| Description |
we propose to study newly formed dust around a class of ultrahot (ca. 100 000 k) pre-white dwarf stars. such stars are, in general, of considerable importance for a detailed understanding of the phases of late stellar evolution. we intend to deal with a remarkable subgroup, born-again (iben et al. 1983, apj 264, 605) stars, which are hot and hydrogen-deficient and have long defied a quantitative spectroscopic analysis due to their extremely high temperatures and peculiar chemical composition. in recent years, these pg 1159 stars, a part of which is surrounded by planetary nebulae, are considered as objects that suffered a late he shell flash and which are now retracing their post-agb evolutionary track for the second time. the most famous example for this born-again sample is a30, an old planetary nebula harbouring a central star which is surrounded by a tiny, complex, dust-rich nebula. the dust is arranged in a disk-like structure, must have been formed quite recently - and is hot. the question of the highly interesting born-again scenario in context with the origin, generation, location and, eventually, destruction of dust around such ultrahot stars will rest on two essential foundations: first, a suitable instrumentation for detailed observations of the infrared emission of the dust (which is, with exception of a30, usually small due to the small amount of dust) and, second, an accurate knowledge and understanding of the physical parameters of the stars that are responsible for the emitting material. iso will provide excellent conditions for the first found- ation, whereas the sophisticated non-lte model atmospheres by the kiel group will be an outstanding means for the second. |
