IRAS 08544–4431: A new post-AGB star in a binary system surrounded by a dusty disc^*

¹ Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
² School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, Scotland KY16 9SS, UK
³ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁴ Onsala Space Observatory, 43992 Onsala, Sweden
⁵ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa

Corresponding author: T. Maas, Thomas.Maas@ster.kuleuven.ac.be

Received: 13 December 2002
Accepted: 15 April 2003

Abstract

We present an analysis of our extensive data-set on IRAS 08544–4431. It is the first object we discuss of our newly defined sample of stars, selected for their position in the “RV Tauri” box in the IRAS [12]–[25], [25]–[60] two-color diagram. Moreover, our selection criteria included an observed excess in the L-band, indicative of a dusty disc. The SED of IRAS 08544–4431 shows a broad IR excess starting already at H. Our optical photometric data reveal some evidence for deep and shallow minima in the light curve and a pulsation time-scale of around 100 days with a small amplitude ( peak-to-peak = 0.17 mag). Our CORALIE radial velocity measurements show that IRAS 08544–4431 is a binary system with a period of  days and a mass function of 0.02 . Moreover, IRAS 08544–4431 is detected in both the CO (2–1) and (1–0) mm-wave emission lines. The triangular shape of the weak CO profile confirms that part of the circumstellar material is not freely expanding but resides probably in a dusty circumbinary disc. Our chemical abundance analysis of a high resolution spectrum of high S/N reveals that a depletion process has modified the photospheric abundances to a moderate extent (). All these findings confirm that the F-type IRAS 08544–4431 is another good example of a binary Post-AGB star surrounded by a dusty disc. The Hα  P-Cygni profile shows ongoing mass-loss with a very high outflow velocity, the origin of which is not understood. The strength and velocity of the Hα-absorption are modulated with the orbital motion; the maxima of both quantities (~400 , 5 Å  respectively) occur at superior conjunction.