Models of Metal-poor Stars with Gravitational Settling and Radiative Accelerations. II. The Age of the Oldest Stars

By O. Richard, G. Michaud, Jacques Richer, S. Turcotte, S. Turck-Chieze, D. VandenBerg. 2001

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Isochrones for ages between 12 and 18 Gyr have been derived from the evolutionary tracks presented in Paper I (Richard et al.) for masses from 0.5 to 1.0 M☉ and initial chemical abundances corresponding to (1) Y = 0.2352, Z = 1.69 × 10-4 ([Fe/H] = -2.31, [α/Fe] = 0.3) and (2) Y = 0.2370, Z = 1.69 × 10-3 ([Fe/H = -1.31, [α/Fe] = 0.3). These are the first models for Population II stars in which both gravitational settling and radiative accelerations have been taken into account. Allowance for these diffusive processes leads to a 10%-12% reduction in age at a given turnoff luminosity. However, in order for the diffusive models to satisfy the constraints from Li and Fe abundance data (see Paper I) and to reproduce the observed morphologies of globular cluster (GC) color-magnitude diagrams (CMDs) in a straightforward way, extra mixing just below the boundary of the convective envelope seems to be necessary. Indeed, when additional turbulent mixing is invoked, the resultant models are able to satisfy all of these constraints, as well as those provided by the CMDs of local subdwarfs, rather well. Moreover, they imply an age near 13.5 Gyr for M92, which is one of the most metal-deficient (and presumably one of the oldest) of the Galaxy's GCs, if the field subgiant HD 140283 is used to derive the cluster distance. Comparisons of field subdwarfs and subgiants with a recently published fiducial for M5 suggests that the cluster has [Fe/H] ≲ -1.4, in conflict with some estimates based on high-resolution spectroscopy, if the metallicities of the field stars are to be trusted. In addition, an age of ≈11.5 Gyr is found for M5, irrespective of whether diffusive or nondiffusive isochrones are employed in the analysis. The implications of our results for the extragalactic distance scale and for the Hubble constant are briefly discussed in the context of the presently favored ΩM ≈ 0.35, ΩΛ ≈ 0.65 cosmological model.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/59222678e79ea0f7811df67a3d85359da4abb3ad

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Citations in the corpus

Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 , (2009, background): “Note that some slow macroscopic process is also required to reproduce the observed morphologies f g obular cluster colour-magnitude diagrams (VandenBerg et al. 2002).”
Distances and ages of NGC 6397, NGC 6752 and 47 Tuc (2003, background): “A careful discussion of this topic is given by Richard et al. (2002), who presented results from more sophisticated models that account for partial ionization and the eff ct of levitation due to radiation pressure.”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “Radiative acceleration may impede gravitational settling (see e.g., Richard et al. 2002).”
Evolution of Stars and Stellar Populations (2005, background): “Introductions to this subject can be found in [46], [52] and [161].”
The Adequacy of Stellar Evolution Models for the Interpretation of the Color-Magnitude Diagrams of Resolved Stellar Populations (2005, background): “Chaboyer et al. (2001) and Richard et al. (2002) have proposed two alternative reconciling scenarios.”
On the self-enrichment scenario of galactic globular clusters: constraints on the IMF (2006, background): “Additionally the spread of abundances cannot be explained by in situ mechanisms like gravitational setlling and thermal diffusion (Richard et al. 2002, Michaud et al. 2004).”
The Victoria-Regina Stellar Models: Evolutionary Tracks and Isochrones for a Wide Range in Mass and Metallicity that Allow for Empirically Constrained Amounts of Convective Core Overshooting (2005, background): “…chemical abundances in TO and lower RGB stars in GCs (see Gratton et al. 2001) and by the lack of any variation of Li abundance with Teff in warm field halo dwarfs (see Ryan et al. 1999), unless some competing process, such as turbulence, is invoked (Richard et al. 2002; VandenBerg et al. 2002).”
The Hubble Constant: A Summary of the Hubble Space Telescope Program for the Luminosity Calibration of Type Ia Supernovae by Means of Cepheids (2006, background): “Avalue of H0 ¼ 62:3 corresponds in an M ¼ 0:3, ¼ 0:7 universe to an expansion age of 15.1 Gyr, which may be compared with the age of M92 of 13.5 Gyr (VandenBerg et al. 2002) and the Th/Eu age of the Galactic halo of 15Gyr (Pagel 2001).”
THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, background): “Finally, we note that radiative levitation and gravitational settling (sometimes called atomic diffusion) is suspectedto play a role in altering the photospheric abundances between dwarf and giant stars at low metallicity (e.g., Richard et al. 2002a,b).”
THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED METHOD ALONG WITH COLOR–MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES (2013, background): “A free parameter, Dturb also appears in formulations of the extra mixing (see Richard et al. 2002, VBD12) that appears to be needed below outer convection zones in order for diffusive models to account for the low abundance of lithium in the Sun and to successfully predict the near independence of…”
The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy (2006, background): “These isochrones do not include core He diffusion, which would decrease their ages at a given turnoff luminosity by∼ 10% (VandenBerg et al. 2002).”
Abundance Variations within Globular Clusters (2004, background): “F or p er so na l u se o nl y. 5 Aug 2004 22:25 AR AR222-AA42-10.tex AR222-AA42-10.sgm LaTeX2e(2002/01/18) P1: IKH 424 GRATTON SNEDEN CARRETTA stars with respect to red giants, a result in agreement with the predictions of models with complete ionization, but not with those of Richard et al. (2002).”
Implications of WMAP Observations on Li Abundance and Stellar Evolution Models (2004, methodology): “Examples of those turbulent diffusion coefficients and how they compare to atomic diffusion coefficients are shown in Figure 6 of Richard et al. (2002b).”
A Driving Mechanism for the Newly Discovered Long-Period Pulsating Subdwarf B Stars (2003, background): “The problem of treating radiative levitation in a multielement mixture is extremely complex and has not yet been solved in a fully satisfactory way, although some important progress has been made (e.g., Chayer et al. 1995 and, more recently, Schuh, Dreizler, & Wolff 2002 and Richard et al. 2002).”