Our search for NIR counterparts of IRAS point sources in the direction of the LMC was aimed at finding obscured AGB stars in the LMC: we found 13 of them. The K- versus J-K diagram is shown to be well suited for discriminating between mass-losing oxygen and carbon stars. Comparison between the luminosity distributions of the oxygen and carbon stars suggests that the fraction of carbon stars is smaller at bolometric luminosities around mag than around mag. Oxygen and carbon stars co-exist at all luminosities, but the new sample of obscured AGB stars does not include luminosities as bright as mag. Analysis of the combined sample of known obscured AGB stars in the LMC will be presented in the next paper of this series.
Besides the obscured AGB stars, we found a probable Galactic halo star or very luminous LMC red supergiant. We also found 7 probable IRAS counterparts which have large J-K colours, but not as large as the obscured AGB stars. Comparison with a sample of post-AGB objects in the Milky Way as compiled by van der Veen et al. (1989) suggests that these NIR detections are likely to be post-AGB stars, with luminosities around mag. We investigated the possibility that they are AGB stars recovering from the occurrence of a thermal pulse. Although this cannot be ruled out completely, we found it difficult to reconcile their NIR and IRAS colours with those of stars in the Milky Way that are suspected to experience this short evolutionary phase.
We also detected several objects with J-K colours larger than for normal stars, but not as large as the IRAS counterparts. They are not related to the IRAS source in the field. Although two of them are identified with known LPVs in the LMC, the remaining few stars may be LMC stars that are reddened by interstellar extinction inside the LMC. This suggestion is supported by the serendipitous detection of a few galaxies with larger J-K colours than normal galaxies, indicating reddening by the LMC corresponding to a visual extinction of a few mag. This would have severe consequences for the study of stars inside the LMC, that are often assumed not to suffer significant interstellar extinction.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998