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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint 12/97

On the red edge of the GW Vir instability strip

K. Werner(1), S. Dreizler(1), U. Heber(2), T. Rauch(1)

(1)Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2)Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, D-96049 Bamberg, Germany

Contribution to A Half Century of Stellar Pulsation Interpretations: A Tribute to Arthur N. Cox, Los Alamos, New Mexico, June 16-20, 1997

Abstract. The position of the red edge of the GW Vir instability strip has been debated since many years. Model calculations suggested that pulsations should persist to much cooler temperatures than actually observed. Analyses of new UV spectroscopic data obtained with HST and HUT reveal that no PG1159 stars exist with effective temperatures below 75 000K. The coolest pulsating class member (PG 0122+200) is found at the cool end of the PG1159 star sequence indicating that the red edge is caused by the transformation of PG1159 stars into DO or DA white dwarfs by gravitational settling of C, N and O and possibly other heavy elements. Hence pulsational models with T_eff below this critical value and PG 1159 composition are meaningless. Pulsator masses are close to 0.6 M_sun and no pulsator is found with M > 0.7 M_sun, except in the high luminosity central star region. The unexpected detection of nitrogen suggests that at the cool end of the GW Vir strip N might be an essential ingredient for pulsation driving or at least pointing at a particular chemical composition which could be responsible for the pulsations.

Paper (62k gzip'ed Postscript including figures)

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