Florian Kerber 1, Joris Blommaert 2, Stefan Kimeswenger 3, Martin Groenewegen 3, & Hans-Ulrich Kaeufl 4
1 University of Innsbruck, Innsbruck, Austria
2 VILSPA ESA, Villafranca, Spain
3 Max-Planck-Institut für Astrohysik, Garching, Germany
4 European Southern Observatory ESO, Garching, Germany
We present ISOCAM observations of Sakurai's Object covering the wavelength range of 4 to 15 mum in seven filter bands. Sakurai's Object (V4334 Sgr) was discovered in February of 1996 and was originally classified as a slow nova. Optical spectra showed that the photosphere was poor in hydrogen but C, N, O and the s-process elements were strongly overabundant. This and the discovery of a planetary nebula surrounding Sakurai's Object lead to the conclusion that this is a star undergoing a late helium-flash. The most recent example observed was V605 Aql the central star of A58 which brightened in 1919; it is a most fortunate coincidence that the flash in Sakurai's Object happened during the lifetime of ISO. Theory of stellar evolution predicts that a late helium flash occurs in about 10 percent of all low mass stars. Still observationally this is an extremely rare event due to its brief duration.
In 1997 our group was the first to report the emergence of strong molecular bands of C2 and CN and also discovered the development of a strong IR excess. We were then given the opportunity to enhance our ISO program to monitor the evolution of Sakurai's Object with a discretionary time proposal. This enabled us to observe Sakurai's Object at four epochs covering just more than a year. The repeated ISOCAM photometry shows that over the period from February 1997 to February 1998 the flux in the 4 to 15 mum range has increased by a factor of about ten! This is certainly due to the formation of hot dust. The comparison of the spectral energy distribution (SED) for different epochs shows that there is a general cooling trend, which can be expected for an expanding dust shell. Yet the ratios of the different filters for a given epoch vary. This certainly is an indication of structural changes in the SED. One conclusion is that the formation of dust is continuing but at variable rates or in an episodic manner. An other explanation is the formation of dust features. Sakurai's object evidently is very poor in hydrogen, therefore the freshly formed dust is likely to be amorphous carbon. To further investigate this topic we have started ground-based monitoring using TIMMI at ESO's 3.6m.
Sakurai's object is an unique opportunity to witness stellar evolution in ``real time''. It is also the first late helium flash that can be studied over a large wavelength range using modern instrumentation. The ISOCAM data presented here clearly show the importance of this approach. We will continue to monitor the evolution of Sakurai's Object from the ground until the next generation of space telecopes in the IR becomes available.