Collaborateurs: Stefan Dreizler (Tübingen); Klaus Werner (Tübingen).
Project description: The majority of stars (above 90\%) end their live as a white dwarf (WD). On the tip of the Asymptotic--Giant--Branch (AGB) the star ejects a planetary nebula (PN). The remnant evolves at constant luminosity to high effective temperatures ($>$100\,000\,K). When nuclear processes in the shell cease, the star enters the WD cooling sequence on which it contracts and cools down. This scenario explains the majority of the post--AGB stars ($\approx$ 80\%) which are hydrogen rich. These stars probably end their lifes as hydrogen rich (DA) WDs. On the other hand there is a large variety of hydrogen deficient stars all the way from the AGB to the coolest WDs: R\,Corona\,Borealis stars, Extreme Helium (EHe) stars, luminous helium rich sdOs, O(He) central stars of PN, Wolf--Rayet central stars of PN ([WC] CSPN), PG\,1159 stars, hot and cool helium rich WDs (DO, DB, DC). Canonical evolution theory fails to explain the existence of the hydrogen deficient post--AGB stars, since standard mass--loss rates are too small to corrode the H--rich envelope completely. Our research concentrated on the hottest objects of theses hydrogen deficient post--AGB stars, namely the PG\,1159 stars, O(He) CSPN, and the DO white dwarfs (Werner et al. 1998, Review in Modern Astronomy 10, p. 219; Dreizler \& Heber 1998, A\&A 334, 618; Rauch et al. 1998, A\&A 338, 651; Dreizler \& Werner 1996, A\&A 314, 217). The former are extremely hot (75000\,K$<$ T$_{\rm eff}$ $<$ 180\,000\,K), post--AGB stars in the transition from the CSPN to the WDs. Their surface composition (He:C:O $\approx$ 30:50:20, Werner et al. 1991, A\&A 244, 437) points to non--canonical stellar evolution, removing the hydrogen rich envelope by enhanced mass loss. e.g. during a late helium shell flash (Iben 1984, ApJ 277, 333). Closely connected are the hot helium rich DO white dwarfs, which are the supposed successors of the PG\,1159 stars after gravitational settling removed most of the heavy elements from the atmosphere. The O(He) stars populate the same region in the HRD as the PG\,1159 stars, they are however significantly less metal rich. Our proposal is designed to significantly enlarge our database of PG\,1159 stars and DO white dwarfs to clarify several aspects of post--AGB evolution which have been addressed by our previous work. Open questions are:\newline --- The evolutionary connection among the hydrogen deficient post--AGB stars. When does the gravitational settling start to transform PG\,1159 stars into DOs and can we understand this process quantitatively (Dreizler 1999, in White Dwarfs, http://astro.uni-tuebingen.de/$\sim$werner/jcam\_paper.html)? Are all DOs successors of PG\,1159 stars or do we have a parallel sequence (Rauch et al. 1998, A\&A 338, 651)? Why is the helium rich white dwarf sequence interrupted between 30000\,K and 45\,000\,K? Do we find DOA white dwarfs (helium rich white dwarfs with significant admixture of hydrogen) shortly above that gap? \newline --- PG\,1159 stars as well as DO white dwarfs have significant individual differences in the chemical composition. What information about previous evolution can be extracted? Interesting in that respect are the small subgroups of PG\,1159 stars showing detectable amounts of hydrogen (Dreizler et al. 1995, A\&A 309, 820) or nitrogen (Dreizler \& Heber 1998, A\&A 334, 618). Both elements are difficult to explain in the enhanced mass loss scenario but rather require deep mixing and burning processes.\newline --- How frequent are the new hot wind DOs (Werner et al. 1995, A\&A 293, L75, Dreizler et al. 1993, A\&A 303, L53) first detected in the Hamburg/ESO survey (HES, Wisotzki et al. 1996, A\&AS 115, 227)?\newline --- A third of the PG\,1159 stars is unstable against non-radial g--mode pulsations. The asteroseismologic analyses of these variations allows a direct insight in the stellar interior (e.g. Kawaler \& Bradley 1994, ApJ 427, 415). More pulsating PG1159 stars, especially of the type of HS\,2324+3944 (Dreizler et al. 1996, A\&A 309, 820) are necessary to understand the underlying physical processes. \newline All this aspects require larger samples than the presently known 24 DOs and 31 PG\,1159 stars. Our aim is an enlargement of the sample size by 50\,\% and almost 100\,\% of the DO and PG\,1159 star sample, respectively. Quasar surveys like the Palomar Green Survey (PG, Green et al. 1986, ApJS 61, 605) and the Hamburg Schmidt Survey (HS, Hagen et al. 1995, A\&AS 1111,195, Engels et al. 1998, A\&AS 128, 507) have been shown to be also extremely valuable sources of faint blue stars like hot white dwarfs and subdwarfs. This is especially true for rare groups like the hot, hydrogen deficient (pre-) white dwarfs which are found in reasonable numbers only in large surveys. The PG survey led to the detection of a new class of pre-white dwarfs, the PG\,1159 stars, which turned out to be Rosetta Stones in the understanding of post--AGB evolution. It also contributed significantly to other rare classes of faint blue stars. The HS survey was equally successful, but by an order of magnitude more efficient due to the better information about the spectral type of the objects from Schmidt spectra. During the recent decade we successfully cooperated with the colleagues in Hamburg in the follow-up spectroscopy of the stellar component of the HS survey (Heber et al. 1991, NATO ASI Series C, 109; Jordan et al. 1991, NATO ASI Series C, 121; Dreizler et al. 1994, in Hot Stars in the Galactic Halo, CUP, 228). Analyses of interesting individuals or classes of stars significantly widened our present understanding of the late stages of stellar evolution (Heber et al. 1993, A\&A 267, L31, 1996,A\&A 311, L17; Jordan et al. 1993, 273, L27 1998, A\&A 330, 277; Dreizler et al. 1994, A\&A 286 463, 1995, A\&A 303, L53, 1996, A\&A 309, 820, Dreizler \& Werner 1996, A\&A 314, 217; Lemke et al. 1998, in The Third Conference on Faint Blue Stars, L. Davis Press, in press; Homeier et al. 1998, A\&A 338, 563). While both of these surveys are restricted to the northern hemisphere the HES provides access to the southern hemisphere. Compared to the HS, the HES allows an even more efficient candidate selection due to the higher spectral resolution of the objective prism (15\,\AA\ at H$\gamma$ instead of 45\,\AA). The resolution is sufficient to allow a pre--classification of the Schmidt spectra extracting hot hydrogen deficient objects (titled ``sdO''). Since the southern faint blue stars are less intensively explored by surveys, the success rate will be further increased (only about 25\% of the known hot hydrogen deficient (pre-) white dwarfs are southern objects). This is further underlined by the fact, that out of very few follow-up spectra of stellar candidates from the HES, the detection of a new class of extremely hot white dwarfs with ongoing mass loss was detected (Werner et al. 1995, A\&A 293, L75). We propose to observe a list of ``sdO'' candidates selected from the HES. Extensive simulations using theoretical spectra transformed to Schmidt spectra, showed that the ``half power point'', the wavelength dividing the Schmidt spectrum in two equal areas, is a reliable selection criterium to obtain hot, hydrogen deficient stars. Comparison of this list with known PG\,1159 and DO stars reveals that we can reliably select such objects. A sufficient blue half power point enables the separation of these stars from the ordinary sdO stars. One of our aims is to check the reliability of the target selection (PhD of N. Christlieb) and provide a much larger database to improve the selection criterium for future observing runs. The main emphasis, however, lies on achieving a significantly larger sample of hot, hydrogen deficient objects (PG\,1159 stars, DO white dwarfs). In two runs we plan to observe a total of 100 objects. During our follow--up spectroscopy of the HS survey we obtained 20\% PG\,1159 stars and DO white dwarfs out of the ``sdO'' candidates. Taking into account the more reliable candidate selection of the HES, we expect to detect about 10 new PG\,1159 stars and 20 new DO white dwarfs in the two runs, which would be an increase by 50\% and nearly 100\%, respectively. In the context of this project the subsample of compact (i.e. PN free) PG\,1159 stars is more interesting. Since in the HES a separation of point-like sources from extend objects is possible by using direct plate scans from the Digitized Sky Survey, we will be able to restrict ourselfes mainly to PN free objects. We also expect to find 3-5 new O(He) stars which would double this sample. The selected sample for this run contains 50 candidates observable during the ESO winter for which we will obtain medium resolution spectra (6\,\AA) with a $S/N$ between 30 and 50 (ESO 1.52m, B\&C, grating 25). They will allow a reliable spectral identification as well as a first spectral analysis.} \TelescopeJustification{The 1.52m telescope in connection with the B\&C spectrograph is an ideal instrument for our purpose. It provides a huge spectral coverage in one exposure significantly improving the classification by coverage of all important optical lines. The overall efficiency allows to obtain a large number of medium resolution spectra (6\,\AA) in reasonable time (average 60\,min/object). The spectral resolution does not only allow a classification but also enables a first determination of stellar parameters.
Last modified 15.1.1999 by
nchristlieb@hs.uni-hamburg.de