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Astron. Astrophys. 325, 178-188


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The WO stars

IV. Sand 5: a variable WO star*

V.F. Polcaro1, R. Viotti1, C. Rossi2, and L. Norci3

1Istituto di Astrofisica Spaziale, CNR, Via E. Fermi 21, I-00044 Frascati RM, Italy
2Istituto Astronomico dell'Università di Roma "La Sapienza", Via G.M. Lancisi 29, I-00161 Roma, Italy
3Dunsink Observatory, Castleknock, Dublin 15, Ireland

Received 24 September 1996 / Accepted 16 December 1996

Abstract

We report the results of 10 years (August 1986-July 1996) of intermediate and low resolution spectroscopic observations of the WR star Sand 5 (WR 142), the nearest and less studied member of the small WO-subgroup which is located in the peculiar open cluster Berkeley 87. The wide spectral coverage (306-716 nm) and high S/N ratio, allowed us to measure in Sand 5 numerous emission features, and to separate most line blends. We have identified emission lines belonging to a very wide ionization range, from He II, C III, C IV, O IV, up to O V, O VI, and, notably, O VIII. The line broadness varies from 1600 km s-1 for O VIII to 3000/4600 km s-1 (O VI), 4100 km s-1 (O V), 4700/6200 km s-1 (C IV), and 5600 km s-1 (He II), which implies a ionization stratification in the wind and an acceleration of the outflowing matter from FORMULA 1600 km s-1 up to the wind terminal velocity of ~ 6000 km s-1. During the extensive period covered by our observations the spectrum of Sand 5 has remained essentially constant. However we have noticed a marked fading of the contribution of He II to the 467 nm emission in 1992-1996, while the O VIII 606 nm doublet was not detected in 1987 and in 1989, suggesting that both events might be associated with irregular long term wind structure variation. From the analysis of the continuum energy distribution we argue for a large interstellar extinction of EB-V=2.1, with a reddening excess of about FORMULA EB-V=0.4 local to the star, and a steep power-law FORMULA distribution of the stellar continuum. We finally discuss the possible evolutionary state for Sand 5.

Key words: line: formation, identification - stars: evolution - stars: individual (Sand 5) - stars: mass-loss - stars: Wolf-Rayet

*Based on observations collected at the 1.5 m telescope of the Bologna Astronomical Observatory at Loiano

Send offprint requests to: V.F. Polcaro


© European Southern Observatory (ESO) 1997

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