4. Evolutionary status and outlook
In order to understand Sakurai's object it might be helpful to compare it to other objects that experienced a very late Helium flash. Given the small number of examples known and the dynamic nature of the flash only qualitative comparison can be made. Both A 30 and A 78 show extended second PNe, as result of a Helium flash thousands of years ago. Optical images taken with HST (Borkowski et al. 1993 , 1995) beautifully demonstrate the knotty structure of this hydrogen-poor ejecta. In both cases the knots show "cometary" tails, signs of erosion of these knots by the fast wind of the central star, which has become hot again. They seem to form an equatorial disk with some polar knots also apparent. This is strong evidence that the matter lost in connection with the Helium flash was not ejected in a uniform shell. HST observations of V605 Aql/A58 reveal a very small (0.7 arcsec) inner knot (Bond et al. 1993; Clayton & de Marco 1997). For an ejection velocity of about 100 km s-1 (Pollacco et al. 1992) the observed size is in agreement with the matter being ejected in 1917 when the flash presumably happened. Using ISOCAM Kimeswenger et al. (1998) have been able to observe a hot dust component in both A 78 and A 58, thereby strengthening the connection between a very late Helium flash, mass loss and dust formation. With our ISO observations we have undoubtedly witnessed an episode of heavy mass loss associated with the very late Helium flash in Sakurai's object. The evolution of Sakurai's object is similar to the example set by V605 Aql in 1921 to 1924 based on the sketchy data we have for that event. It also developed molecular features in the spectrum a few years after the flash (Clayton & de Marco 1997). It has shown R CrB like dimmings while declining (Harrison 1996) and is now deeply enshrouded in an envelope of hot dust. Most recent (March and June 99) observations by Jacoby et al. (1999) and us find Sakurai at a V magnitude of 20 indicating that dust formation continues and might lead to a similarly "buried" star. One word of caution concerning the status of Sakurai's object in terms of stellar evolution seems to be in order. While Sakurai's object could definitely be called an R CrB star right now, the star it resembles most, V605 Aql is a very hot, possibly a WR-type, star today. In contrast the known R CrB stars show only limited evolution (e.g. shortening of pulsational period in RY Sgr, Kilkenny 1982).
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999