Issue |
A&A
Volume 369, Number 3, April III 2001
|
|
---|---|---|
Page(s) | 1058 - 1077 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20010185 | |
Published online | 15 April 2001 |
Stellar and circumstellar activity of the
Be star
Centauri*
III. Multiline nonradial pulsation modeling
1
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2
Landessternwarte Königstuhl, 69117 Heidelberg, Germany
3
Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic
4
Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
5
European Southern Observatory, Casilla 19001, Santiago 19, Chile
Corresponding author: Th. Rivinius, triviniu@eso.org
Received:
13
December
2000
Accepted:
31
January
2001
After the description and time series analysis of the
variability of the circumstellar and stellar lines,
respectively, in Papers I and II of this series, this paper
sets out to model the stellar variability in terms of
multi-mode nonradial pulsation (nrp), but also adds
another 109 echelle spectra to the database, obtained in 1999.
While the near-circumstellar emission has faded further, the
six periods and the associated line profile variabilities
(lpv) have remained unchanged.
For the modeling, of the periods
-
close to 0.5 day, and
of
the two periods
and
near 0.28 day
were selected, because they have the largest amplitude in
their respective groups, which are characterized by their own
distinct phase-propagation pattern. Permissable ranges of
mass, radius, effective temperature, projected equatorial
rotation velocity, and inclination angle were derived from
calibrations and observations available in the literature. A
total of 648 different combinations of these parameters were
used to compute a number of trial series of line profiles for
comparison with the observations. Next to reproducing the
observed variability, the primary constraint on all models was
that the two finally adopted solutions for
and
had to be based on only one common set of values
of these quantities. This was, in fact, accomplished.
Townsend's [CITE]
code Bruce was deployed to model the pulsational
perturbations of the rotationally distorted stellar surface.
With the help of Kylie, from the same author, these
perturbations were converted into observable quantities. The
local flux and the atmosphere structure were obtained from a
grid of Atlas9 models
with solar metallicity, while the formation of 5967 spectral
lines was calculated with the LTE code of [CITE].
An initial coarse grid of models using all these ingredients
was computed for all 12 nrp modes with
≤ 3
and m
≠ 0. Comparison with the observed variability
of C ii 4267, which is the best compromise between
contamination by circumstellar emission and significance of
the variability, yielded (
= 2, m = +2) for
(and, by implication,
-
)
and (
= 3, m = +3) for
(and
) as the best matching nrp modes. At
/
and 440 km s-1,
respectively, the mass-to-radius ratio and the equatorial
velocity are on the high side, but not in fundamental conflict
with established knowledge. The photometric variations of all
six modes combine at most to a maximal peak-to-peak amplitude
of 0.015 mag, consistent with the non-detection of any of the
spectroscopic periods by photometry.
Without inclusion of additional physical processes,
present-day linear nrp models are fundamentally unable
to explain major red-blue asymmetries in the power
distribution, which however seem to be limited to only some
lines and the modes with the highest amplitudes.
Nevertheless, the model reproduces very well a wide range of
observed details. Most notable among them are: (i) Although
all modeling was done on the residuals from the mean
profiles only, the mean spectrum predicted by the
model closely fits the observed one. (ii) Dense series of
high-quality spectra obtained as early as 1987
and as recently as 1999,
published independently but not included in the modeling
efforts of this paper, are matched in great detail by the
multiperiodic nrp model.
As in omegaCMa,
the inferred modes are retrograde in the corotating frame
and in the observer's frame appear prograde only because of
the rapid rotation. This has implications for models of the
ejection of matter during line emission outbursts, which in
μ Cen are correlated with the beating of modes in the
0.5-d group of periods. The length of the corotating
periods as well as the horizontal-to-vertical velocity
amplitude ratios suggest a g-mode character.
Key words: line: profiles / line: formation / stars: oscillation / stars: Be / stars: individual: μ Cen
© ESO, 2001
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