Thibault Lejeune

Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture

We have constructed a new set of isochrones, called the Y2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of α-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main-sequence birthline instead of from the zero-age main sequence and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the α-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages.

Database of Geneva stellar evolution tracks and isochrones for (UBV)

We have used an updated version of the empirically and semi-empirically calibrated B a S e L library of synthetic stellar spectra of Lejeune et al.([CITE], [CITE]) and Westera et al.([CITE]) to calculate synthetic photometry in the (UBV) J (RI) C JHKLL ′ M , HST-WFPC2, Geneva, and Washington systems for the entire set of non-rotating Geneva stellar evolution models covering masses from 0.4-0.8 to 120-150

_{\mathsf{J}}

M_{\odot}

(RI)

and metallicities

_{\mathsf{C}}

Z=0.0004

JHKLL'M, HST-WFPC2, Geneva and Washington photometric systems

(1/50

The helium content and age of the Hyades: - Constraints from five binary systems and Hipparcos parallaxes

Z_{\odot}

Convective core mixing: A metallicity dependence?

) to 0.1 (5

A standard stellar library for evolutionary synthesis - III. Metallicity calibration

Z_{\odot}

Observed HR Diagrams and Stellar Evolution

). The results are provided in a database which includes all individual stellar tracks and the corresponding isochrones covering ages from 10 3 yr to 16-20 Gyr in time steps of