Variations of the He II λ1640 line in B0e-B2.5e stars

Abstract

Using the International Ultraviolet Explorer data archive, we have examined the SWP echellograms of 74 B0-B2.5e stars for statistically significant fluctuations in the He II ("Hα") λ1640 line profile. In this sample we found that the He II line is occasionally variable in 10 stars over short to long timescales. The He II-variable stars discovered are λ Eri, ω Ori, μ Cen, 6 Cep, HD 67536, ψ¹ Ori, η Cen, π Aqr, 2 Vul, and 19 Mon. The most frequent two types of variability are an extended blue wing absorption and a weakening of the line along the profile. Other types of variability are a weak emission in the red wing and occasionally a narrow emission feature. In the overwhelming number of cases, the C IV resonance doublet exhibits a similar response; rarely, it can exhibit a variation in the opposite sense. Similar responses are also often seen in the Si IV doublet, and occasionally even the Si III λ1206 line. We interpret the weakenings of He II and of high-velocity absorptions of C IV to localized decreases in the photospheric temperature, although this may not be a unique interpretation. We discuss the variable blue wing absorptions and red wing emissions in terms of changes in the velocity law and mass flux carried by the wind. In the latter case, recent experimental models by Venero, Cidale, & Ringuelet require that during such events the wind must be heated by 35 000 K at some distance from the star.