Abstract details

The impact on the predicted Teff scale of using the latest MARCS model atmospheres as boundary conditions, instead of a fixed atmospheric structure (e.g., the gray T-tau relation) is examined. The former have been fitted to stellar interior models at both the photosphere and at tau=100 to ascertain the sensitivity of the results to the chosen fitting point. At the outset, the adopted physics in both the atmosphere and interior codes were shown to be very similar when the same mixing-length parameter and chemical abundances were assumed. Evolutionary tracks and isochrones were computed for [Fe/H] = 0.0 and -2.0. In the case of solar abundances, the Teff of the giant branch at a given luminosity varied by up to 100-150 K, depending on how the outer layers were treated. Similar variations were found for metal-poor giants while they were ~75 K for [Fe/H] = -2 main-sequence stars. However, there were clear systematic differences between the two cases. Interestingly, models for the low solar Z favored by Asplund et al (Z = 0.0122) were unable to reproduce the gap near the turnoff in the C-M diagram of the old open cluster M67, in contrast to models that assume Z = 0.018.