THE PHOTOSPHERIC ABSORPTION LINES IN THE ULTRAVIOLET SPECTRUM OF THE MULTIPLE SYSTEM HD 5980

摘要

We search for radial velocity variations in the ultraviolet spectra of the erupting Wolf-Rayet/luminous blue variable system HD 5980, located in the Small Magellanic Cloud. We obtain an average radial velocity of 22 +- 22 km s~(-1) for five observations in 1999 at different orbital phases, -39 +- 24 km s~(-1) for one observation in 2001, and 20 +- 45 km s~(-1) for one observation in 2002. These velocities are with respect to the SMC O3 star MPG 355, which is used as one of the templates. Hence, radial velocity variations on the 19.265 day (star A+star B) orbital timescale attributable to the erupting star (star A) of the system are not detected, thus confirming that the visible absorption lines in the spectrum have their origin in a third stellar component (star C). We propose that star A has very broad (v sin i ～ 250 km s~(-1)) absorption lines, which would escape detection in the complicated emission-line spectrum of the system. Such broad lines, combined with the 128 km s~(-1) orbital motion, would lead to a lower amplitude fictitious radial velocity curve when the stationary set of absorption lines is measured. We conclude that line profile variations at orbital phase 0.0 from one epoch of observations to another are associated with these broad underlying absorption lines, arising in the unstable photosphere of star A. The analysis is performed with the aid of synthetic UV spectra that are calculated from LTE line-blanketed atmosphere models, and the HST Space Telescope Imaging Spectrograph spectra of four O-type stars in the cluster NGC 346, to which HD 5980 is believed to belong. We estimate T_(eff) and obtain values of v sin i and the mean radial velocity for MPG 324 [O4 V((f))], MPG 368 [O4-5 V((f))], MPG 355 [O3 V(f~*)], and MPG 113 (OC 6Vz).