed Z=0.0125 (Asplund et al.), though a few models were computed for Z=0.0165 (Grevesse & Sauval) to determine the dependence of the tracks on Z_\odot. Grids of "scaled solar, differentially corrected" (SDC) atmospheres were also computed to try to improve upon theoretical MARCS models. When they were used as boundary conditions, the resultant tracks agreed very well with those based on a standard scaled-solar (e.g., Krishna Swamy) T(tau,T_eff) relation, independently of the assumed metal abundance. Fits of isochrones to the C-M diagram of the [Fe/H] = -2 globular cluster M68 were examined, as was the possibility that the mixing-length parameter varies with stellar parameters., Comment: 54 pages, including 20 figures and 3 tables; accepted (July 2007) for publication in the Astrophysical Journal

The mass at which a transition is made between stars that have radiative or convective cores throughout the core H-burning phase is a fairly sensitive function of Z (particularly the CNO abundances). As a consequence, the ~4 Gyr, open cluster M67 provides a constraint on Z_\odot (and the solar heavy-element mixture) because (i) high-resolution spectroscopy indicates that this system has virtually the same metal abundances as the Sun, and (ii) its turnoff stars have masses just above the lower limit for sustained core convection on the main sequence. In this study, evolutionary tracks and isochrones using the latest MARCS model atmospheres as boundary conditions have been computed for 0.6-1.4 solar masses on the assumption of a metals mix (implying Z_\odot = 0.0125) based on the solar abundances derived by M. Asplund and collaborators using 3-D model atmospheres. These calculations do not predict a turnoff gap where one is observed in M67. No such difficulty is found if the analysis uses isochrones for Z_\odot = 0.0165, assuming the Grevesse & Sauval (1998) mix of heavy elements. Our findings, like the inferences from helioseismology, indicate a problem with the Asplund et al. abundances. However, it is possible that low-Z models with diffusive processes taken into account will be less problematic., Comment: 13 pages, including 2 figures and 1 table; accepted (July 2007) for publication in the Astrophysical Journal Letters

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

Seventy-two grids of stellar evolutionary tracks, along with the capability 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. (2000, ApJ, 532, 430) for 17 [Fe/H] values from -2.31 to -0.30 and alpha-element abundances corresponding to [alpha/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 solar masses) 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 solar masses 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 modelled using a parameterized version of the Roxburgh (1989, A&A, 211, 361) 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 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. [All of the model grids may be obtained from the Canadian Astronomy Data Center (http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/cvo/community/VictoriaReginaModels ).], Comment: Accepted for the ApJS (Feb. 2006); 39 pages including 14 figures, 3 tables

We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1 < [Fe/H] < +0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster, we provide a ridge line tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses., Comment: 14 pages, 6 black & white figures, 1 color figure, latex. Accepted for publication in The Astronomical Journal. Figures 1-5 have been degraded to meet size restrictions. Smoothing of line art should be disabled in PDF viewers for correct display of figures. Full-resolution version and machine-readable tables available from T.M. Brown (tbrown@stsci.edu)

We present new BVRI broad-band photometry for the old open cluster NGC188 based upon analysis of 299 CCD images either obtained by us, donated by colleagues, or retrieved from public archives. We compare our results on a star-by-star basis with data from eleven previous photometric investigations of the cluster. We homogenize and merge the data from all the photometric studies, and also merge membership probabilities from four previous proper-motion studies of the cluster field. Fiducial cluster sequences in the BV (Johnson) RI (Cousins) photometric system of Landolt (1992, AJ, 104, 340) represent the principal result of this paper. These have been compared to reference samples defined by (a) Landolt's standard stars, (b) the old open clusters M67 and NGC6791, and (c) stars within 25 pc having modern photometry and precise Hipparcos parallaxes. In a companion paper we show that our derived cluster results agree well with the predictions of modern stellar-interior and -evolution theory, given reasonable estimates of the cluster chemical abundances and foreground reddening. The individual and combined datasets for NGC188 have been made available through our web site., Comment: Accepted for PASP

We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312 in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The cluster was included in deep observations taken to measure the star formation history of the M31 halo. Overcoming a very crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag below the main sequence turnoff. These are the first observations to allow a direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD and luminosity function using a finely-spaced grid of isochrones that have been calibrated using observations of Galactic clusters taken with the same camera and filters. The luminosity difference between the subgiant and horizontal branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison to isochrones yields an age of 10 +2.5/-1 Gyr.

Evolutionary models taking into account radiative accelerations, thermal diffusion, and gravitational settling for 28 elements, including all those contributing to OPAL stellar opacities, have been calculated for solar metallicity stars of 0.5 to 1.4 solar masses. The Sun has been used to calibrate the models. Isochrones are fitted to the observed color-magnitude diagrams (CMDs) of M67 and NGC188, and ages of 3.7 and 6.4 Gyr are respectively determined. Convective core overshooting is not required to match the turnoff morphology of either cluster, including the luminosity of the gap in M67, because central convective cores are larger when diffusive processes are treated. This is due mainly to the enhanced helium and metal abundances in the central regions of such models. The observation of solar metallicity open clusters with ages in the range 4.8--5.7Gyr would further test the calculations of atomic diffusion in central stellar regions: according to non-diffusive isochrones, clusters should not have gaps near their main-sequence turnoffs if they are older than ~4.8Gyr, whereas diffusive isochrones predict that gaps should persist up to ages of ~5.7Gyr. Surface abundance isochrones are also calculated. In the case of M67 and NGC188, surface abundance variations are expected to be small. Abundance differences between stars of very similar Teff are expected close to the turnoff, especially for elements between P and Ca. Moreover, in comparison with the results obtained for giants, small generalized underabundances are expected in main-sequence stars. The lithium to beryllium ratio is discussed briefly and compared to observations., Comment: 34 pages, 15 figures, Accepted by the 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., Comment: 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/

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., Comment: 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)

An improved algorithm for interpolating isochrones and isochrone population functions (IPFs), based on the scheme introduced by Bergbusch & VandenBerg (1992, APJS, 81, 163), is described. The computer programs (FORTRAN 77) and the grids of evolutionary tracks, for 17 [Fe/H] values between -2.31 and and -0.3, with three choices of [alpha/Fe] (0.0, 0.3, and 0.6) at each iron abundance, are freely available to users interested in producing isochrones and IPFs. We add to the evidence presented in previous papers in this series in support of the Teff and color scales of our models. The temperatures derived by Gratton et al. (1996, A&A, 314, 191) for local Population II subdwarfs with accurate {Hipparcos} parallaxes are in excellent agreement with those predicted for them, when the Gratton et al. [Fe/H] scale is also assumed. The locus defined by local subdwarfs and subgiants on the [M_V, log Teff] plane and the morphologies of globular cluster C-M diagrams are well matched by the present models, despite the neglect of diffusion. The three field halo subgiants in our sample all have ages >15 Gyr, which is also favored for the Galaxy's most metal-poor globular clusters (GCs). Our isochrones accurately reproduce the Da Costa & Armandroff (1990, AJ, 100, 162) red-giant branch fiducials for M15, NGC 6752, NGC 1851, and 47 Tuc on the [M_I, (V-I)_0]-diagram. However, our models fail to predict the observed luminosities of the red-giant bump by ~0.25 mag: this may indicate inward overshooting of convective envelopes in red giants. The Zinn & West (1984, ApJS, 55, 45) metallicities for intermediate metal-poor GCs (-1.8 > [Fe/H] > -1.1) are preferred over recent spectroscopic results, suggesting an inconsistency between current subdwarf and GC [Fe/H] scales., Comment: 44 pages in total, including 7 tables and 12 figures

We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$\alpha$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present., Comment: 20 figures, 6 tables, 59 postscript pages

A (V,V - I)--diagram for the intermediate-age open cluster NGC 7789 has been derived from CCD observations of more than 15,000 stars within ~ 18 arcmin of the cluster center. From the brightest giants and blue stragglers at V ~ 11 to the faintest lower main-sequence stars that were observed (at V ~ 21, M_V ~ 9), the C-M diagram is well defined. A prominent clump of core helium-burning stars is evident at V = 13.0 and the upper end of the main sequence shows a fairly pronounced curvature to the red, which is indicative of significant convective core overshooting. Indeed, comparisons with up-to-date stellar models show that it is not possible to explain the observed morphology in the vicinity of the turnoff unless the overshooting is quite extensive. Interestingly, if sufficient overshooting is assumed in order to match the main-sequence data, it is not possible to reproduce the cluster's extended giant branch unless the cluster age is at least 1.6 Gyr (assuming a metallicity in the range -0.2 <= [Fe/H] <= 0.0). This, in turn, requires that the cluster have an apparent distance modulus (m-M)_V <= 12.2. Thus, sometime within the past few hundred million years, the ignition of helium burning in NGC 7789 has switched from a quiescent to an explosive (``flash'') phenomenon, and the length of the cluster's red-giant branch has been steadily increasing with the passage of time since then. From main-sequence fits to models that have been carefully normalized to the Sun, we infer a reddening 0.35 <= E(V-I) <= 0.38., Comment: 28 pages, 21 figures, 5 tables. be accepted for publication in the November 1998 issue of the PASP

We present deep CCD photometry of the globular cluster M13 (NGC 6205) in the Stromgren uvby-Beta system, and determine a foreground reddening of E(b-y)= 0.015mag. From a fit to the main-sequence of metal-poor subdwarfs with Hipparcos parallaxes, we derive (m-M)_0=14.38 +- 0.10 which implies an age near 12 Gyr assuming [Fe/H]=-1.61 and [Alpha/Fe]=0.3. The distance independent ((b-y)_0, c_0) diagram indicates that M13 and metal-poor field subdwarfs of similar metallicity must be coeval to within +-1 Gyr. In addition, we find that, at any given (b-y)_0 color, there is a large spread in the c_0 index for M13 red-giant branch (RGB) stars. We suspect that this scatter, which extends at least as faint as the base of the RGB, is most likely due to star-to-star variations in the atmospheric abundances of the CNO elements. We also note the existence of what appears to be two separate stellar populations on the HB of M13. Among other possibilities, it could arise as the result of differences in the extent to which deep mixing occurs in the precursor red giants., Comment: 14 pages, including two encapsulated postscript figures. Accepted for publication in ApJL, Uses aasms4.sty

We discuss the present state of knowledge and thought concerning the spread in age found among Galactic globular clusters, with some discussion of the implications for what happened during the earliest stages of the formation of the Milky Way Galaxy. Differential observational techniques to derive the relative age differences among clusters of similar metallicity are discussed in detail. We conclude that as of the current date (mid-April 1996) the state of the field is still somewhat muddled. However, we believe that there is now a substantial body of evidence --- including a particularly revealing intercomparison of the color-magnitude diagrams of \ngc{1851}, \ngc{288}, and \ngc{362} presented here --- indicating that age is {\it not\/} the dominant second parameter determining the shape of globular clusters' horizontal branches. If our assertion is correct, then apart from a handful of anomalous clusters that may well have been captured from a satellite dwarf galaxy, there is no strong evidence either for a significant spread in age among clusters of a given metal abundance or for a systematic variation of mean age with Galactocentric distance. On the question of whether there is a significant age difference between metal-poor and metal-rich clusters, we feel compelled to fall back on the Scottish verdict: ``Unproven.'' Data now being collected by numerous groups in various subdisciplines may resolve the remaining controversy within a few years., Comment: Eight postscript figures Also available at Also available at http://www.dao.nrc.ca/DAO/SCIENCE/science.html

We establish a framework for determing absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The clusters span Galactocentric distances from 4 through 100 kpc and cover a metallicity range from $[Fe/H] = -0.6$ to $-2.3$. Adopting currently plausible choices for the relation between cluster metallicity and horizontal-branch luminosity, and alpha-enhancement ratios, we find that the majority of the globular clusters form an age distribution with a dispersion $\sigma(t)$ about $10^9$ years, and a total age spread smaller than 4 Gyr. Clusters in the lowest metallicity group ($[Fe/H] < -1.8$) appear to be the same age to well within 1 Gyr at all locations in the Milky Way halo, suggesting that star formation began throughout the halo nearly simultaneously in its earliest stages. We find no statistically significant correlation between mean cluster age and Galactocentric distance (no age gradient) from 4 to 100 kpc. The correlation between cluster age and horizontal-branch type suggests that causes in addition to metallicity and age are required to understand the distribution of stars along the horizontal branches in globular cluster color-magnitude diagrams., Comment: 26 pages, AASTeX v3.0 with 3 Postscript figures