X-ray image by Chandra Observatory
Epoch J2000 Equinox J2000
|Right ascension||00h 59m 26.569s|
|Declination||–72° 09′ 53.91″|
|Apparent magnitude (V)||11.31|
|Spectral type||WNL + WN4h + O|
|B−V color index||−0.18|
|Proper motion (μ)||RA: −3.5 mas/yr
Dec.: −2.4 mas/yr
|Absolute magnitude (MV)||−7.3 (combined), −6.3/−5.8/−6.1|
|Mass||58–79 (A) / 51–67 (B) M☉|
|Radius||23–280 (A) / 16–17 (B) R☉|
|Luminosity||2,000,000 / 2,500,000 / 2,000,000 L☉|
|Temperature||21,000–53,000 (A) K|
|Rotational velocity (v sin i)||250 (A) / 75 (C) km/s|
HD 5980, RMC 14, Sk 78, SMC AB 5, AAVSO 0056-72.
HD 5980 is the brightest star in the Small Magellanic Cloud (SMC) and is located in NGC 346. It has three components, all amongst the most luminous stars known: an unusual primary with a Wolf–Rayet spectrum that has produced a luminous blue variable (LBV) eruption; a secondary, also a Wolf–Rayet star, forming an eclipsing spectroscopic binary; and an O-type supergiant that may not be physically associated.
The three components of the system have similar bolometric luminosities, although all the physical parameters of the three stars are uncertain because of the difficulties of resolving their spectra, the partial eclipses, apparent intrinsic variations with the orbital phase, and the strong variability of at least one component. The calibration of spectral features to physical characters such as temperature has historically been complicated by the low metallicity of objects in the SMC.
The primary star, generally referred to as A, is visually the brightest component of the three. It was apparently a hydrogen-poor [Wolf–Rayet star#Classification|WN3]]-type until about 1990, but then underwent an LBV-type eruption that saw its radius increase ten-fold and its temperature drop dramatically so that now it could be classified as WN11, and obviously showing strong hydrogen lines.
The secondary, known as B, is also a Wolf–Rayet star. It is of type WN4, but shows hydrogen in its spectrum. It is a spectroscopic double in orbit around A with a period of 19 days. The orbital parameters indicate that B is a little less massive. The orbit is inclined at 88° to us, and partial eclipses occur twice per orbit, with timings that indicate an eccentricity of 0.30.
Component C is a distant hot conventional star, most likely an early O-type supergiant. The luminosity and temperature are comparable to the other two components but this is a less evolved hydrogen-rich star. It is not known if C is physically associated with the other two stars, and it has been suggested that it may itself be a close binary.
In 1991, HD 5980 was observed to have changed spectral type and decreased in temperature after a slow increase in brightness. In 1993 it had further decreased in temperature and then in 1994 it dramatically increased in visual brightness while the temperature decreased further. This is referred to as an eruption, although in reality it is an increase in the radius of the star which eventually becomes unstable and leads to huge mass loss. This behavior is the classic indicator of a luminous blue variable star. Close examination showed that the eruption occurred in the primary component.
At one stage in 1994, the spectrum was that of a blue hypergiant: B1.5Ia+. The bolometric luminosity stayed approximately constant during the eruption, as is typical for LBVs, but the visual brightness increased by over two magnitude during the brief peak. One study suggests an increase of 3-6 times in luminosity to 10000000 times the sun at its peak but this may simply be due to different analysis techniques, and others find a fairly consistent luminosity of a few million times the sun. The eruption peaked in 1994 and by 2009 the star had returned to near "minimum" with a spectrum of WN4/5. This would make it the hottest LBV and as might be expected one of the most luminous. An earlier eruption may have occurred around 1960.
Although HD 5980 is treated as an LBV, it does not follow the normal pattern which would be an effective temperature during eruption of around 8500 K and an A type spectrum. It is speculated that the close companion causes this particular star to exhibit the LBV-type instability at much higher temperatures. Romano's star and Var 83 may be similar, and the little-studied Var 2 is even hotter, all of them in M33.
Although the LBV star is several million times as luminous as the sun and more luminous than any LBV except Eta Carinae, the eruption was not one of the giant eruptions also referred to supernova imposters. Normal LBV eruptions show little change in bolometric luminosity while the rare great eruptions are a further 5—15 times more luminous than HD 5980.
- Pasemann, Diana; Rühling, Ute; Hamann, Wolf-Rainer; Rühling; Hamann (2011). "Spectral analyses of the Wolf-Rayet stars in the Small Magellanic Cloud". Société Royale des Sciences de Liège, Bulletin 80: 180–184. Bibcode:2011BSRSL..80..180P.
- Foellmi; Koenigsberger; Georgiev; Toledano; Marchenko; Massey; Dall; Moffat et al. (2007). "New insights into the nature of the SMC WR/LBV binary HD 5980". arXiv:0711.4858v1 [astro-ph].
- Koenigsberger, G.; Georgiev, L.; Hillier, D. J.; Morrell, N.; Barbá, R.; Gamen, R. (2010). "A ∼ 40 Year Variability Cycle in the Luminous Blue Variable/wolf-Rayet Binary System Hd 5980?". The Astronomical Journal 139 (6): 2600. Bibcode:2010AJ....139.2600K. doi:10.1088/0004-6256/139/6/2600.
- Georgiev, L.; Koenigsberger, G.; Hillier, D. J.; Morrell, N.; Barbá, R.; Gamen, R. (2011). "Wind Structure and Luminosity Variations in the Wolf-Rayet/luminous Blue Variable Hd 5980". The Astronomical Journal 142 (6): 191. Bibcode:2011AJ....142..191G. doi:10.1088/0004-6256/142/6/191.
- Bibcode: 2002ASPC..260..507K
- Drissen, L.; Crowther, P. A.; Smith, L. J.; Robert, C.; Roy, J. R.; Hillier, D. J. (2001). "Physical Parameters of Erupting Luminous Blue Variables: NGC 2363‐V1 Caught in the Act". The Astrophysical Journal 546: 484. arXiv:astro-ph/0008221. Bibcode:2001ApJ...546..484D. doi:10.1086/318264.
- Koenigsberger, G.; Auer, L. H.; Georgiev, L.; Guinan, E. (1998). "Wind Velocity Variations in the Luminous Blue Variable–Type Erupting Star of the Wolf‐Rayet Binary HD 5980". The Astrophysical Journal 496 (2): 934. Bibcode:1998ApJ...496..934K. doi:10.1086/305398.