Hubble Space Telescope Observations of the Luminous Blue Variable/W-R Eclipsing Binary System HD 5980

Abstract

We report the results of Hubble Space Telescope STIS observations of the intriguing luminous blue variable/Wolf-Rayet binary system HD 5980 in the Small Magellanic Cloud. Although its spectral characteristics (WN6) are currently very similar to those observed in 1995 in the FUV, some of the line fluxes continue with the increasing trend observed since the 1980s. The erupting star still dominates the emission-line spectrum, and a radial velocity curve from UV lines is derived, supporting previous estimates of its mass (50 M⊙). A rough estimate of 2 × 10-4 M⊙ yr-1 for the current mass-loss rate of this star is derived. The only spectral lines attributable to the close companion are the very extended P Cygni absorption components of the strong UV lines, present only during the eclipse of the eruptor, implying that the wind-wind collision shock cone winds tightly around this companion. A third stellar component contributing to the spectrum is detected through the presence of stationary photospheric absorption lines in the UV spectrum. This object contributes ∼30% to the UV continuum luminosity. A new set of ISM absorption components at -680 km s-1 has appeared as a result of the shock interface between the slow wind (∼400 km s-1) that emerged during the 1994 eruption and the subsequently emerging fast wind (∼2000 km s-1) phases.