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(var) + 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||61(A), 66(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 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: the unusual primary has a Wolf–Rayet spectrum and has produced a luminous blue variable (LBV) outburst; the secondary, also a Wolf–Rayet star, forms an eclipsing spectroscopic binary with the primary star; and an O-type supergiant 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 WN3-type until about 1990, but then underwent an LBV outburst 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 19.3 days. The orbital parameters indicate that the two stars are approximately equally massive, within the margin of error. The orbit is inclined at 86° to us, and partial eclipses occur twice per orbit, with timings that indicate an eccentricity of 0.27.
Component C is a distant single-line spectroscopic binary system. The primary is a 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. A period of 96.5 days has been derived from the radial velocity variations. This is five times the period of the A/B system, suggesting the four stars form a gravitational trapezium system.
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 outburst, 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 outburst 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 outburst, 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 outburst 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 outburst 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 outburst of around 8,500K 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 outburst was not one of the giant eruptions also referred to supernova imposters. Normal LBV outbursts show little change in bolometric luminosity while the rare great eruptions are a further 5—15 times more luminous than HD 5980.
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