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 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 × 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, 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.

UV Spectroscopy of Massive Stars

“…In general, disentangling techniques do not work since the line profile shapes vary with orbital phase. This effect is clearly seen, for example, in the WR binaries WR 79 [120] and HD 5980 (e.g., [121,122]), and in Eta Carinae (e.g., [123]).…”

“…During 1994 it underwent an LBV-like outburst, which was very unusual since its effective temperature during minimum is much higher than for other LBVs. The two WN stars are estimated to have masses of ∼60 M [206], and effective temperatures of around 50,000 K [122,207,208]. UV monitoring shows complex variability of the main resonance profiles as a function of orbital phase and in time [122].…”

“…The two WN stars are estimated to have masses of ∼60 M [206], and effective temperatures of around 50,000 K [122,207,208]. UV monitoring shows complex variability of the main resonance profiles as a function of orbital phase and in time [122]. These variations are not well understood but will provide insights into the wind structures of the two stars.…”

Origins of Type Ibn SNe 2006jc/2015G in interacting binaries and implications for pre-SN eruptions

“…Observationally, eruptions from WR stars are also very rarely detected (e.g. HD 5980 in the Small Magellanic Cloud has been witnessed to undergo an LBV-like eruption in 1993-1994 and exhibit a WR-type spectrum during the quiescent phase; also this star is hydrogen-rich; Barba et al 1995, Koenigsberger 2004, Hillier et al 2019. The research of SNe Ibn has led to the speculation that some WR stars may still contain residual LBV-like instabilities, leading to pre-SN eruptions with significant mass loss (F07; P07; P08).…”

The winking eye of a hefty star. WR 21a revealed as a very massive eclipsing binary by TESS

“…As we show in Table 4, the most significant frequencies are n-times the orbital frequency, being the orbital harmonics 𝑛 = 5, 7, 9, 14, 17, 18, 25, 27, 33, 54, that means all these frequencies could be related to TEOs. Kołaczek-Szymański et al (2021) proposed that the massive WN+WN+O multiple system HD 5980 in the Small Magellanic Cloud (Koenigsberger et al 2014) is the most massive star known with a heartbeat effect, and probably with TEOs (Hillier et al 2019) with a frequency of 3.96 d −1 . The present analysis shows that WR 21a is one of the most massive systems known to present TEOs and a heartbeat system as well.…”