1 research outputs found
Grids of stellar models with rotation - III. Models from 0.8 to 120 Msun at a metallicity Z = 0.002
(shortened) We provide a grid of single star models covering a mass range
from 0.8 to 120 Msun with an initial metallicity Z = 0.002 with and without
rotation. We discuss the impact of a change in the metallicity by comparing the
current tracks with models computed with exactly the same physical ingredients
but with a metallicity Z = 0.014 (solar). We show that the width of the
main-sequence (MS) band in the upper part of the Hertzsprung-Russell diagram
(HRD), for luminosity above log(L/Lsun) > 5.5, is very sensitive to rotational
mixing. Strong mixing significantly reduces the MS width. We confirm, but here
for the first time on the whole mass range, that surface enrichments are
stronger at low metallicity provided that comparisons are made for equivalent
initial mass, rotation and evolutionary stage. We show that the enhancement
factor due to a lowering of the metallicity (all other factors kept constant)
increases when the initial mass decreases. Present models predict an upper
luminosity for the red supergiants (RSG) of log (L/Lsun) around 5.5 at Z =
0.002 in agreement with the observed upper limit of RSG in the Small Magellanic
Cloud. We show that models using shear diffusion coefficient calibrated to
reproduce the surface enrichments observed for MS B-type stars at Z = 0.014 can
also reproduce the stronger enrichments observed at low metallicity. In the
framework of the present models, we discuss the factors governing the timescale
of the first crossing of the Hertzsprung gap after the MS phase. We show that
any process favouring a deep localisation of the H-burning shell (steep
gradient at the border of the H-burning convective core, low CNO content)
and/or the low opacity of the H-rich envelope favour a blue position in the HRD
for the whole or at least a significant fraction of the core He-burning phase.Comment: 17 pages, 15 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic