During the past 10 years, the observations of the IRAS satellite (Neugebauer et al. 1984) have led to the discovery of a few thousand mass-loosing stars on the Asymptotic Giant Branch (AGB). Some of them are heavily "obscured'', in the sense that they lose mass at such a rate (/yr or more) that their circumstellar envelope becomes optically thick to the stellar radiation. IRAS observations, as well as observations of the millimeter lines of CO and of the OH maser emission, have considerably improved our knowledge on their mass-loss rates. However, further studies are severely hampered by the lack of knowledge of the distances, and hence of the luminosities.
The Magellanic Clouds have reasonably well known distances, and they are far enough to consider that all the stars belonging to the same galaxy are at the same distance, with a small uncertainty. In the optical and NIR range, a huge amount of work has been performed in the Small and Large Magellanic Clouds (SMC and LMC) to search for M supergiants and AGB stars. In the LMC, the most complete works, spatially speaking, have been performed by Westerlund and co-workers (Westerlund 1960, 1961; Westerlund et al. 1978; Westerlund et al. 1981), Sanduleak & Philip (1977), and Rebeirot et al. (1983), leading to the discovery of several hundred M stars and a few hundred C stars. These surveys were, however, limited in sensitivity and could only detect the brightest stars (I<13.5). A deeper survey (I<17), but spatially limited, has been performed by Blanco and co-workers (Blanco et al. 1980; Blanco & McCarthy 1983; Frogel & Blanco 1990). The SMC has been less studied. The work of Blanco et al. (1980) was the first objective prism survey of this galaxy. This, and the subsequent work of Blanco & McCarthy (1983) turned up a few hundred carbon and M-type stars. Reid & Mould (1990) selected AGB star candidates from V and I-band photometry in a area. Recent works concentrated mainly on carbon stars (Westerlund et al. 1986; Rebeirot et al. 1993) and lead to the discovery of about 2000 of them.
Complementary to previous surveys, people started to search for long-period variables (LPVs) through IJHK(L) photometry (see e.g. Feast et al. 1980; Glass & Lloyds Evans 1981; Glass & Feast 1982; Wood et al. 1983; Wood et al. 1985; Glass & Reid 1985; Reid et al. 1988; Hughes 1989; Feast et al. 1989; Hughes & Wood 1990). There are now about 1000 LPVs known in the LMC. The most important result of these works is certainly the relations between the luminosity and the period.
As previous studies were based on optical or near-infrared observations, the resulting samples of stars do not contain optically very thick sources which would be hardly detectable at such wavelengths. In the following we will distinguish between optically identified stars, i.e. stars with optically thin dust shells (typically J-K>2.5) easily detectable in the optical range, and "obscured'' stars, i.e. stars with optically thick dust shells (typically J-K>2.5) and without optical counterpart. Note that such a separation between optical and obscured stars is partly arbitrary as the transition between optically thin and thick dust shells is of course continuous. The separation is more historical as, complementary to optical surveys, people started to search for AGB stars with high mass-loss rates selecting candidates in the IRAS survey, and confirming (or not) the nature of the selected IRAS sources through JHKLM photometry.
In 1986, Elias et al. selected IRAS sources from the Point Source Catalogue (PSC) with a flux density, S12, larger than 2 Jy, and among them discovered two supergiants similar to Galactic OH/IR stars, PSC 04553-6825 and PSC 05346-6949. The same year, Wood et al. selected IRAS-PSC sources with and a S25/S12 ratio similar to those of Galactic OH/IR stars. They detected the maser emission of OH in PSC 04553-6825. This star has an optical counterpart with spectral type M7.5 (Elias et al. 1986). Its optical counterpart was in fact previously known, one can find it as number 64 in Table II of Westerlund et al. (1981). Wood et al. (1992) extended the previous study and detected OH emission in 5 IRAS-PSC sources. They also determined the period of 9 objects in the LMC. In total, they present a list of 3 SMC and 16 LMC sources that they believe to be late-type stars with thick dust shells. However, in Sect. 4.2. we show that, among these 19 sources, only 9 are actually good candidates being obscured AGB stars or late-type supergiants, the others beeing associated with optically known M supergiants, or with blue supergiants, or even with an HII region or a galaxy. The work by Whitelock et al. (1989) in the SMC is also based on the Point Source Catalogue. They monitored in the JHK(L) bands 5 sources with S25/S12 ratios corresponding to a colour temperature of a few 100 K. Among these 5 sources, 2 are long-period variables without optical counterparts, 1 is associated with an M star, 1 is a peculiar carbon star, and 1 is associated with a blue supergiant.
In addition to the survey observations, IRAS also made pointed observations, with orthogonal scan directions, notably in the direction of the Magellanic Clouds. The corresponding detection limits are fainter than those of the PSC. The IRAS pointed observations cover the major part of the SMC and the LMC, except the outer regions. These data have been reduced and published in a catalogue by Schwering & Israel (1990). Part of these data in the LMC has also been reduced by Reid et al. (1990) with the aim of searching for obscured AGB stars. They also made photographic I plates and give possible optical counterparts of some IRAS sources. With additional JHK observations, Reid (1981) discovered 10 "cocoon'' stars, i.e. AGB stars with optically thick dust shells, associated with IRAS sources. He also showed that, for these "cocoon'' stars, the optical counterpart proposed by Reid et al. was in fact not associated with the IRAS source as he found a much redder object close to the IRAS position. More recently, based on the source selections presented here, Zijlstra et al. (1996), in the second paper of this series (hereafter called Paper II), identified 16 additional AGB stars with optically thick dust shells and estimated their mass-loss rates.
In this paper, we will first adress the question: "What are the properties of AGB stars detected by IRAS in the LMC?'' (Sect. 2). In Sect. 3, we present our selection of IRAS sources, from the catalogues of Schwering & Israel (1990) and Reid et al. (1990) in order to find AGB stars candidates. We systematically searched for optical identifications of all the selected IRAS sources. In Sect. 4 we present final tables, optical stars with an IRAS counterpart in Table 1 (click here), obscured AGB stars or supergiants without optical counterpart in Table 2 (click here), planetary nebulae in Table 3 (click here), unidentified IRAS sources that we think to be good obscured AGB (or post-AGB) stars candidates in Table 4 (click here), and foreground stars in Table 6 (click here). In Sect. 5 we discuss on the reliability of IRAS observations at flux levels close to the detection limits and compare both catalogues.