Updated pre-main sequence tracks at low metallicities for 0.1

Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Rome, Italy e-mail: [dicrisci;dantona;ventura]@oa-roma.inaf.it

Received: 17 October 2008
Accepted: 21 December 2008

Abstract

Context. Young populations at are being examined to understand the role of metallicity in the first phases of stellar evolution. For the analysis it is necessary to assign mass and age to Pre-Main Sequence (PMS) stars. While it is well known that the mass and age determination of PMS stars is strongly affected by the convection treatment, extending any calibration to metallicities different from solar is very artificial, in the absence of any calibrators for the convective parameters. For solar abundance, Mixing Length Theory models have been calibrated by using the results of 2D radiative-hydrodynamical models (MLT-), that are very similar to those computed with non-grey ATLAS9 atmosphere boundary conditions and a full spectrum of turbulence (FST) convection model both in the atmosphere and in the interior (NEMO-FST models).

Aims. While MLT- models are not available for lower metallicities, we extend to lower Z the NEMO-FST models, under the hypothesis that in such a way we are simulating the results of MLT- models also at smaller Z.

Methods. We use standard stellar computation techniques in which the atmospheric boundary conditions are derived making use of model atmosphere grids. This allows us to take into account the non greyness of the atmosphere, but adds a new parameter to the stellar structure uncertainty, namely the efficiency of convection in the atmospheric structure, if convection is computed in the atmospheric grid by a model different to the model adopted for the interior integration.

Results. We present PMS models for low mass stars from 0.1 to 1.5  for metallicities [Fe/H] = -0.5, -1.0 and -2.0. The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models. At fixed luminosity more metal poor isochrones are hotter than solar ones by Δ  T_eff/Δ  ~ 0.03-0.05 in the range in Z from 0.02 to 0.0002 and for ages from 10⁵ to 10⁷ yr.