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Astron. Astrophys. 350, L27-L30 (1999)

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1. Introduction

Sakurai's object (V4334 Sgr) most likely is a star undergoing a final Helium flash, the first example since V605 Aql/A 58 in 1919. Details of its discovery and evolution are found in Duerbeck et al. (1996, 1997), Kerber et al. (1999a) and references therein. This is the first such event that can be studied with modern technology and at wavelengths other than optical. Most recently two studies of the old planetary nebula (PN) surrounding Sakurai's object (Kerber et al. 1999a; Pollacco 1999) have demonstrated that a very late Helium flash is under way, i.e. one that happened when the star was highly evolved (T [FORMULA] 100 000 K) and had already entered the white dwarf cooling track. This result is in full agreement with the quantitative analysis of Asplund et al. (1999) who show that Sakurai's object is hydrogen-poor and hydrogen is still being removed from the photosphere as it is ingested and burned, whereas s-process elements become more abundant.

The theory of a very late Helium flash (Iben et al. 1983) predicts that such stars will return to the AGB and retrace their own post-AGB evolution for a second time ("born-again"). This will lead to the formation of a second PN composed of hydrogen-poor material and dust (Harrington 1996). The prototypes of these PNe are A 30 and A 78 (Jacoby & Ford 1983; Hazard et al. 1980) but also V605 Aql/A 58 (Seitter 1987), GIJC-1 (Gillett et al. 1989) in M 22 and IRAS 15154-5258 (Manchado et al. 1989) belong to this class. Therefore both theory and the known examples of very late Helium flashes suggest that significant mass loss is associated with such an event.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999