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Astron. Astrophys. 328, 219-228


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The orbital motion of 2 Velorum*

W. Schmutz1, J. Schweickhardt2, O. Stahl2, B. Wolf2, T. Dumm1, Th. Gäng2,3, I. Jankovics4, A. Kaufer2, H. Lehmann5, H. Mandel2, J. Peitz2, and Th. Rivinius2

1Institut für Astronomie, ETH-Zentrum, CH-8092 Zürich, Switzerland
2Landessternwarte Heidelberg Königstuhl, D-69117 Heidelberg, Germany
3STScI, 3700 San Martin Drive, Baltimore MD 21218, USA
4Gothard Astrophysical Observatory, H-9707 Szombathely, Hungary
5Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, D-07778 Tautenburg, Germany

Received 26 May 1997 / Accepted 11 July 1997

Abstract

We analyze the orbital motion of $\gamma^2$ Velorum based on high resolution optical spectra obtained in 1995 and 1996. By combining our data with values from the literature we find a period $P=78.53\pm 0.01$ d. We determine radial velocity semi-amplitudes $K_{\rm WR}= 122\pm 2$ km s-1 and $K_{\rm O}= 38.4\pm 2$ km s-1 for the Wolf-Rayet star and the O star, respectively. The given errors are the standard deviations of the results from individual lines.

The inclination of the system is $i=65^\circ$. This result is obtained by combining $M_{\rm O} sin^3 i$ from the orbital analysis with the mass $M_{\rm O}= 29$ M$_{\odot}$, obtained from its luminosity and stellar evolution tracks. The mass of the Wolf-Rayet component is $M_{\rm WR}=9$ M$_{\odot}$.

Our O star velocity curve disagrees by 15 $\sigma$ from a previously published result. We have identified the reason for the disagreement in the failure to correct for the WR emission that affects the measurements of the absorption line centers. The correction could have introduced systematic errors that may be larger than the given precision of the amplitudes. However, we can set an upper limit of $K_{\rm O}<50$ km s-1 from uncorrected measurements. This yields an upper limit for the Wolf-Rayet mass of $M_{\rm WR}<12$ M$_{\odot}$.

We find systematic phase-shifts between the velocity curves of some emission lines. We also observe that some (but not all) emission lines deviate significantly in the 1995 data set from the line's orbital solution defined by the 1996 observations. These phenomena may indicate that the radial velocities of the emission lines could be subject to systematic distortions and that the errors of the orbital motion are larger than the internal precision given here.

- stars: Wolf-Rayet

Key words: binaries: spectroscopic - stars: early-type - stars: fundamental parameters - stars: individual: $\gamma^2$ Vel

*Based on observations collected at the European Southern Observatory at La Silla, Chile. ESO proposals Nr.56.D-327 and 57.D-517.

Send offprint requests to: W. Schmutz


© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998
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