Issue |
A&A
Volume 375, Number 1, August III 2001
|
|
---|---|---|
Page(s) | 54 - 69 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20010824 | |
Published online | 15 August 2001 |
Long-term spectroscopic monitoring of the Luminous Blue Variable AG Carinae*
1
Landessternwarte Königstuhl, 69117 Heidelberg, Germany
2
Gothard Astrophysical Observatory, 9707 Szombathely, Hungary
3
PMOD/WRC, 7260 Davos Dorf, Dorfstrasse 33, Switzerland
4
European Southern Observatory, 85748 Garching, Karl-Schwarzschild-Str. 2, Germany
Corresponding author: O. Stahl, O.Stahl@lsw.uni-heidelberg.de
Received:
10
April
2001
Accepted:
22
May
2001
We have extensively monitored the Luminous Blue Variable
AG Car (HD 94910) spectroscopically. Our data cover the years 1989
to 1999. In this period, the star underwent almost a full S Dor
cycle from visual minimum to maximum and back. Over several seasons,
up to four months of almost daily spectra are available. Our data
cover most of the visual spectral range with a high spectral
resolution (). This allows us
to investigate the variability in many lines on time scales from
days to years. The strongest variability occurs on a time scale of
years. Qualitatively, the variations can be understood as changes
of the effective temperature and radius, which are in phase with the
optical light curve. Quantitatively, there are several interesting
deviations from this behaviour, however. The Balmer lines show
P Cygni profiles and have their maximum strength (both in equivalent
width and line flux) after the peak of the optical light
curve, at the descending branch of the light curve. The line-width
during maximum phase is smaller than during minimum, but it has a
local maximum close to the peak of the visual light curve. We
derive mass-loss rates over the cycle from the Hα line and
find the highest mass loss rates (
, about a factor of five higher than in the
minimum, where we find
) after the visual maximum. Line-splitting is very
commonly observed, especially on the rise to maximum and on the
descending branch from maximum. The components are very long-lived
(years) and are probably unrelated to similar-looking line-splitting
events in normal supergiants. Small apparent accelerations
of the components are observed. The change in radial velocity could
be due to successive narrowing of the components, with the
absorption disappearing at small expansion velocities first. In
general, the line-splitting is more likely the result of missing
absorption at intermediate velocities than of excess absorption at
the velocities of the components. The Hei lines and other
lines which form deep in the atmosphere show the most peculiar
variations. The Hei lines show a central absorption with
variable blue- and red-shifted emission components. Due to the
variations of the emission components, the Hei lines can
change their line profile from a normal P Cyg profile to an inverse
P Cyg-profile or double-peak emission. In addition, very broad
(±1500 km s-1) emission wings are seen at the strongest
Hei lines of AG Car. At some phases, a blue-shifted
absorption is also present. The central absorption of the Hei
lines is blue-shifted before and red-shifted after maximum.
Possibly, we directly see the expansion and contraction of the
photosphere. If this explanation is correct, the velocity of the
continuum-forming layer is not dominated by expansion but is only
slightly oscillating around the systemic velocity.
Key words: stars: individual: AG Car / stars: early-type / stars: emission-line, Be / stars: variables: general / stars: mass-loss
© ESO, 2001
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