The enigmatic binary system HD 5980. Issue 1 (19th March 2019)

• Record Type: Journal Article
• Authors: Hillier, D John; Koenigsberger, Gloria; Nazé, Yaël; Morrell, Nidia; Barbá, Rodolfo H; Gamen, Roberto
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 486:Issue 1(2019)
• Page Start: 725
• Page End: 742
• Publication Date: 2019-03-19
• Subjects: binaries: eclipsing -- stars: individual(HD 5980) -- stars: Wolf–Rayet -- stars: winds, outflows -- stars: mass-loss; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz808
• Languages: English
• ISSNs: 0035-8711

Abstract

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 Star A (the LBV) eclipses Star B indicate that the fitted mass-loss rate and luminosity have reached the lowest values ever determined for such spectra: $\dot{M}$ = 4.5 × 10 −5 $\mathrm{M}_\odot \, \hbox{yr}^{-1}$, L = 1.7 × 10 6 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, HST UV observations obtained in 2016, when Star A is eclipsed by Star B, 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.