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

Does SEGUE/SDSS indicate a dual Galactic halo?

We re-examine recent claims of observational evidence for a dual Galactic halo in SEGUE/SDSS data, and trace them back to improper error treatment and neglect of selection effects. In particular, the detection of a vertical abundance gradient in the halo can be explained as a metallicity bias in distance. A similar bias, and the impact of disk contamination, affect the sample of blue horizontal branch stars. These examples highlight why non-volume complete samples require forward-modelling from theoretical models or extensive bias-corrections. We also show how observational uncertainties produce the specific non-Gaussianity in the observed azimuthal velocity distribution of halo stars, which can be erroneously identified as two Gaussian components. A single kinematic component yields an excellent fit to the observed data, when we model the measurement process including distance uncertainties. Furthermore, we show that sample differences in proper motion space are the direct consequence of kinematic cuts, and are enhanced when distance estimates are less accurate. Thus, their presence is neither a proof for a separate population, nor a measure of reliability for the applied distances. We conclude that currently there is no evidence from SEGUE/SDSS that would favour a dual Galactic halo over a single halo full of substructure.Comment: 13 pages, 10 figures accepted for publication in Ap

The Cosmic Production of Helium

Siirretty Doriast

Connecting the local stellar halo and its dark matter density to dwarf galaxies via blue stragglers

The Gaia HR diagram shows the presence of apparently young stars at high tangential velocities. Using a simple analytical model I show that these stars are likely to be blue stragglers. Once normalized to red giant stars, the fraction of nearby halo blue stragglers is of order 20 percent, and remarkably close to that measured in dwarf galaxies. Motivated by this similarity, I apply to field blue stragglers scaling relations inferred from blue stragglers in dwarf galaxies. Doing this for the Milky Way halo returns an average stellar density of 3.4 x 10^-5 Msun/pc^3 and a dark matter density of ~0.006 Msun/pc^3 ~ 0.22 GeV/cm^3 within 2 kpc from the Sun. These values compare favourably to other determinations available in the literature, but are based on an independent set of assumptions. A few considerations of this methodology are discussed, most notably that the correlation between the dark matter halo core-density and stellar mass seen in dwarf galaxies seems to hold also for the nearby Milky Way halo.Comment: ApJ accepted. Video of Figure 1 at https://www.dropbox.com/s/3z183bjti0p7vi4/video.mp4?dl=

Bayesian analysis of ages, masses, and distances to cool stars with non-LTE spectroscopic parameters

For studies of Galactic evolution, the accurate characterization of stars in terms of their evolutionary stage and population membership is of fundamental importance. A standard approach relies on extracting this information from stellar evolution models but requires the effective temperature, surface gravity, and metallicity of a star obtained by independent means. In previous work, we determined accurate effective temperatures and non-LTE logg and [Fe/H] (NLTE-Opt) for a large sample of metal-poor stars, -3<[Fe/H]<-0.5, selected from the RAVE survey. As a continuation of that work, we derive here their masses, ages, and distances using a Bayesian scheme and GARSTEC stellar tracks. For comparison, we also use stellar parameters determined from the widely-used 1D LTE excitation-ionization balance of Fe (LTE-Fe). We find that the latter leads to systematically underestimated stellar ages, by 10-30%, but overestimated masses and distances. Metal-poor giants suffer from the largest fractional distance biases of 70%. Furthermore, we compare our results with those released by the RAVE collaboration for the stars in common (DR3, Zwitter et al. 2010, Seibert et al. 2011). This reveals -400 to +400 K offsets in effective temperature, -0.5 to 1.0 dex offsets in surface gravity, and 10 to 70% in distances. The systematic trends strongly resemble the correlation we find between the NLTE-Opt and LTE-Fe parameters, indicating that the RAVE DR3 data may be affected by the physical limitations of the 1D LTE synthetic spectra. Our results bear on any study, where spectrophotometric distances underlie stellar kinematics. In particular, they shed new light on the debated controversy about the Galactic halo origin raised by the SDSS/SEGUE observations.Comment: 13 pages and 15 figures. Accepted for publication in MNRA

Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion

We present NLTE Li abundances for 88 stars in the metallicity range -3.5 < [Fe/H] < -1.0. The effective temperatures are based on the infrared flux method with improved E(B-V) values obtained mostly from interstellar NaI D lines. The Li abundances were derived through MARCS models and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, and complemented with reliable equivalent widths from the literature. The less-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall into two well-defined plateaus of A_{Li} = 2.18 (sigma = 0.04) and A_{Li} = 2.27 (sigma = 0.05), respectively. We show that the two plateaus are flat, unlike previous claims for a steep monotonic decrease in Li abundances with decreasing metallicities. At all metallicities we uncover a fine-structure in the Li abundances of Spite plateau stars, which we trace to Li depletion that depends on both metallicity and mass. Models including atomic diffusion and turbulent mixing seem to reproduce the observed Li depletion assuming a primordial Li abundance A_{Li} = 2.64, which agrees well with current predictions (A_{Li} = 2.72) from standard Big Bang nucleosynthesis. Adopting the Kurucz overshooting model atmospheres increases the Li abundance by +0.08 dex to A_{Li} = 2.72, which perfectly agrees with BBN+WMAP.Comment: A&A Letters, in pres