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Astron. Astrophys. 325, 178-188 (1997)

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The WO stars

IV. Sand 5: a variable WO star *

V.F. Polcaro 1, R. Viotti 1, C. Rossi 2 and L. Norci 3

1 Istituto di Astrofisica Spaziale, CNR, Via E. Fermi 21, I-00044 Frascati RM, Italy
2 Istituto Astronomico dell'Università di Roma "La Sapienza", Via G.M. Lancisi 29, I-00161 Roma, Italy
3 Dunsink 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 [FORMULA] 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 [FORMULA] =2.1, with a reddening excess of about [FORMULA] [FORMULA] =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

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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