INVESTIGADORES
GRANADA Anahi
artículos
Título:
Evolution of single B-type stars with a large angular momentum content
Autor/es:
GRANADA, ANAHÍ; HAEMMERLE, LIONEL
Revista:
ASTRONOMY AND ASTROPHYSICS
Editorial:
EDP SCIENCES S A
Referencias:
Lugar: Paris; Año: 2014 vol. 570 p. 1 - 15
ISSN:
0004-6361
Resumen:
Context. The Geneva Stellar Evolution Group has recently presented an
extended database of rotating stellar models at three different
metallicities for nine different initial rotation parameters and ten
different masses corresponding to spectral types from early-F to late-O.
With these grids we have contributed to the understanding of the
evolution of single rotating stars, and we intend to use them to produce
synthetic stellar populations that fully account for the effects of
stellar rotation. However, up to now we still lacked stellar
evolutionary tracks that rotate close to the critical limit during the
whole main-sequence (MS) phase. This occurs because the flat internal
profile of rotation imposed at the zero-age main sequence (ZAMS) is
modified by the action of meridional currents immediately after the
ZAMS, causing the surface rotational velocity to decrease abruptly until
it reaches a quasi-stationary state. Aims: We compute stellar
models with non-solid rotation at the ZAMS, which allows us to obtain
stellar evolutionary tracks with a larger content of angular momentum
that rotate close to the breakup limit throughout the whole MS.
Methods: We produced stellar models by removing the assumption that
stars rotate as solid bodies at the ZAMS. We obtained the stellar
structure at the ZAMS with a differentially rotating profile for three
different metallicities by performing pre-MS calculations and by
proposing ad hoc initial rotational profiles. We then computed the MS
evolution and later phases of stellar evolution of these models, which
attain rotational equatorial velocities close to the critical limit
throughout their whole MS phase. Results: Stellar models with
solid rotation at the ZAMS adequately represent the overall
characteristics and evolution of differentially rotating models of
identical angular momentum content, but with a lower initial surface
rotational velocity rate, at Z = 0.014, Z = 0.006, and Z = 0.002. For
models with solid rotation at the ZAMS we therefore recommend to use as
the initial rotational rate the values derived once the quasi-stationary
state is reached, that is, after the abrupt decrease in surface
velocity. By producing stellar structures at the ZAMS with
differentially rotating profiles and larger angular momentum content
than in our previous works, we obtain models that rotate close to the
critical limit throughout the whole MS. These models have a longer MS
lifetime and a higher surface chemical enrichment already at the end of
the MS, particularly at Z = 0.002. Interestingly, the initial equatorial
rotational velocities are virtually metallicity independent for all
stellar models we computed in the B-type star range with the same mass
and angular momentum content at the ZAMS. If, as some observational
evidence indicates, B-type stars at Z = 0.002 rotate with a higher
equatorial velocity at the ZAMS than stars with Z = 0.014, our finding
would indicate that the angular momentum content of B-type stars in the
SMC is higher than their Galactic counterparts.