Low resolution spectroscopy of ISOGAL sources: Search for early-type stars with infrared excess

Abstract

An analysis of low resolution spectra and infrared data of 29 ISOGAL-DENIS sources with mid-IR excess is presented. Eight ISOGAL sources from our sample with 7-15 mu m excesses are found to be B and A-type stars, some of them with emission lines. Two ISOGAL sources, J175614.4-240831 (B3-4IIIe) and J173845.3-312403 (B7IIIe), show a bump between 5000 and 6000 Å which may be attributed to extended red emission (ERE). Some of the B, A and F-type giants with a large infrared excess might be in the post-AGB phase. For about 50% of the sources in this preliminary study, a nearby second (or even multiple) component was found. Such sources, in particular two B-stars, are not discussed when the probability of the optical spectrum being associated with the ISOGAL source is low. These results confirm that the DENIS-ISOGAL I-J/K-[15] diagram is the most suitable diagram to distinguish between early (AB) and late spectral types (KM). It provides the most useful tool to systematically search for nearby early-type stars with an infrared excess among the background of distant AGB stars in ISOGAL fields of the Galactic disk. This is paper No. 9 in a refereed journal based on data from the ISOGAL project. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. Based on observations collected at the European Southern Observatory, La Silla Chile (63.L-0319).