Recent Science Highlights

8 June 1995

(following information provided by Dr. Antonella Fruscione)

Drs. K.Werner, S.Dreizler, U.Heber and T.Rauch from Kiel, Bamberg (Germany) report their recent results in confining the edges of the GW Vir instability strip.:

• The so-called PG1159 stars (from the prototype PG1159-035 or GW Vir) represent the hottest stage of post Asymptotic Giant Branch stellar evolution. Surface temperatures reach up to 170000 K. The photospheres are hydrogen-free and the stars essentially expose the deep interior of their former Red Giant phase.
• Their evolutionary paths through the Hertzsprung-Russell diagram to the hot end of the white dwarf cooling sequence leads them through an instability region in which the stars are performing non-radial pulsations. These objects hold the key for our understanding of late stellar evolution because their interior is accessible to asteroseismologic methods.
• A prerequisite for such investigations are reliable analyses of photospheric parameters, particularly in order to find the exact location of the instability strip. Combined campaigns of optical observations and spectroscopy from space based telescopes (EUVE, HST, ROSAT) were started for this project. The EUVE satellite turns out to be particularly useful to determine the surface temperature. The analysis of a spectrum of the non-pulsator PG1520+525 taken with EUVE in comparison with HST data of the pulsating prototype PG1159-035 (GW Vir) locates the blue edge of the instability strip near Teff=140000 K for stars in the respective luminosity range.

Figures (272 Kb post-script):

Figure 1: shows the EUVE spectrum of PG 1520+525 together with two synthetic spectra from model atmospheres.

Figure 2: shows the location of the instability strip in the surface gravity vs. temperature diagram together with theoretical evolutionary tracks.