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Born-again stars

During the past century, three stars have been observed as being both hot subdwarfs and RCB stars at different times.

V605Aql is now the hydrogen-deficient central star of a planetary nebula (Abell58). In 1919 it brightened as a slow nova, and there is a report that its spectrum at one time resembled an RCB star ([van den Bergh 1971]). A hydrogen-deficient knot in the inner nebula could have been ejected around 1919 ([Bond et al. 1993]).

FGSge was a faint blue star in 1900. Since then, it has become progressively redder and brighter so that now it has the spectrum of an F or early-G type giant ([Herbig & Boyarchuk 1968], [Smolinski et al. 1976], [Montesinos et al. 1990]). Since the 1930s, it has shown somewhat irregular pulsations with a quasi-period increasing from around 5 days in 1934 to about 120 days in the 1990s ([van Genderen 1994]). In 1992 it started to show RCB type fading events ([Jurcsik 1992]). Although irregular, their onset appears to occur at a particular phase (light minimum) in the pulsation cycle ([Arkhipova 1996]). At the same time the spectrum has become very carbon-rich ([Iijima & Strafella 1993]). The surface layers now show the products of many nuclear reaction processes, and are thought to be hydrogen poor ([Gonzales et al. 1998]). Thus there is evidence to suggest that FGSge may be a new-born RCB star.

In 1996, an unremarkable faint blue star suddenly brightened and within three years had become a very cool and luminous carbon star, now known as Sakurai's Object or V4334Sgr ([Nakano et al. 1996], [Duerbeck et al. 1997]). This star holds the record for being the fastest evolving star known; in addition to its rapid expansion, the surface composition has been observed to change as convection in the stellar envelope dredges more and more procesed material from the stellar interior ([Asplund et al. 1999]). The similarities of some abundance ratios has prompted speculation that V4334Sgr too may be a new-born RCB star. However its continued evolution to even lower effective temperatures, the development of a high mass-loss rate ([Kerber et al. 1999]), the presence of a planetary nebula ([Benetti et al. 1996]) photosphere and the potential emergence of a hot core with a fast wind ([Eyres et al., 1999]) may equally presage the ejection of new nebula material as the expanding envelope detaches itself from the core, followed by a rapid return to the PN phase.

The spectacular evolution of these three stars has been explained in terms of the final-flash model. Their similarities with RCB stars may eventually help to explain the origin of RCB stars. Alternatively, the fact that V605Aql contracted to become a hot central star as quickly as it became a giant, whilst RCrB itself has not changed significantly over the last 200 years, may mean that these three stars represent entirely separate evolutionary pathways.