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Astron. Astrophys. 347, 194-202 (1999)

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1. Introduction

HD101584 belongs to a class of peculiar stars at moderate to high galactic latitudes with supergiant-like spectra (Waters et al. 1993). They have been intensively studied during the last decade, and there is a growing belief that most of them are low-mass post-AGB objects (Parthasarathy & Pottasch 1986; Trams et al. 1990; van der Veen et al. 1993; Bakker et al. 1996b). HD101584, was originally classified as an F-type supergiant (Humphreys 1976), but more detailed observations, of high-excitation absorption lines in the red part of the spectrum and photometry, revealed a late B-type star of higher surface gravity (Bakker et al. 1996b). The F-type spectrum probably originates in a dense stellar wind of low ionization degree. Bakker et al. (1996a) infer a [FORMULA] period from photometry data, and possibly also from Doppler velocities of high-exitation absorption lines. According to the authors, the most likely explanation is the presence of a low-mass companion in a close, eccentric orbit (separation[FORMULA]10 [FORMULA]), but this must be regarded as only tentative. Optical emission and low-excitation absorption lines suggest the presence of a circumsystem disk (size[FORMULA]100 [FORMULA]) that is seen nearly edge-on (Bakker et al. 1996b). The CO first vibrational overtone bands that are seen in emission may also originate in such a disk if it is dense enough (Oudmaijer et al. 1995).

The P Cygni-profile components of the Balmer lines show that the primary star is still losing mass at a significant rate, but the estimate is uncertain, [FORMULA] (Trams et al. 1990; Bakker et al. 1996b). The mass loss occurs in the form of a wind with a maximum outflow velocity of [FORMULA]100[FORMULA]. The presence of additional absorption dips may indicate a variable mass loss. The huge far-infrared excess emission may be dominated by dust emission from an escaping circumstellar AGB-envelope. Assuming this Bakker et al. (1996b) estimate a total dust mass of 0.02[FORMULA], and hence a total circumstellar mass probably in excess of 2[FORMULA].

Loup et al. (1990) and Trams et al. (1990) detected a remarkable CO(J=[FORMULA]) line profile from HD101584, and van der Veen et al. (1993) detected the CO(J=[FORMULA]) line. The CO emission probably comes from a number of separate regions, and part of the molecular material have expansion velocities at least as high as 130[FORMULA], i.e., considerably higher than in normal AGB-winds. A much narrower, double-peaked OH 1667 MHz maser line (maximum expansion velocity of [FORMULA]40[FORMULA]) was detected by te Lintel Hekkert et al. (1992). The integrated OH emission is centered on the star (to within 0[FORMULA]3), but the data show that the gas velocity increases systematically along PA[FORMULA]-60o with the most blueshifted emission [FORMULA]2" to the SE and the most redshifted emission at [FORMULA]2" to the NW. There is no OH emission within [FORMULA]1" from the star. The presence of OH emission and a 10[FORMULA]m feature (Bakker et al. 1996b) suggest that the circumstellar material is O-rich (i.e., C/O[FORMULA]1).

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999