On the use of Gaia magnitudes and new tables of bolometric corrections

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-correction

Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems

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/444/39

Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. III. Grids of Isochrones for −2.5≤-2.5 \le [Fe/H] ≤−0.5\le -0.5 and Helium Abundances Y=0.25Y = 0.25 and 0.290.29 at Each Metallicity

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 figure

Zero-Age Horizontal Branch Models for -2.5 <= [Fe/H] <= -0.5 and Their Implications for the Apparent Distance Moduli of Globular Clusters

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 figure

The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy

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 MNRA