Mass-Loss Rate and He/H Abundance of the Erupting Component in the Small Magellanic Cloud System HD 5980

Abstract

The binary Wolf-Rayet system HD 5980 in the Small Magellanic Cloud underwent a major luminous blue variable-type eruptive event in 1994. It is the first such recorded event in which the apparent precursor transits from WNE to WNL spectral types prior to the eruption. In this paper we analyze the spectrum of the system obtained when the outburst was declining (1994 December), but the dominant spectrum was that of the eruptor. From non-LTE analysis we obtain Ṁ=10^-3 M_⊙ yr^-1, N[He]/N[H] = 0.43 (by number; Y = 0.63 by mass), v_∞ = 600 km s^-1, T_* = 35,500 K, and L_eruptor = 3 × 10⁶L_⊙. A comparison of the He/H abundance derived here and an estimate obtained from published data of 20 yr ago leads us to conclude that the member of the binary system that underwent the eruption is the star formerly classified as an O7 supergiant. The considerable amount of He in this star indicates that it is in transition to becoming an H-poor W-R. By comparing the stellar parameters with single-star evolutionary tracks, we derive that the progenitor was more massive than 120 M_⊙, and that its current mass is close to 80 M_⊙.