Astronomical Science h Carinae 2009.0: One of the Most Remarkable Stars in the Sky Christiaan Sterken1 Arnout van Genderen 2 Gerd Weigelt 3 Andreas Kaufer 4 1 Vrije Universiteit Brussel, Brussels, Belgium 2 Leiden University Observatory, Leiden, the Netherlands 3 Max-Planck-Institut für Radioastronomie, Bonn, Germany 4 ESO h Carinae is one of the most luminous and massive stars in the Galaxy. The star underwent a major eruption in 1838, followed by a second maximum a few decades later and a low-gradient brightening to the present. The central source of h Car is a highly-eccentric binary with a period of 5.54 years. The photometric and interferometric moni- toring programmes with ESO tele- scopes are summarised. On the occa- sion of the forthcoming periastron passage in 2009.0, the star will be the target of intensive photometric, spec- troscopic and interferometric monitor- ing from Chile and other southern ob- servatories. h Car: an extreme LBV S Doradus variables – commonly known Figure 1: Images of h Carinae: HST image from gradient brightening during more than a as Luminous Blue Variables (LBVs) – are Morse et al. (1998) (top left); VLT/UT1 First Light century. Photo No. 5, (top right); artist’s impression of the bi- evolved massive stars that undergo four nary configuration, courtesy S. Ivarsson (bottom). major types of intrinsic photometric varia- It became obvious that the central star of bility: microvariations, S Doradus phases, P Cygni and h Carinae (Figure 1) are h Car is a true S Dor variable: the star stochastic variability, and eruptions. the historically best-documented cases migrates in the Hertzsprung-Russell dia- For a detailed discussion of these types of eruptive behaviour of S Dor stars. gram (at constant luminosity) so that of va riabilities, and for a very complete P Cygni was discovered by the Dutch brightness increases are accompanied by review of the state of affairs at the end of chartmaker Willem Adriaensz Blaeu in redder colours, and vice versa (see Fig- the second millennium, we refer to van 1600. The star faded, becoming invisible ure 3 for a partial light curve and colour Genderen (2001). These stars were la- to the naked eye until 1658, when it curve between 1992 and 2000). Such belled S Dor variables in the General Cat- flared again to a secondary maximum, a migration cycle is called an S Doradus alogue of Variable Stars on the basis of followed by another return to oblivion and phase. their behavioural similarities with the pro- a subsequent very weak increase dur- totype star S Doradus. The Armagh 2000 ing a centuries-long brightening phase. h Carinae is one of the most luminous definition (see ASP Conference Series h Carinae was observed in a similar erup- and massive stars in the Galaxy, with 6 233, page 288) states: “S Doradus varia- tion by John Herschel at the Cape in a luminosity of L ~ 5 × 10 LA and a bles are hot, luminous stars that show 1838, and the historical light curve (Fig- mass of M ~ 100 MA, a mass loss rate of · –3.3 photometric and/or spectroscopic varia- ure 2) illustrates a pattern very similar M ~ 10 MA for a distance of d = tions like S Doradus and which have un- to the light history of P Cygni: a major 2.3 kpc. The system consists of a bright dergone – or possibly will undergo – an eruption (also called the Great Eruption), hollow bipolar nebula, called the Homun- h Carinae or P Cygni-type outburst.” followed by a second maximum a few culus, and an apparently much fainter decades later, and then a steady low- (currently 2.4 mag) central star. In reality, 32 The Messenger 130 – December 2007 –2.0 Figure 2: Visual light curve of h Cari- scope of the Leiden Southern Station nae. The Great Eruption extended –1.0 (South Africa) was moved to Chile, where from 1837 to about 1856. Source: Frew 0.0 (2004). it became known as the Dutch Tele- 1.0 scope. The telescope, equipped with the 2.0 Walraven VBLUW system, was decom- 3.0 missioned in 1999. Between 1983 and v m 2000 the observations were then contin- 4.0 ued with the ESO 50-cm and Danish 5.0 SAT telescopes in other photometric sys- 6.0 tems, mainly as part of the Long-Term 7.0 Photometry of Variables (LTPV) Group 8.0 (see The Messenger 33, 10), between 9.0 1820 1840 1860 1880 1900 1920 1940 1960 1980 2000 1994 and 1998 with the 70-cm Swiss Tel- Year escope, and later on with the Danish 1.54-m. The combination of the data, 1993 1994 1995 1996 1997 1998 1999 2000 Figure 3: Vy (•, based on Geneva V obtained over such a long time base of and Strömgren y) light curve (left Y- 5.4 5.2 an extended emission-line object meas- axis) and b–y colour index of h Car, ured with different detectors and photo- 5.5 5.3 with an approximate indication of vis- ual magnitude on the Johnson V-scale metric systems, is a real challenge. 5.6 5.4 y V (right Y-axis). The dashed line in the V 5.7 light curve is based on broadband vis- 5.5 Between 1992 and 1994, when h Car was ual estimates and CCD photometry 5.8 5.6 and fills in the seasonal gap in 1998; observed very intensively in the optical, the dashed line in the colour curve microvariations typical for S Dor variables 5.9 5.7 is based on broadband colour data. were identified with a quasi-period of 0.5 ° re present measurements through 58.6 days. Note that oscillation periods of b–y a V-filter. The narrow peak at HJD- 0.6 24410800 coincides with the time of this order of magnitude are often present 9000 9500 10 000 10 500 11 000 11 500 the 1997.9 event. Source: Sterken et during the low-brightness stages of S Dor HJD-2440000 al. (2000). variables. Peculiar UV colour-variations indicate that part of the light variations this central star is much brighter, but ap- nary configuration (for the 2003.5 perias- must be due to non-stellar causes. For pears fainter due to nebular extinc tion. tron passage, based on orientation B of example, the flux of the relatively strong The Homunculus is now 16 arcsec across, Akashi et al. 2006). Balmer continuum glow (coming from the which corresponds to 0.2 pc. During the equatorial disc) shows a modulation of S Dor-eruption phase in the 1840s, one 0.1 mag in concert with the 5.5-year bi- peak reached an exceptional bright- Photometric monitoring at ESO nary revolution, but is asymmetric with ness of Mbol ~ –14 (van Genderen and respect to the periastron passages. How- Thé 1984). Photometric monitoring of the integrated ever, it is not unlikely that short-cycle brightness of h Car at ESO La Silla was oscillations in the Balmer-continuum radi- started in 1979 when the 90-cm tele- ation (200–400 days) are, in fact, caused h Car, a highly-eccentric binary 2885 end X-ray min A variety of observations have suggested 2870 that the central source of h Car is a high- optical dip ly eccentric binary. Augusto Damineli first 2840 min dip noticed the 5.5-year periodicity in the optical + NIR NIR dip 2826 2823 spectroscopic changes of this object (see 2820 X-min Damineli et al. 2000 and Whitelock et al. 2818 2004). The periastron separation is only start NIR ~ 2 AU, whereas at apastron the separa- declines 2810 Figure 4: A schematic drawing of the Major axis tion is ~ 20 AU, corresponding to ~ 9 mil- binary system (for the 2003.5 peri- liarcseconds. Smith et al. (2004), on the 2808 = PP astron passage and e = 0.9, seen basis of Hubble Telescope images, re- pole-on and not to scale, viewing moved most doubts on the existence of a angle ~ 45˚) based on orientation B described by Akashi et al. (2006). hot companion. Iping et al. (2005) state Central circle and small dot: the pri- 2790 start that the companion star was seen in X-ray decline mary and secondary, respectively. the far-UV spectrum shortly before the Large circle (dashed): the wind enve- 2003.5 event. The orbital parameters lope of the primary. Hatched area: the shocked secondary wind. A however are still a matter of debate. Fig- 2700 start peak number of features are indicated by optical + NIR ure 4 is a schematic drawing of the bi - time marks (JD-2450000). The Messenger 130 – December 2007 33 Astronomical Science Sterken C. et al., h Carinae 2009.0: One of the Most Remarkable Stars in the Sky by stellar oscillations. Some starlight va- 1998 1999 2000 2001 2002 2003 Figure 5: Schematic light curves be- riability seems to be induced by tidal tween 1997 and 2004. The V scale 5.0 is close to the UBV system. The arrows forces of the secondary acting on the pri- at the top indicate the computed mary and its extended envelope, like the 5.2 events (or periastron passages). The long timescale optical and near-IR light short full curves on the left and right maxima coinciding with the periastron 5.4 and the blue squares are for the whole Homunculus (van Genderen et al. V passages and the shorter light peaks (van 5.6 2006). For the central region, points Genderen et al. 2006, and references refer to HST data (aperture 0.1?): red therein).