P1: ARK/MBL/plb P2: MBL/plb QC: MBL/agr T1: MBL January 9, 1998 15:7 Annual Reviews AR037-01 Annu. Rev. Astron. Astrophys. 1997. 35:1–32 Copyright c 1997 by Annual Reviews Inc. All rights reserved ETA CARINAE AND ITS ENVIRONMENT Kris Davidson and Roberta M. Humphreys Astronomy Department, University of Minnesota, Minneapolis, Minnesota 55455; e-mail: [email protected] KEY WORDS: massive stars, variable stars, LBVs or luminous blue variables ABSTRACT Eta Carinae (Eta) is one of the most remarkable of all well-studied stars and per- haps the most poorly understood. Observations with the Hubble Space Telescope and other modern instruments have solved a few of the mysteries concerning this object while opening a comparable number of new ones. In this review we first recount some essential background information concerning Eta, then we sketch most of the observational developments of the past few years, related to the star itself and to its ejecta. Throughout, we propose a series of specific unsolved ob- servational and theoretical problems that seem especially interesting or important at this time. 1. INTRODUCTION A strange field of speculation is opened by this phenomenon...here we have a star fitfully variable to an astonishing extent, and whose fluctuations are spread by Deutsche Forschungsgemeinschaft on 07/02/07. For personal use only. over centuries, apparently with no settled period and no regularity of progression. What origin can we ascribe to these sudden flashes and relapses? What conclu- sions are we to draw as to the comfort and habitability of a system depending for Annu. Rev. Astro. Astrophys. 1997.35:1-32. Downloaded from arjournals.annualreviews.org its supply of light and heat on so uncertain a source? —JFW Herschel (1847) Eta Carinae (η Car) is arguably the most remarkable stellar object that is close enough to be observed in great detail. It is so special that enthusiasts often refer simply and familiarly to “Eta”—a usage we would deplore for any other star with a Bayer designation. Eta’s fame and uniqueness rest on a combination of three different but related lists of attributes, which we merely sketch here at the outset of this review, postponing references until later. 1 0066-4146/97/0915-0001$08.00 P1: ARK/MBL/plb P2: MBL/plb QC: MBL/agr T1: MBL January 9, 1998 15:7 Annual Reviews AR037-01 2 DAVIDSON & HUMPHREYS First, a list of superlatives. The most luminous evolved star that can be studied closely, Eta is also the survivor of the greatest well-documented non- terminal stellar explosion. Its mass probably exceeds 100 , which would make it rare even if it were a more stable star near the mainM sequence. For two decades of the nineteenth century, Eta was visually one of the dozen brightest stars in the sky, even at a distance of more than 2 kpc; the total luminous energy radiated during the whole outburst was comparable to that of a supernova event. In recent years, its luminosity has been several million times that of the Sun. Today it is the brightest extra–solar-system infrared source in the sky at λ 20 µm, and it has the brightest known stellar wind at millimeter or centimeter∼ wavelengths. Eta has produced one of the most elegant of all known bipolar nebulae. Second, as John Herschel (1847) wrote with only mild exaggeration, “There is perhaps no other sidereal object which unites more points of interest than this.” Stellar physics?—The interplay of radiation pressure and opacity (maybe rotation too) causes Eta to be irregularly, chaotically unstable, with occasional titanic eruptions. Stellar evolution?—This star appears to be an extreme ex- ample showing how instability limits evolution at the top of the Hertzsprung- Russell (H-R) diagram. Star formation?—Eta was formed in a rich association that includes several other very massive (and therefore very unusual) stars. Gas dynamics?—Bipolar flows created by the aforesaid instability are beautifully symmetric, but at the same time they have conspicuous localized asymmetric features that no one predicted. Microphysics and radiative transfer?—Certain bright emission lines in the ejecta are excited by fluorescent processes that are uniquely effective around Eta. Dust formation? Chemical enrichment of the surrounding medium?—Eta is relevant to these and to other topics as well, in nonroutine largely unexplored ways. Third, this object is very poorly understood. We do not know why or how Eta’s giant eruptions occurred, and its energy budget has never been properly audited. Although we do know that Eta is evolved, we are not sure whether it by Deutsche Forschungsgemeinschaft on 07/02/07. For personal use only. is typical of the most massive stars; it might be a rapid rotator or a close binary system. Some authors suggest that the star exhibits surprising periodicities. Some of the most obvious characteristics of its bipolar ejecta are difficult to Annu. Rev. Astro. Astrophys. 1997.35:1-32. Downloaded from arjournals.annualreviews.org explain, as are the brightnesses and ratios of some of its most conspicuous emis- sion lines. As new instruments have settled particular observational questions about Eta, they have opened new questions to replace the old ones—a classic tendency shared with other unusually interesting astrophysical topics. We recommend Eta as a subject for all sorts of theoretical and observational research projects. Therefore, throughout the review we mark some of the most crucial or interesting questions with Q labels, e.g. Herschel’s classic mystery that still deserves top billing: { } P1: ARK/MBL/plb P2: MBL/plb QC: MBL/agr T1: MBL January 9, 1998 15:7 Annual Reviews AR037-01 ETA CARINAE 3 Q1 What origin can we ascribe to these sudden flashes and relapses? { } Eta and its vicinity were the topics of a meeting held in 1993, whose pro- ceedings contain many useful papers (see Niemela et al 1995). For a review of the class of star to which Eta most likely belongs, see Humphreys & Davidson (1994). 2. AN UPPER-CLASS NEIGHBORHOOD From our vantage point, the constellation Carina marks the most spectacular direction in the Milky Way. There our line of sight passes along the Carina spiral arm, which can be traced to distances of more than 10 kpc at visual wavelengths (Humphreys 1976) and as far as 23 kpc with CO observations (Grabelsky et al 1988). This major Galactic feature is defined by large numbers of massive stars and prominent star-formation regions, such as the Carina Nebula (NGC 3372) and its surrounding association Car OB1 (` 284.2 to 288.3 , b 2.2 to ≈ ◦ ◦ ≈− ◦ 0.9◦, D 2.5 kpc). With a more impressive concentration of very luminous +stars than any≈ other place within a few kiloparsecs of the Sun, the Carina Nebula is one of the best available sites for studying the formation and evolution of the most massive stars. Its two central clusters, Trumpler 14 and Trumpler 16 (Tr 14 and Tr 16), are particularly unusual. When we refer here to Tr 16, we also include a set of stars called Collinder 228 (Cr 228). Together these are now considered to be a single cluster, with an illusory separation between Tr 16 and Cr 228 caused by a superimposed dust lane (see Walborn 1995). A census of hot stars in Tr 14 and Tr 16, based on the work of Walborn (1982a,b), Morrell et al (1988), Levato & Malaroda (1981, 1982), and Massey & Johnson (1993), includes 36 stars earlier than spectral type B0 and 3 WN-A stars. These provide enough ultraviolet (UV) photons to ionize the large nebula and to heat the dust seen in infrared observations (Harvey et al 1979; Ghosh et al 1988). Far more remarkably, Tr 14 and Tr 16 contain 6 of the 17 known O3-type stars in our by Deutsche Forschungsgemeinschaft on 07/02/07. For personal use only. Galaxy; because O3 stars are at the hot end of the normal spectral sequence, they are very massive and quite rare. Knowing this, perhaps one is not very surprised to learn that Eta is a member of Tr 16. Annu. Rev. Astro. Astrophys. 1997.35:1-32. Downloaded from arjournals.annualreviews.org The larger association Car OB1 and the outer clusters IC 2581, NGC 3293, and NGC 3324 contain numerous OB stars and evolved supergiants, including many M supergiants, whose presence implies an age of about 10 million years for the oldest massive stars in this stellar complex. Published distance moduli for the outer clusters range from 11.9–12.5 mag, and the distance to the as- sociation is usually quoted as 2.5 kpc (see Humphreys 1978). The clusters Bochum 10 and 11 are apparently∼ close to the Carina Nebula and contain sev- eral OB stars; Fitzgerald & Mehta (1987) derived ages of 7 and 3 million years P1: ARK/MBL/plb P2: MBL/plb QC: MBL/agr T1: MBL January 9, 1998 15:7 Annual Reviews AR037-01 4 DAVIDSON & HUMPHREYS and distance moduli of 12.2 and 12.7 mag for Bo 10 and Bo 11, respectively. Tr 15 is usually considered to be behind the H II region (Walborn 1995). Distances to Tr 14 and Tr 16 have been made more uncertain by anomalous extinction within or near the Carina Nebula. Feinstein et al (1973), Herbst (1976), and Forte (1978) reported abnormally high ratios of extinction to red- dening there, i.e. the wavelength dependence of the obscuration is relatively weaker than it is in the normal interstellar medium. In a multiwavelength study, Th´e et al (1980) found that the wavelength dependence of the extinction varies from star to star in the vicinity of the Carina Nebula.