The enigmatic binary system HD 5980

D. J. Hillier

doi:10.1093/mnras/stz808

The enigmatic binary system HD 5980

2019-03-19

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D. J. Hillier *

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Gloria Koenigsberger *

DOI：10.1093/mnras/stz808

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The Small Magellanic Cloud multiple system HD5980 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:. M = 4.5 x 10(-5) M-circle dot yr(-1), L = 1.7 x 10(6) L-circle dot. 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 similar to 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 emissionline 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.

Keywords:

binaries: eclipsing

stars: individual(HD 5980)

stars: Wolf-Rayet

stars: winds, outflows

stars: mass-loss

The enigmatic binary system HD 5980

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