Probing the weak wind phenomenon in Galactic O-type giants. (arXiv:1903.07937v1 [astro-ph.SR])

Probing the weak wind phenomenon in Galactic O-type giants. (arXiv:1903.07937v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Almeida_E/0/1/0/all/0/1">E. S. G. de Almeida</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Marcolino_W/0/1/0/all/0/1">W. L. F. Marcolino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bouret_J/0/1/0/all/0/1">J.-C. Bouret</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pereira_C/0/1/0/all/0/1">C. B. Pereira</a>

Analyses of Galactic late O dwarfs (O8-O9.5V stars) raised the `weak wind
problem’: spectroscopic mass loss rates ($dot{M}$) are up to two orders of
magnitude lower than the theoretical values. We investigated the stellar and
wind properties of Galactic late O giants (O8-O9.5III stars). We performed a
spectroscopic analysis of nine O8-O9.5III stars in the ultraviolet (UV) and
optical regions using the model atmosphere code CMFGEN. From the UV region, we
found $dot{M}$ $sim$ $10^{-8}-10^{-9}$ $mathrm{M_odot}$ $mathrm{yr^{-1}}$
in overall. This is lower by $approx 0.9 – 2.3$ dex than the predicted values
based on the (global) conservation of energy in the wind. $dot{M}$ predicted
from first principles, based on the moving reversing layer theory, agrees
better with our findings, but it fails to match the spectroscopic $dot{M}$ for
the most luminous OB stars. The region of $log(L_star/L_odot) approx 5.2$
is critical for both sets of predictions in comparison with the spectroscopic
mass-loss rates. CMFGEN models with the predicted $dot{M}$ (the former one)
fail to reproduce the UV wind lines for all stars of our sample. We reproduce
the observed H$alpha$ profiles of four objects with our $dot{M}$ derived from
the UV. Hence, low $dot{M}$ values (weak winds) are favored to fit the
observations (UV + optical), but discrepancies between the UV and H$alpha$
diagnostics remain for some objects. Our results indicate weak winds beyond the
O8-9.5V class, being the region of $log(L_star/L_odot) approx 5.2$ indeed
critical to the weak wind phenomenon. Since O8-O9.5III stars are more evolved
than O8-9.5V, evolutionary effects do not seem to play a role on the onset of
the weak wind phenomenon. These findings support that $dot{M}$ (for low
luminosity O stars) in use in the majority of modern stellar evolution codes
must be severely overestimated up to the end of H-burning phase.

Analyses of Galactic late O dwarfs (O8-O9.5V stars) raised the `weak wind
problem’: spectroscopic mass loss rates ($dot{M}$) are up to two orders of
magnitude lower than the theoretical values. We investigated the stellar and
wind properties of Galactic late O giants (O8-O9.5III stars). We performed a
spectroscopic analysis of nine O8-O9.5III stars in the ultraviolet (UV) and
optical regions using the model atmosphere code CMFGEN. From the UV region, we
found $dot{M}$ $sim$ $10^{-8}-10^{-9}$ $mathrm{M_odot}$ $mathrm{yr^{-1}}$
in overall. This is lower by $approx 0.9 – 2.3$ dex than the predicted values
based on the (global) conservation of energy in the wind. $dot{M}$ predicted
from first principles, based on the moving reversing layer theory, agrees
better with our findings, but it fails to match the spectroscopic $dot{M}$ for
the most luminous OB stars. The region of $log(L_star/L_odot) approx 5.2$
is critical for both sets of predictions in comparison with the spectroscopic
mass-loss rates. CMFGEN models with the predicted $dot{M}$ (the former one)
fail to reproduce the UV wind lines for all stars of our sample. We reproduce
the observed H$alpha$ profiles of four objects with our $dot{M}$ derived from
the UV. Hence, low $dot{M}$ values (weak winds) are favored to fit the
observations (UV + optical), but discrepancies between the UV and H$alpha$
diagnostics remain for some objects. Our results indicate weak winds beyond the
O8-9.5V class, being the region of $log(L_star/L_odot) approx 5.2$ indeed
critical to the weak wind phenomenon. Since O8-O9.5III stars are more evolved
than O8-9.5V, evolutionary effects do not seem to play a role on the onset of
the weak wind phenomenon. These findings support that $dot{M}$ (for low
luminosity O stars) in use in the majority of modern stellar evolution codes
must be severely overestimated up to the end of H-burning phase.

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