Eta Carinae is an extreme star, even by the standards of LBVs. If single, h Carinae is one of the most massive stars known, having a mass of 120 M¤ (see Damineli 1996 for references). It is now considered more likely to be a binary, however, comprising a ~70 M¤ primary, and a much smaller secondary of unknown mass and type (Damineli et al. 1997, Damineli et al. 2000).
It is one of the most luminous stellar objects of our Galaxy, having a luminosity of 5 × 106 L¤.
Eta Carinae is a very strongly mass losing star, even by LBV or Wolf-Rayet standards. It “probably lost 2 – 3 M¤ during its famous 1840’s outburst and its current mass loss rate is estimated at 10-4 to 10-3 M¤ /yr” (Humphreys 1989, p. 5).
The spectrum is obscured by surrounding shells of ejected material. It was recorded as an F type supergiant in the 1890s; now it displays neither absorption nor emission lines in the optical part of the spectrum but it is almost certainly type O or possibly B. The ejected material absorbs much of the energetic short wavelength radiation from the star, and re-radiates it as red and infrared, thereby disguising the ‘true’ spectrum.
The spectrum of h Carinae indicates nitrogen enrichment, suggesting the presence of core material in the photosphere.
LBVs in general exhibit some periodicity, and the mean periods may be inversely proportional to the stars’ luminosities. Damineli 1996 describes spectroscopic and near-infrared studies of h Carinae itself, deriving strong evidence for a stable 5.52 year cycle in h Carinae which is roughly consistent with Stothers and Chin’s (1995) ‘3 – 5 years’ period.
The last shell event of 1992 (see Damineli 1996 for references) was followed by an enhancement of flux in the radio wavelength range and by the reappearance of the stellar source in hard X-rays.
WRONG: A binary scenario was sometimes invoked to explain some features of the optical light curve, but this hypothesis is not supported by the observations.
In addition to many smaller events h Carinae has undergone giant bursts in the last centuries. A Sumerian recording of a ‘new star’ in 3000 B.C. is possibly attributable to Eta Carinae (Naeye 1997). In 1837, Eta Carinae flared up, peaking second only to Sirius at magnitude -0.8, in 1843. It remained at first magnitude for around 20 years, but has since settled back around 6 to 8.
“The putative binary system believed to constitute h Carinae survived an outburst in the previous century that lasted 20 years; and which created a nebula with pronounced bipolar lobes that together contain about 2.5 solar masses of material. The nebula also exhibits an equatorial ‘waist’ containing 0.5 solar masses. the physical mechanisms responsible for the outburst and bipolar geometry are not understood. Here we report infrared observations (spectroscopy and imaging) that reveal the presence of about 15 solar masses of material, located in an equatorial torus. the massive torus may have been created through highly non-conservative mass transfer, which removed the entire envelope of one of the stars, leaving an unstable core that erupted in the nineteenth century. The collision of the erupted material with the pre-existing torus provides a natural explanation for the bipolar shape of the nebula” (Morris et al. 1999, p. 502).
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The star is now surrounded by a shell of gas ejected in the 1837 outburst, known as the Homunculus Nebula, spectacularly revealed in the now-famous HST photograph of June 1996. The Homunculus is mainly a reflecting nebula comprising some 2 to 3 M¤, which is apparently produced by a bipolar outflow from Eta Carinae, and expanding at around 650 km/sec (Naeye 1997, Frank 1997, see Damineli 1996 for further references).
Damineli, A.; Conti, P.S.; Lopes, D.F. 1997: Eta Carinae: a long period binary?. New Astronomy 2: 107.
Damineli, A.; Kaufer, A.; Wolf, B.; Stahl, O.; Lopes, D.F.; de Ara&uacute;jo, F.X. 2000: Eta Carinae: binarity confirmed. Astrophysical Journal 528: L101-L104.
Frank, A. 1997: Where's the disk?: LBV bubbles and aspherical fast winds. In Nota, A.; Lamers, H.J.G.L.M. (ed.) 1997: Luminous Blue Variables: Massive Stars in Transition. Astronomical Society of the Pacific Conference Series 120 120: 338-344.
Humphreys, Roberta M. 1989: What are LBVs? - Their Characteristics and Role in the Upper H-R Diagram. In K. Davidson et al. (ed.) 1989: Physics of Luminous Blue Variables, pp. 3-12.: 3-12.
Kaler, J.B. 2001: Extreme stars: at the edge of creation. Cambridge: 1-236.
Morris, P.W.; Waters, L.B.F.M.; Barlow, M.J.; Limk, T.; de Koter, A.; Voors, R.H.M.; Cox, P.; de GraauwI, T.; Henning, T.; Hony, S.; Lamers, H.J.G.L.M.; Mutschke, H.; Trams, N.R. 1999: Discovery of a massive equatorial torus in the η Carinae stellar system. Nature 402: 502-504.
Naeye 1997: . Sky & Telescope.
Puls, J.; Vink, J.S.; Najarro, F. 2008: Mass loss from hot massive stars. Astronomy and Astrophysics Review 16: 209-325.
Stothers, R.B.; Chin, C. 1995: A period-luminosity relation for the slow variation of luminous blue variables. Astrophysical Journal 451: L61-L64.