We propose to take advantage of the opportunity to witness the development
of a star undergoing a late He-flash right now. Several months ago an exciting
new object that most likely belongs to this rare class has been discovered
(Sakurai's object; see IAU Circ. 6322). Originally classified as a slow
nova it now has to be considered as a star undergoing its late He-flash
at the moment. An old (30 arcsec) planetary nebula (PN) has been discovered
in narrow-band images taken in Halpha and [OIII]. The spectrum of Sakurai's
object resembles that of an F-type supergiant, albeit with anaoously strong
lines of C, N and O and weak H-Balmer lines (Duerbeck & Benetti, 1996,
ApJ 468, L111; Kerber et al. 1996 Proc. IAU Symp. 180 on "Planetary Nebulae",
in press). The only other examples in historical times are V605Aql (A 58) in
1919, and possibly FG Sag. To take full advantage of such an event taking
place, we have initiated a coordinated monitoring effort with partners in Chile,
South Africa, and New Zealand. Talks are underway with additional colleagues
in Brazil and Australia as well as colleagues involved in DeNIS.-
It should be noted that such an object is not a curiosity in the context of
stellar evolution, but a process that should be experienced by 10 to 20% of
all low-mass stars (Iben et al., 1983, ApJ 264, 605).
Observations with ISO will be done with PHOT. We request two observations of
Sakurai's object: one as soon as possible and the second late in the mission
in order to assess the development of the object. In particular the question
if and when dust is beginning to form can be addressed by these measurements
in combination with our ground-based network. Calculations of the exposure times
have been done based on high precision IJKs photometry derived from DeNIS survey
data. For the second observation updated input will be provided to account for
any developments in brightness.
This proposal has to be regarded as a valuable supplement of our ISO projects
on dust in PNe (RWEINBER.DIBAPNE, RWEINBER.EODDIPNE), particularly of "Dust in
born-again planetary nebulae".