Issue |
A&A
Volume 503, Number 3, September I 2009
|
|
---|---|---|
Page(s) | 973 - 984 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200912083 | |
Published online | 22 June 2009 |
Atmospheric velocity fields in tepid main sequence stars *,**
1
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland
2
Department of Physics & Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada e-mail: jlandstr@astro.uwo.ca
3
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
4
Observatoire de Paris, LESIA, CNRS UMR 8109, 92195, Meudon, France
5
Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
6
Australia Telescope National Facility, CSIRO, Epping, NSW 1710, Australia
Received:
16
March
2009
Accepted:
11
June
2009
Context. The line profiles of the stars with below a few
can
reveal direct signatures of local velocity fields such as
convection in stellar atmospheres. This effect is well established
in cool main sequence stars, and has been detected and studied in
three A stars.
Aims. This paper reports observations of main sequence B, A and F stars
(1) to identify additional stars with sufficiently low values of
to search for spectral line profile signatures of local
velocity fields and (2) to explore how the signatures of the
local velocity fields in the atmosphere depend on stellar
parameters such as effective temperature and peculiarity type.
Methods. We have carried out a spectroscopic survey of B and A stars of low
at high resolution. Comparison of model spectra with those
observed allows us to detect signatures of the local velocity
fields such as asymmetric excess line wing absorption, best-fit
parameter values that are found to be larger for strong
lines than for weak lines, and discrepancies between observed and
modelled line profile shapes.
Results. Symptoms of local atmospheric velocity fields are always detected
through a non-zero microturbulence parameter for main sequence
stars having below about 10 000 K, but not for hotter
stars. Direct line profile tracers of the atmospheric velocity
field are found in six very sharp-lined stars in addition to
the three reported earlier. Direct signatures of velocity fields
are found to occur in A stars with and without the Am chemical
peculiarities, although the amplitude of the effects seems larger
in Am stars. Velocity fields are also directly detected in
spectral line profiles of A and early F supergiants, but without
significant line asymmetries.
Conclusions. We confirm that several atmospheric velocity field signatures,
particularly excess line wing absorption which is stronger in the
blue line wing than in the red, are detectable in the spectral
lines of main sequence A stars of sufficiently low . We triple
the sample of A stars known to show these effects, which are found
both in Am and normal A stars. We argue that the observed line
distortions are probably due to convective motions reaching the
atmosphere. These data still have not been satisfactorily
explained by models of atmospheric convection, including numerical
simulations.
Key words: convection / stars: atmospheres / stars: chemically peculiar / stars: abundances / stars: rotation / line: profiles
© ESO, 2009
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