Eta (η) Carinae

	h Carinae		Bayer designation (1603)
	HR 4210		Harvard Revised Photometry designation (1879+)
	HD 93308		Henry Draper Catalogue (1918+)
	SAO 238429		Smithsonian Astrophysical Observatory (1966+)
	n/a		Hipparcos catalogue (1997)

Synopsis

Multiple star, probably double; luminous blue variable (LBV) primary and early Of-type supergiant secondary; located in Carina; R.A. 10:45:03.591, dec. -59:41:04.26; culmination 15 April; apparent visual magnitude ~8; distance 2300 pc (approx 7500 light years).

Introduction

The enigmatic h Carinae is one of the most massive and most luminous binary systems observable from Earth. The primary is believed to represent an important though short-lived, unstable phase in the life of the most massive stars: the Luminous Blue Variable (LBV) phase.

Luminous Blue Variables represent a short-lived (~10⁴ -10⁵ years) phase of massive star evolution in which the stars are subject to significant effective temperature changes. They come in two kinds: S Doradus variables and giant eruptors. The largest population of ~30 LBVs in the Galaxy and the Magellanic Clouds is that of the S Doradus variables with magnitude changes of 1-2 magnitudes on timescales of years to decades. The general understanding is that the S Doradus cycles occur at approximately constant bolometric luminosity (which has yet to be proven) – principally representing temperature variations. The second type of LBV instability involves objects that show truly giant eruptions with magnitude changes of order 3-5 during which the bolometric luminosity most certainly increases. In the Milky Way it is only the cases of P Cygni and Eta Carinae which have been witnessed to experience such extreme behaviour. Whether these two types of variability occur in similar or distinct objects is not yet clear, however in view of the “unifying” properties of the LBV P Cygni it is highly probable that the S Doradus variables and giant eruptors are related, that they are in a similar evolutionary state, and that they are subject to the same type of instabilities near the Eddington limit. (Abridged from Puls et al. 2008.)

h Carinae is a long period eccentric binary. It became famous after the “great eruption” of the 1840s, in which the bipolar Homunculus Nebula was created. “Yet determination of its orbital and physical parameters is hampered by obscuring winds. … the effects of the strong, colliding winds change with phase due to the high orbital eccentricity” (Damineli et al. 2016, p. 186).

“Eta Carinae is one of the most luminous evolved stars in the local Universe (Davidson & Humphreys 1997). It became famous after the great eruption in the 1840’s, which created the beautiful bipolar Homunculus flow and there were additional smaller mass ejections. The discovery that it is long period eccentric binary (Damineli 1996, Damineli et al. 1997) provided the potential to determine the mass of the companions and to explain a number of complex, variable features. One very interesting is the connection between the sharp photometric peaks during the great eruption and timing of periastron passages, indicating that at periastron, there is strong interaction between the companion stars (Damineli 1996, Prieto et al. 2014). Moreover, the shape of the homunculus displays features formed when the ejecta was a thousand times smaller than at the time of the eruption (Steffen et al. 2014).

…

“A recent discovery (Steiner & Damineli 2004) of a “burst” in the He ii l4686 line, because of its sharp and phase-locked behaviour (Teodoro et al. 2012), turned out to be a key phenomenon to study the binary system. As shown by Madura & Owocki (2010) and Madura et al. (2013), the ‘bore hole’ effect can account for the escaping of UV radiation from the inner interacting binary to the external regions, through the cavity (filled by the sparser secondary wind) produced by the WWC.” (Damineli et al. 2016, p. 187).

Eta Carinae is surrounded by the Homunculus Nebula. “According to basic concepts of stellar evolution, the strong N enhancement and the strong C and O depletion in the nebula show that it was ejected from a star whose surface layers consist of CNO-cycle products. … After the ejection of the nebula, the remnant star should show either the same surface composition as the nebula (if only a part of the layer with CNO-equilibrium products was ejected) or the subsequent more advanced composition, i.e., an He-rich, N-rich, and C-depleted layer (as in Wolf-Rayet stars of type WN) or an He-rich, N-depleted, and C-rich layer (as in Wolf-Rayet stars of type WC). The latter composition occurs when the products of triple-a burning reach the surface. … We conclude that the spectrum of the apparent nucleus of h Car is not in agreement with that expected of a star that has ejected a nebula with CNO-equilibrium composition. This conclusion implies that the star that produces the spectrum of the apparent nucleus is not the one from which the nebula was ejected. So there are at least two stars in the system” (Lamers et al. 1998, p. L132, L133).

Description

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).

In practice, however, “variations of the system do not easily lead to the physical parameters: both companions have strong winds, with especially the primary’s known to be strongly variable, leading to strong and variable wind-wind collision (WWC), which give rise to many complex and variable phenomena like free-free and free-bound emission, radio emission, high and low energy emission lines” (Damineli et al. 2016, p. 187).

Spectrum

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.

Periodicity

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 period length defined by the He ii l4686 line during the last 3 cycles is 2022.9±1.6 d. When combined with other features across the electromagnetic spectrum (Damineli et al. 2008) it is better constrained to P= 2022.7 ± 0.3 d” (Damineli et al. 2016, p. 190).

Eruptions

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.

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.

Summary

Interpretation

“The star was previously suspected to be a binary on the basis of its light variations with a period of about 60 days (van Genderen et al. 1995); however, since this period is not stable and since it is similar to the pulsation period of other LBVs (Lamers et al. 1998), this is probably not a binary period. A much more convincing period of 5.5250.01 yr has been found by Damineli (1996, 1997) on the basis of large variations in the strength of the He i 10830 Å emission line and in the H band magnitude. The evidence has been traced back as far as 1948. The prediction and the subsequent verification of the decrease in line strength at the end of 1997 December (Jablonski, Lopes, & Damineli 1998) is a beautiful confirmation of this periodicity” Lamers et al. 1998, p. L131).

“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).

Observing

Locating

locating info needs to be inserted here

Appearance

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).

References

Damineli, A. 1997: called from Astronomy, objEtaCar. 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 .

Damineli, A.; Conti, P.S.; Lopes, D.F. 1997: Eta Carinae: a long period binary? New Astronomy 2: 107.

Damineli, A.; Hillier, D. J.; Corcoran, M. F.; Stahl, O.; Levenhagen, R. S.; Leister, N. V.; Groh, J. H.; Teodoro, M.; Albacete Colombo, J. F.; Gonzalez, F.; Arias, J.; Levato, H.; Grosso, M.; Morrell, N.; Gamen, R.; Wallerstein, G.; Niemela, V. 2008: The periodicity of the η Carinae events. Monthly Notices of the Royal Astronomical Society 384 (4): 1649-1656.

Damineli, A.; Kaufer, A.; Wolf, B.; Stahl, O.; Lopes, D.F.; de Araújo, F.X. 2000: Eta Carinae: binarity confirmed. Astrophysical Journal 528: L101-L104.

Damineli, A.; Teodoro, M.; Richardson, N.D.; Gull, T.R.; Corcoran, M.F.; Hamaguchi, K.; Groh, J.H.; Weigelt, G.; Hillier, D.J.; Russell, C.; Moffat, A.; Pollard, K.R.; Madura, T.I. 2016: The wind-wind collision hole in eta Car. In Eldridge, J.J.; Bray, J.C.; McClelland, L.A.S.; Xiao, L. (ed.) 2016: The Lives and Death-Throes of Massive Stars. Proceedings IAU Symposium 329: 186-190.

Damineli. A. 1996: . Astrophysical Journal 460: L49.

Davidson, K.; Humphreys, R.M. 1997: Eta Carinae and its environment. Annual Review of Astronomy and Astrophysics 35: 1-32.

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, R.M. 1989: What are LBVs? - Their characteristics and role in the upper H-R diagram. In Davidson, K.; Moffat, A.F.J.; Lamers, H.J.G.L.M. (ed.) 1989: Physics of Luminous Blue Variables. Kluwer Academic Publishers: 3-12.

Jablonski, F.J.; Lopes, D.F.; Damineli, A. 1998: . IAU Circ. 6874.

Kaler, J.B. 2001: Extreme stars: at the edge of creation. Cambridge University Press: 1-236.

Lamers, H.J.G.L.M.; Livio, M.; Panagia, N.; Walborn, N.R. 1998: On the multiplicity of η Carinae. The Astrophysical Journal 505: L131-L133.

Madura, T.I.; Gull, T.R.; Okazaki, A.T.; Russell, C.M.P.; Owocki, S.P.; Groh, J.H.; Corcoran, M.F.; Hamaguchi, K.; Teodoro, M. 2013: Constraints on decreases in η Carinae’s mass-loss from 3D hydrodynamic simulations of its binary colliding winds. MNRAS 436: 3820-3855.

Madura, T.I.; Owocki, S.P. 2010: Signatures of the 3-D wind-wind collision cavity in <$ctag obj Symbol:Eta> Car. Revista mexicana de astronomía y astrofísica (Serie de Conferencias) 38: 52-53.

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.

Prieto, J.; Rest, A.; Bianco, F.; Matheson, T.; Smith, N.; Walborn, N.; Hsiao, E.; Chornock, R.; Álvarez, L.P.; Campillay, A.; Contreras, C.; González, C.; James, D.; Knapp, G.; Kunder, A.; Margheim, S.; Morrell, N.; Phillips, M.; Smith, R.; Welch, D.; Zenteno, A. 2014: Light echoes from η Carinae’s great eruption: Spectroscopic evolution and the rapid formation of nitrogen-rich molecules. Astrophysical Journal Letters, 787:L8.

Puls, J.; Vink, J.S.; Najarro, F. 2008: Mass loss from hot massive stars. Astronomy and Astrophysics Review 16: 209-325.

Steffen, W.; Teodoro, M.; Madura, T.I.; Groh, J.H.; Gull, T.R.; Mehner, A.; Corcoran, M.F.; Damineli, A.; Hamaguchi, K. 2014: The three-dimensional structure of the Eta Carinae Homunculus. MNRAS 422: 3316-3328.

Steiner, J.E.; Damineli, A. 2004: Detection of He II λ4686 in η Carinae. Astrophysical Journal Letters 612: L133-L136.

Stothers, R.B.; Chin, C. 1995: A period-luminosity relation for the slow variation of luminous blue variables. Astrophysical Journal 451: L61-L64.

Teodoro, M.; Damineli, A.; Arias, J.; de Araújo, F.; Barbá, R.H.; Corcoran, M.; Fernandes, M.; Fernández-Lajús, E.; Fraga, L.; Gamen, R.; González, J.; Groh, J.; Marshall, J.; McGregor, P.; Morrell, N.; Nicholls, D.; Parkin, E.; Pereira, C.; Phillips, M.; Solivella, G.; Steiner, J.; Stritzinger, M.; Thompson, I.; Torres, C.; Torres, M.; Herencia, M.Z. 2012: He II λ4686 in η Carinae: Collapse of the wind-wind collision region during periastron passage. Astrophysical Journal 746: 73.

van Genderen, A.M.; Sterken, C.; de Groot, M.; Stahl, O.; Andersen, J.; Andersen, M.I.; Caldwell, J.A.R.; Casey, B.; Clement, R.; Corradi, W.J.B.; Cuypers, J.; Debehogne, H.; de Maria, J.M.G.; Jønch-Sørensen, H.; Vaz, L.P.R.; Stefl, S.; Lopez, J.S.; Beele, D.; Eggenkamp, I.M.M.G.; Göcking, K.D.; Jorissen, A.; de Koff, S.; Kuss, C.; Schoenmakers, A.P.; Vink, J.; Wälde, E. 1995: A pulsating star inside η Carinae. I. Light variations, 1992-1994. Astronomy and Astrophysics 304: 415-430.