Spectroscopic study of the outflowing disk winds of B[e] supergiants in the Magellanic Clouds

We report on UV high resolution spectroscopic observations of R 50 in the Small Magellanic Cloud, and R 82 and Hen S22 in the Large Magellanic Cloud obtained with the International Ultraviolet Explorer. The observed stars are supposed to represent edge-on cases of B[e] supergiants for which a two-component stellar wind model has previously been suggested. The spectra are characterized by P Cygni-type lines of Fe II. The observations show that the three stars have very slowly expanding winds with terminal velocities derived from the blue absorption edges of 75, 100, and 120 km s^-1, respectively. Fits of the Fe II lines of Hen S22 and R 82 using the SEI method lead to even slower velocities of about 60 to 80 km s^-1, respectively. This is about a factor of ten slower than the terminal velocity of normal B-type supergiants. The results are consistent with the assumption that the observed stars are viewed edge-on. We derived optical depths of the absorption components of the Fe II resonance lines of Hen S22 and R 82 of larger than about 5, yielding lower limits for the disk mass-loss rates of the order of 6 10^-7 and 5 10^-7M_⊙ yr^-1, respectively. The very low terminal velocity of the disk can be explained by the fact that the disks of the B[e] supergiants are on the low-velocity side of the bi-stability jump of radiation driven winds (which reduces υ_∞/υ_esc) and a rotational velocity of about 0.75 of the critical rotation velocity (which reduces the effective υ_esc). The effective gravity derived from υ_∞ and υ_∞/υ_esc = 1.3 is very low. It is on the order of log g_eff = 0.2 to 0.7.