Evolution of rotation in rapidly rotating early-type stars during the main sequence with 2D models. (arXiv:1904.05219v1 [astro-ph.SR])

Evolution of rotation in rapidly rotating early-type stars during the main sequence with 2D models. (arXiv:1904.05219v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gagnier_D/0/1/0/all/0/1">D. Gagnier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rieutord_M/0/1/0/all/0/1">M. Rieutord</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Charbonnel_C/0/1/0/all/0/1">C. Charbonnel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Putigny_B/0/1/0/all/0/1">B. Putigny</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lara_F/0/1/0/all/0/1">F. Espinosa Lara</a>

The understanding of the rotational evolution of early-type stars is deeply
related to that of anisotropic mass and angular momentum loss. In this paper,
we aim to clarify the rotational evolution of rapidly rotating early-type stars
along the main sequence (MS). We have used the 2D ESTER code to compute and
evolve isolated rapidly rotating early-type stellar models along the MS, with
and without anisotropic mass loss. We show that stars with $Z=0.02$ and masses
between $5$ and $7~M_odot$ reach criticality during the main sequence provided
their initial angular velocity is larger than 50% of the Keplerian one. More
massive stars are subject to radiation-driven winds and to an associated loss
of mass and angular momentum. We find that this angular momentum extraction
from the outer layers can prevent massive stars from reaching critical rotation
and greatly reduce the degree of criticality at the end of the MS. Our model
includes the so-called bi-stability jump of the $dot{M}-T_{rm eff}$ relation
of 1D-models. This discontinuity now shows up in the latitude variations of the
mass-flux surface density, endowing rotating massive stars with either a
single-wind regime (no discontinuity) or a two-wind regime (a discontinuity).
In the two-winds-regime, mass loss and angular momentum loss are strongly
increased at low latitudes inducing a faster slow-down of the rotation.
However, predicting the rotational fate of a massive star is difficult, mainly
because of the non-linearity of the phenomena involved and their strong
dependence on uncertain prescriptions. Moreover, the very existence of the
bi-stability jump in mass-loss rate remains to be substantiated by
observations.

The understanding of the rotational evolution of early-type stars is deeply
related to that of anisotropic mass and angular momentum loss. In this paper,
we aim to clarify the rotational evolution of rapidly rotating early-type stars
along the main sequence (MS). We have used the 2D ESTER code to compute and
evolve isolated rapidly rotating early-type stellar models along the MS, with
and without anisotropic mass loss. We show that stars with $Z=0.02$ and masses
between $5$ and $7~M_odot$ reach criticality during the main sequence provided
their initial angular velocity is larger than 50% of the Keplerian one. More
massive stars are subject to radiation-driven winds and to an associated loss
of mass and angular momentum. We find that this angular momentum extraction
from the outer layers can prevent massive stars from reaching critical rotation
and greatly reduce the degree of criticality at the end of the MS. Our model
includes the so-called bi-stability jump of the $dot{M}-T_{rm eff}$ relation
of 1D-models. This discontinuity now shows up in the latitude variations of the
mass-flux surface density, endowing rotating massive stars with either a
single-wind regime (no discontinuity) or a two-wind regime (a discontinuity).
In the two-winds-regime, mass loss and angular momentum loss are strongly
increased at low latitudes inducing a faster slow-down of the rotation.
However, predicting the rotational fate of a massive star is difficult, mainly
because of the non-linearity of the phenomena involved and their strong
dependence on uncertain prescriptions. Moreover, the very existence of the
bi-stability jump in mass-loss rate remains to be substantiated by
observations.

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