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Astron. Astrophys. 325, 585-600 (1997)

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1. Introduction

Population studies of Asymptotic Giant Branch (AGB) stars in the Large Magellanic Cloud (LMC) have led to a number of interesting results, using the advantage of its known distance. The period-luminosity relation for Mira variables was deduced from photometric monitoring of Long Period Variables (LPVs) in the LMC (Feast et al. 1989). Objective prism survey studies of the LMC showed a clear lack of carbon stars amongst the most luminous AGB stars ([FORMULA] [FORMULA] mag; see Iben 1981 for a review). These studies have been largely based on samples of optically visible AGB stars (e.g. Blanco et al. 1980; Cohen et al. 1981; Westerlund et al. 1981).

Stars near the tip of the AGB experience mass-loss rates as high as [FORMULA] [FORMULA]  yr-1, giving rise to circumstellar envelopes (CSEs) in which dust forms, making them optically thick at optical wavelengths. Hence these AGB stars may only be observable at infrared (IR) wavelengths and were missed in earlier studies. Samples of candidate obscured AGB stars in the LMC have only recently become available (Reid 1991; Wood et al. 1992; Zijlstra et al. 1996). The AGB nature of some of these stars still needs to be confirmed, and their (circum-)stellar properties are yet to be explored in detail. The small existing samples should be extended both in number and in (circum-)stellar parameter space, to allow a population study of mass-losing AGB stars in the LMC.

In paper I (Loup et al. 1997) we constructed an IRAS-selected sample of AGB and post-AGB star candidates. In paper II (Zijlstra et al. 1996) we confirmed the AGB nature for a subsample thereof, on the basis of their bolometric luminosities and their near-IR (NIR) and IRAS colours. We also showed that the chemical type - oxygen or carbon star - may be deduced from the position of a mass-losing AGB star in the K-[12] versus H-K diagram. Considering the difficulty of obtaining spectra of these stars, the IR colour diagnostics are a powerful tool in determining the nature of the IR stars (e.g. van der Veen & Habing 1988; Guglielmo et al. 1993; Le Bertre 1993; Le Sidaner & Le Bertre 1994; Loup & Groenewegen 1994).

Here, in paper III of our series on the study of obscured AGB stars in the Magellanic Clouds, we present the results of a NIR search for counterparts of IRAS point sources in the direction of the LMC, aiming at enlarging the sample of known obscured AGB stars in the LMC. After classifying the stars as being mass-losing AGB stars, or not, we briefly comment on their properties. We discuss not only the AGB stars, but also the nature of other positive identifications, and serendipitous detections. Amongst them are stars that may be explained as being in the post-AGB phase, and we compare their properties with those of a sample of IRAS-selected post-AGB star candidates in the Milky Way. We also find a few galaxies, which may be used as probes to measure the interstellar extinction inside the LMC.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998