EDP Sciences
Free Access
Volume 405, Number 1, July I 2003
Page(s) 271 - 283
Section Stellar atmospheres
DOI https://doi.org/10.1051/0004-6361:20030613

A&A 405, 271-283 (2003)
DOI: 10.1051/0004-6361:20030613

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

T. Maas1, H. Van Winckel1, T. Lloyd Evans2, L.-Å. Nyman3, 4, D. Kilkenny5, P. Martinez5, F. Marang5 and F. van Wyk5

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

(Received 13 December 2002 / Accepted 15 April 2003)

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 ( $\Delta V$ peak-to-peak = 0.17 mag). Our CORALIE radial velocity measurements show that IRAS 08544-4431 is a binary system with a period of  $499 \pm 3$ days and a mass function of 0.02  $M_{\odot}$. 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 ( $\rm [Zn/Fe]=+0.4$). 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 H $\alpha$ 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 H $\alpha$-absorption are modulated with the orbital motion; the maxima of both quantities (~400 , 5 Å respectively) occur at superior conjunction.

Key words: stars: abundances -- stars: AGB and post-AGB -- stars: evolution -- stars: individual: IRAS 08544-4431 -- stars: individual: RV Tauri -- stars: binaries: spectroscopic

Offprint request: T. Maas, Thomas.Maas@ster.kuleuven.ac.be

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© ESO 2003