Near-IR spectra of ISOGAL sources in the inner galactic bulge

Abstract

We present near-IR spectra (HK-band) of a sample of 107 sources with mid-IR excesses at 7 and 15 µm detected during the ISOGAL survey. Making use of the DENIS interstellar extinction map from Schultheis et al. (1999) we derive luminosities and find that the M_bol vs. ¹²CO and M_bol vs. H₂O diagrams are powerful tools for identifying supergiants, AGB stars, giants and young stellar objects. The majority of our sample are AGB stars (∼80%) while we find four good supergiant candidates, nine young stellar objects and 12 RGB candidates. We have used the most recent K₀ − [15] relation by Jeong et al.(2002) based on recent theoretical modeling of dust formation of AGB stars to determine mass-loss rates. The mass-loss rates of the supergiants are comparable with those in the solar neighbourhood while the long-period variables cover a mass-loss range from −5 < log M˙ < −7. The red giant candidates lie at the lower end of the mass-loss rate range between −6.5 < log M˙ < −9. We used the equivalent width of the CO bandhead at 2.3 µm, the NaI doublet and the CaI triplet to estimate metallicities using the relation by Ramırez et al. (2000b). The metallicity distribution of the ISOGAL objects shows a mean [Fe/H] ∼ −0.25 dex with a dispersion of ±0.40 dex which is in agreement with the values of Ramirez et al. (2000b) for Galactic Bulge fields between b = −4° and b = −1.3°. A comparison with the solar neighbourhood sample of Lancon & Wood (2000) shows that our sample is ∼0.5 dex more metal-rich on average.