Issue |
A&A
Volume 434, Number 2, May I 2005
|
|
---|---|---|
Page(s) | 637 - 647 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20041437 | |
Published online | 11 April 2005 |
Rotation velocities of white dwarfs
III. DA stars with convective atmospheres
1
Dr.-Remeis-Sternwarte, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany e-mail: karl@sternwarte.uni-erlangen.de
2
Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany
3
Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
4
Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany
5
Space Telescope Science Institute, Baltimore, USA
Received:
9
June
2004
Accepted:
12
November
2004
The sharp Hα NLTE line cores of hydrogen-rich (DA) white dwarfs
allow their projected rotational velocities to be determined.
High resolution optical spectra of 22 stars obtained with the Keck I telescope
are matched by synthetic spectra computed from a grid of NLTE model
atmospheres. In this paper, the third in a series on white
dwarf rotation, we concentrate preferentially on DA white dwarfs
with convective atmospheres, i.e. with < 14 000 K. Previous
analyses found DA white dwarfs hotter than 14 000 K to be very slow
rotators and rarely show any spectroscopically detectable rotation.
For 19 of our programme stars we were able to derive projected rotational
velocities or upper limits.
No rotation could be detected for seven
stars in our sample.
However twelve stars show significant line broadening.
In the case of the ZZ Ceti star G 117-B15A, the observed Hα line
profile cannot be matched by a rotationally broadened profile as
its line core is too narrow.
Combining our results with those from two similar studies,
we have obtained information on the rotation or other line broadening
mechanisms
(such as caused by magnetic fields) of 56 DA white dwarfs.
The fraction of rotating DA white dwarfs whose line profiles can be matched
for a vanishing projected rotation velocity is high for hot white dwarfs
with radiative atmospheres (25 out of 28).
Amongst the cool white dwarfs with presumedly convective
atmospheres, only for a few stars (8 out of 22) has no additional
line broadening
to be invoked to explain their observed Hα line profiles.
We conjecture that the physics of Hα line formation in convective
DA white dwarf atmospheres is not yet sufficiently well understood and
additional observational and theoretical efforts have to be made.
Key words: stars: white dwarfs / stars: variables: general / stars: rotation / stars: convection / stars: atmospheres
© ESO, 2005
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