The location of AGB stars, post-AGB stars, planetary nebulae, HII regions, and galaxies, in the IRAS two-colour diagram (12-25-60) has been described by many authors (see e.g. van der Veen & Habing 1988). In this paper we use the work by Pottasch et al. (1988, their Fig. 1 (click here)) which gives an overview. We are not only interested in AGB stars and M supergiants, we also would like to include possible post-AGB objects and planetary nebulae. Therefore the problem is mainly to eliminate HII regions and galaxies. According to Fig. 1 (click here) in Pottasch et al., most HII regions have (S12/S25)<0.4 (C21>+0.08) and (S25/S60)<0.3 ( ), and most galaxies have (S12/S25)<1 (C21>-0.32) and (S25/S60)<0.3. The knowledge of the 12, 25, and IRAS flux densities would allow us to select AGB stars, post-AGB stars and planetary nebulae with great confidence.
The sensitivity limits given by Schwering & Israel (1990) are 0.15, 0.22, and 0.41 Jy at 12, 25, and respectively. From the discussion in section 2, or from the work by Reid (1991), we expect that most AGB stars detected by IRAS in the LMC will not be detected at , and often will be detected only in one band, at 12 or . The selection of sources is then not so straightforward, and we have to use limits on the IRAS flux ratios. In practice, we have to consider two possiblities at : detected or not detected, and three possibilities at 25, 60, and : detected, not detected, or contaminated by the surroundings. Non detections provide us upper limits on the IRAS flux densities. Contaminated fluxes do not give us any information.
As a first selection, we have removed all the sources that Schwering & Israel find extended (23 sources in the LMC, 49 sources in the SMC). Next we have removed all the sources with S100>2S60 (945 sources in the LMC, 58 sources in the SMC); note that this eliminates all the sources detected at but not detected at . This criterion removes many HII regions and galaxies which are "cold'' objects, preferentially detected at long wavelengths. It could, however, also remove some planetary nebulae.
From these two first selections, we obtained a sample of 923
sources in the LMC, and 142 sources in the SMC. We also
added 24 LMC sources, with S100<2S60 or not
detected at 60 and , listed in Reid
et al. (1990) which
have not been found by Schwering & Israel (conversely
some sources mentioned in Schwering & Israel have not
been found by Reid et al.). Then we applied the
following selection criteria:
There are several cases where we do not have any information on one of the flux ratios, or where the limit derived on one of the flux ratios is not significant, so we cannot conclude anything. We have divided the initial sample into three groups. Group 1 contains 256 LMC and 37 SMC sources following the selection criteria, and expected to be red supergiants, AGB stars, post-AGB stars, or PNe candidates. Group 3 contains 415 LMC and 62 SMC sources which do not follow the selection criteria, expected to be HII regions or galaxies. Group 2 contains 276 LMC and 43 SMC sources for which the available IRAS data are not conclusive, they can be a priori anything.