1997A&A...325..178P

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 (306-716nm) 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 1600km/s for O VIII to 3000/4600km/s (O VI), 4100km/s (O V), 4700/6200km/s (C IV), and 5600km/s (He II), which implies a ionization stratification in the wind and an acceleration of the outflowing matter from ≤1600km/s up to the wind terminal velocity of ∼6000km/s. 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 467nm emission in 1992-1996, while the O VIII 606nm 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 E_B–V=2.1, with a reddening excess of about δE_B–V=0.4 local to the star, and a steep power-law λ^–3.85 distribution of the stellar continuum. We finally discuss the possible evolutionary state for Sand 5.