The impact of rotation on the line profiles of Wolf-Rayet stars
T. Shenar, W.-R. Hamann, H. Todt
Massive Wolf-Rayet stars are recognized today to be in a very common,
but short, evolutionary phase of massive stars.
While our understanding of Wolf-Rayet stars has increased
dramatically over the past decades, it remains unclear whether rapid rotators are among them.
There are various indications that
rapidly rotating Wolf-Rayet stars should exist. Unfortunately, due to their expanding atmospheres,
rotational velocities of Wolf-Rayet stars are very difficult to measure.
However, recently observed spectra of several Wolf-Rayet stars reveal peculiarly broad
and round emission lines.
Could these spectra imply rapid rotation?
In this work, we model the effects of rotation on the atmospheres of
Wolf-Rayet stars. We further investigate whether the peculiar spectra of five Wolf-Rayet stars may be
explained with the help of stellar rotation, infer appropriate rotation parameters,
and discuss the implications of our results.
We make use of the Potsdam Wolf-Rayet (PoWR) non-LTE model atmosphere code.
Since the observed spectra of Wolf-Rayet stars are mainly formed in
their expanding atmospheres,
rotation must be accounted for
with a 3D integration scheme of the formal integral. For this purpose,
we assume a rotational velocity field consisting of an inner co-rotating domain and an outer domain,
where the angular momentum is conserved.
We find that rotation can reproduce the unique spectra analyzed here. However,
the inferred rotational velocities at the stellar surface are large (~200 km/s), and the
inferred co-rotation radii (~10 stellar radii) suggest the existence of
very strong photospheric magnetic fields (~20 kG).
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