THE WO STARS .4. SAND-5 - A VARIABLE WO STAR

We report the results of 10 years (August 1986-July 1996) of intermedi ate and low resolution spectroscopic observations of the WR star Sand 5 (WR 142), the nearest and less studied member of the small WO-subgro up 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 meas ure in Sand 5 numerous emission features, and to separate most line bl ends. We have identified emission lines belonging to a very wide ioniz ation range, from He II, C III, C Iv, O Iv? up to O V, O VI, and, nota bly, O VIII. The line broadness varies from 1600 km s(-1) for O VIII t o 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 stratific ation in the wind and an acceleration of the outflowing matter from le ss than or equal to 1600 km s(-1) up to the wind terminal velocity of similar to 6000 km s(-1). During the extensive period covered by our o bservations the spectrum of Sand 5 has remained essentially constant. However we have noticed a marked fading of the contribution of He II t o the 467 nm emission in 1992-1996, while the O VIII 606 nm doubler wa s 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 delta EB-V=0.4 local to the star, and a steep power-law lambda(-3.85) distribution of the stellar continuum. We finally discuss the possibl e evolutionary state for Sand 5.