Near-IR spectra of ISOGAL sources in the Inner Galactic Bulge

Abstract

In this work we present near-IR spectra (HK-band) of a sample of 107 sources with mid-IR excesses at 7 and 15 $\mu$m 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 ${\mathrm{M}}_{\mathrm{bol}}$ vs.~${}^{12}\mathrm{CO}$ and ${\mathrm{M}}_{\mathrm{bol}}vs.{\mathrm{H}}_2\mathrm{O}$ 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 ${\mathrm{K}}_0-[15]$ relation by Jeong et al. (2002) based on recent theoretical modeling of dust formation of AGB stars to determine mass-loss rates. However, the uncertainties in the mass-loss rates are rather large. 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 $-5<\log \dot{M}<-7$. The red giant candidateslie at the lower end of the mass-loss rate range between $-6.5<\log \dot{M}<-9$. We used the equivalent width of the CO bandhead at 2.3 $\mu \mathrm{m}$, the NaI doublet and the CaI triplet to estimate metallicities using the relation by Ram\'ırez et al. (\cite{Ramirez2000}). The metallicity distribution of the ISOGAL objects shows a mean [Fe/H] $\sim$ -0.25 dex with a dispersion of $\pm 0.40dex$ which is in agreement with the values of Ramírez et al. (\cite{Ramirez2000}) for Galactic Bulge fields between $\mathrm{b}=-4^{\mathrm{o}}$ and $\mathrm{b}=-1.3^{\mathrm{o}}$. A comparison with the solar neighbourhood sample of Lançon & Wood (LW) shows that our sample is ~ 0.5 dex more metal-rich on average.

23 pages, 9 figures, 2 appendix with IR spectra. accepted for A&A