Updated pre-main sequence tracks at low metallicities for 0.1 $\mathsf \le\ M/M _\odot\ \le 1.5$

Abstract

Context. Young populations at Z < Z 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-α 2D ), 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-α 2D 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-α 2D 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 M 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 log T eff /Δ log Z ∼ 0.03−0.05 in the range in Z from 0.02 to 0.0002 and for ages from 10 5 to 10 7 yr.

An Extended Theoretical Scenario for Classical Cepheids. I. Modeling Galactic Cepheids in the Gaia Photometric System

“…Moreover, in order to investigate the effect of superadiabatic convection efficiency, whose known main effect is to quench the pulsation driving mechanism, each selected model is computed for three values of the mixing length parameter α = l/H P 4 adopted to close the system of nonlinear hydrodynamical and convective equations (Bono et al 1999), namely α = 1.5 , α = 1.7 and α = 1.9. The choice of the mixing length parameter range was based on specific computations presented in previous papers (Di Criscienzo et al (2004);Fiorentino et al (2007); Natale et al (2008); Marconi et al (2013a)) which suggested that hotter variables are well reproduced with α = 1.5-1.6, whereas variables closer to the red edge of the instability strip often enquire α = 1.8-2.0 due to the most important efficiency of convection in the redder part of the color-magnitude diagram. For each pulsation model the nonlinear equations are integrated until a stable limit cycle is attained in the F or FO mode.…”

The Pisa pre-main sequence tracks and isochrones

“…Thus, we decided to follow the standard approach outlined by the pioneering contribution of Iben (1965) and followed until now by many authors (Baraffe et al 1998;Siess et al 2000;Yi et al 2001;Dotter et al 2007;Di Criscienzo et al 2009), which consists in choosing as an initial model a fully convective structure of very large radius and luminosity, i.e. a point along the Hayashi track, from which the star begins its quasi-static contraction at constant mass, i.e.…”

“…One might speculate that such a large difference in luminosity for such a low-mass star is related to the D-burning phase at the beginning of the Hayashi track that is not included in DVD09 computations. Di Criscienzo et al (2009) discussed how, owing to the lack of atmospheric structures for log g (cm s −2 ) < 3.5, they had to skip the deuterium burning during the early evolutionary phases. The DVD09 0.4 M track begins at log g (cm s −2 ) ≈ 4.00, at a luminosity lower than the other models (log L/L ≈ −1), BCAH98 included, who use the same atmospheric models.…”

A detailed study of the main sequence of the globular cluster NGC 6397: can we derive constraints on the existence of multiple populations?

“…Notice that the grids of model atmospheres available are computed only for solar scaled mixtures. The lack of suitable model atmospheres for α-enhanced populations forces us to use a grid for the boundary atmospheric conditions with larger [Fe/H] to simulate the α-enhancement, following the procedure adopted 5 The same approach has been adopted for pre-main sequence stars in Di Criscienzo et al (2009). 6 The choice of the α parameter is less and less critical when decreasing the mass, as the external layers become so dense that convection becomes more and more adiabatic.…”