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Astron. Astrophys. 332, 25-32 (1998)

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2. Target selection

As AGB stars with significant mass loss radiate predominantly in the mid-infrared the starting point of the selection is the IRAS faint source catalog (FSC; Moshir et al., 1989). All stars in the FSC in the direction of the Small Magellanic Cloud were selected that have a flux-quality of 2 or larger at 12 and 25 µm. As AGB stars that have a 60 µm flux larger than their 25 µm flux are extremely rare in our galaxy, all objects which have [FORMULA] and a flux-quality of 2 or larger at 60 µm were eliminated as well. This resulted in a sample of 51 stars, which may still contain foreground objects.

To get a rough estimate of the mass loss rate and luminosity of the objects if they were AGB stars in the SMC, the 12 and 25 µm fluxes were fitted to a pre-calculated grid of simple radiative transfer models (Groenewegen 1993). A central star represented by a black body of 2500 K was assumed to be surrounded by a circumstellar shell containing carbon dust. For the level of modelling used it is of little importance that some of the objects are oxygen-rich and where a model with silicate dust would have been more appropriate. The temperature of the dust at the inner radius was assumed to be 1000 K. In such a model the [FORMULA] ratio is completely determined by the dust optical depth. For a fixed distance the luminosity of the object is immediately derived by comparing the calculated with the observed 12 µm flux.

For every star the luminosity and optical depth (mass loss rate) were estimated in this way. Candidate AGB stars are those that have luminosities below, say, 105 [FORMULA] (to allow for the fact that the luminosity estimate may be wrong by up to a factor of two and that some IRAS sources are Mira variables whose IRAS fluxes may represent the flux close to maximum light) and mass loss rates in the range 10-7 -10-4 [FORMULA] yr-1. In this scheme foreground objects typically have low mass loss rates and very high fiducial luminosities. Every result was inspected individually to check if no potential AGB stars were inadvertently left out. In addition every star was run through the SIMBAD database to look for any additional information. Two stars are included in the sample that are known foreground objects; both are late-type giants and NIR photometry may be of interest. Apart from presenting the photometry these stars are not discussed further. In addition four stars with a flux-quality of only one at 12 or 25 µm were included.

The final sample of candidate AGB stars in the SMC contains 31 objects. This sample is not intended or expected to be complete and may still contain a few foreground objects (apart from the two known ones). Table 1 lists our designation, the name in the FSC, and the IRAS 12 and 25 µm fluxes derived using the GIPSY package as explained in Sect. 5.


Table 1. The sample

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998