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Astron. Astrophys. 330, L41-L44 (1998)

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1. Introduction

Sakurai's Object ([FORMULA] = 17 [FORMULA] 52 [FORMULA] 32 [FORMULA] 69, [FORMULA] = -17 [FORMULA] 41 [FORMULA] 07 [FORMULA] 7, J2000.0) was discovered by the Japanese amateur astronomer Y. Sakurai in February of 1996 (Nakano et al. 1996 ). First classified as a slow nova, this new star showed a highly unusual spectrum dominated by a wealth of absorption lines from carbon, nitrogen and oxygen, but displays only weak hydrogen Balmer lines (Benetti et al. 1996). This finding and the discovery of a faint old planetary nebula (PN) surrounding the star, led Duerbeck & Benetti (1996) to the idea that Sakurai's Object may be a star undergoing a late He-flash. Despite the fact that 10 to 20 % of all low mass stars should experience such a phase (Iben & MacDonald 1996), it is an exceedingly rare observational event due to its extremely short duration. In historic times there are only two other likely candidates: the first is V 605 Aql, the central star of A 58 that showed a strong brightening in 1919 (Harrison 1996, see Clayton & De Marco 1997 for a review). The second example is the peculiar star FG Sge that has crossed the HRD over the last 100 years, while showing an almost continuous increase in brightness; see van Genderen & Gautschy (1995) for a summary of its evolution. FG Sge's behaviour has most recently been explained by Blöcker & Schönberner (1997) by a He-flash that happened during the plateau phase of the PN evolution (late He-flash). He-flashes on the cooling track towards the white dwarf region (very late He-flash) develop much faster and are therefore a very short-lived phase. Sakurai's Object gives us a unique opportunity to study such a very late He-flash with the full array of modern equipment. Monitoring has already led to a number of important discoveries; Asplund et al. (19979 and Kipper & Klochkova (1997) have been able to derive accurate abundances for Sakurai's Object, thereby establishing the hydrogen-poor nature of the object. Kerber et al. (1997a) have discovered dramatic changes in the optical spectrum, where strong molecular bands of C2 and CN were found. Similarly Kimeswenger et al. (1997) identified a strong NIR excess due to hot dust. This was confirmed by recent ISO observations (Kerber et al. 1997b). All these observations seem to be in agreement with the notion that Sakurai's Object is indeed experiencing a very late He-flash and is undergoing an episode of mass loss. A reliable and accurate distance will be vital in order to learn as much as possible from this example of stellar evolution in "real time".

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998