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Astron. Astrophys. 359, 168-180 (2000)

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On the nature of the pulsation of the post-AGB star HD 56126 *

D. Barthès 1, A. Lèbre 2, D. Gillet 3 and N. Mauron 2

1 Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Avda. Diagonal 647, 08028 Barcelona, Spain
2 Groupe de Recherche en Astronomie et Astrophysique du Languedoc-UMR 5024/ISTEEM (CNRS), Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France
3 Observatoire de Haute-Provence (CNRS), 04870 Saint-Michel-l'Observatoire, France

Received 6 December 1999 / Accepted 11 April 2000

Abstract

Spectroscopic and photometric observations of the variable carbon-rich post-AGB star HD 56126 are analysed in order to improve the knowledge of its pulsation and of its fundamental parameters. New high resolution spectra (R=42 000), taken as regularly as possible (every 7-9 days) from October 1995 to April 1996 at the Haute-Provence Observatory, are presented. Important profile variations of the H[FORMULA] line are confirmed, and variations of H[FORMULA] are also shown for the first time. These variations can be significant over time intervals as short as [FORMULA] 7 days.

Photospheric radial velocities were derived and merged with other data from different sources to form a set of 89 values spanning [FORMULA] 8 years. We also considered a completely independent set of 87 V-band Geneva photometric measurements made over 7 years and covering approximatively the same epoch. The results of the Fourier analysis of the radial velocity and photometric series agree remarkably well: a strong main period of [FORMULA] days is found in both data sets. This period supersedes the value of 27 days previously found by us with much fewer data points. Weaker secondary oscillations, probably related to the irregularity of the pulsation, are also detected. There is no prominent Fourier component at [FORMULA] and the pulsation of HD 56126 is certainly quite different from that of RV Tauri variables.

Linear nonadiabatic analysis has been applied. It is found that the only linear radial mode that can fit the main period is the first overtone. This, together with the requirement of some driving of the pulsation and with the core mass-luminosity relation, favours the following range of fundamental parameters: log g = 0.38-0.49, [FORMULA]-[FORMULA], [FORMULA] = 5700-6100 K and [FORMULA]-[FORMULA].

Owing to its higher temperature, the star seems more evolved than post-AGB RV Tauri's.

Hydrodynamical models are also discussed. The spectroscopic and photometric variations of the star appear to be consistent with a radial pulsation yielding shockwaves which propagate through the atmosphere and probably generate a complex, asynchronous motion of the outer hydrogen layers.

Key words: stars: atmospheres – stars: AGB and post-AGB – shock waves – line: profiles – stars: oscillations – stars: individual: HD 56126

* Based on spectroscopic observations performed at the Observatoire de Haute-Provence (France) and at the European Southern Observatory (La Silla, Chile), and on photometric measurements obtained with the 70 cm Swiss Telescope at La Silla.

Send offprint requests to: lebre@graal.univ-montp2.fr

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000
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