Abell 43: longest period planetary nebula nucleus variable^*

¹ Institute of Theoretical Astrophysics, University of Oslo, Box 1029 Blindern, 0315 Oslo, Norway e-mail: j.e.solheim@astro.uio.no
² Universite Paul Sabatier, Observatoire Midi-Pyrénées, CNRS/UMR5572, 14 Av. Belin, 31400 Toulouse, France e-mail: gerardv@ast.obs-mip.fr
³ Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA e-mail: anjum@astro.washington.edu
⁴ Institute of Theoretical Physics and Astronomy of Vilnius University, Gostauto 12, Vilnius 01108, Lithuania e-mail: jr@itpa.lt
⁵ Observatory of Vilnius University, M.K. Ciurlionio 29, Vilnius, 03100, Lithuania e-mail: vidas.dobrovolskas@ff.vu.lt

Received: 27 February 2007
Accepted: 11 April 2007

Abstract

Context.Most PG 1159 stars are hydrogen deficient post-AGB stars on their way to the white dwarf cooling sequence. However, a fraction of them show small amounts of atmospheric hydrogen and are known as the hybrid PG 1159 stars. Both the normal and hybrid PG 1159 stars may reside at the centre of a Planetary Nebula. Some PG 1159 stars exhibit non-radial g-mode pulsations, providing us with a means to constrain their fundamental physical parameters, internal structure, and evolutionary status; asteroseismology is the technique of using global stellar pulsations to probe the star's interior.

Aims. Model calculations demonstrate that the PG 1159 instability strip is driven by the partial ionization of carbon and oxygen through the κ-mechanism. The range of excited frequencies in models is sensitive to the details of the chemical composition in the driving zone. Hydrogen content plays a crucial role in deciding the range of excited frequencies for models of hybrid PG 1159 pulsators. The range of periods observed in all PG 1159 pulsators can then be compared against the range predicted by theoretical models, which are based on observations of their chemical composition and evolutionary status. Our aim is to test the validity of the predicted range of pulsation periods for the hybrid PG 1159 variables, Abell 43 and NGC 7094.

Methods.To achieve this goal, we acquired continuous high speed CCD photometry of the hybrid PG 1159 star Abell 43 using the 2.6 m Nordic Optical Telescope and the 3.5 m telescope at Apache Point Observatory. We also observed NGC 7094, a hybrid PG 1159 star with atmospheric parameters similar to Abell 43. We analyzed our data to obtain the pulsation spectra of the two stars.

Results.We detected six significant pulsations in the light curves of Abell 43 with periods between 2380 s and 6075 s. We were also fortunate to detect low amplitude pulsations in our NGC 7094 data with periods between 2000 s and 5000 s. The observed range of periods for both stars is consistent with the theoretical predictions.

Conclusions.The range of periods observed in both hybrid PG 1159 stars, Abell 43 and NGC 7094, agree with the theoretical model that the pulsations are driven by the κ-mechanism induced by the partial ionization of carbon and oxygen.