The enigmatic binary system HD 5980

Abstract

The Small Magellanic Cloud multiple system HD 5980 contains a luminous blue variable (LBV) that underwent a major eruption in 1994, and whose current spectrum is that of a hydrogen-rich Wolf–Rayet (WR) star. Since the eruption, the wind mass-loss rate has been declining while wind speeds have been steadily increasing. Observations obtained in 2014 when <i>Star A</i> (the LBV) eclipses <i>Star B</i> indicate that the fitted mass-loss rate and luminosity have reached the lowest values ever determined for such spectra: Ṁ = 4.5 × 10⁻⁵ M⊙ yr⁻¹, L = 1.7 × 10⁶ L⊙. In addition, the radius of the LBV’s continuum-emitting region is similar to that derived from the eclipse light curves of the late 1970s. Hence, it appears to have attained a similar ‘low’ state to that of the late 1970s. While a good fit to the emission spectrum is obtained using a cmfgen model, there are discrepancies in the UV. In particular, the extent of the observed absorption profiles is ∼1000 km s⁻¹ greater than predicted by the emission-line intensities. Further, <i>HST</i> UV observations obtained in 2016, when <i>Star A</i> is eclipsed by <i>Star B</i>, show unusual P Cygni profiles that are not easily explained. Surprisingly the 2016 emission-line spectrum is similar to that at the opposite eclipse obtained in 2014. The complex UV profiles are likely to arise as a consequence of the dynamics of the wind–wind collision and radiative braking, both of which will cause significant departures from spherical symmetry, and have a strong orbital phase dependence. However, other scenarios, such as intrinsically aspherical winds, cannot be ruled out.