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

¹ Institut d'Astrophysique de Paris, CNRS, 98bis Bld. Arago, 75014 Paris, France
² Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
⁵ Physical Research Laboratory, Navarangpura, Ahmedabad 380009, India
⁶ DASGAL CNRS UMR 8633, Observatoire de Paris, France

Corresponding author: M. Schultheis, schulthe@iap.fr

Received: 8 August 2001
Accepted: 22 February 2002

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 μm 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 –[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.