EDP Sciences
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Volume 375, Number 1, August III 2001
Page(s) 54 - 69
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20010824

A&A 375, 54-69 (2001)
DOI: 10.1051/0004-6361:20010824

Long-term spectroscopic monitoring of the Luminous Blue Variable AG Carinae

O. Stahl1, I. Jankovics2, J. Kovács2, B. Wolf1, W. Schmutz3, A. Kaufer4, Th. Rivinius4 and Th. Szeifert4

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

(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 ( $\lambda/\Delta\lambda \approx 20 000$). 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 H$\alpha$ line and find the highest mass loss rates ( $\log\dot{M}/({M}_\odot {\rm
yr}^{-1}) \approx -3.8$, about a factor of five higher than in the minimum, where we find $\log\dot{M}/({M}_\odot {\rm yr}^{-1})
\approx -4.5$) 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 ($\pm$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 -- stars: variable -- stars: mass-loss

Offprint request: O. Stahl, O.Stahl@lsw.uni-heidelberg.de

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