Extinction at 7 ${\rm \mu}$m and 15 ${\rm \mu}$m from the ISOGAL survey

The extinction laws at 7µm and 15µm are derived for more than 120 sightlines in the inner Galactic plane based on the ISOGAL survey data and the near-infrared data from DENIS and 2MASS. The tracers are the ISOGAL point sources with (7)-(15)<0.4 which are RGB tip stars or early AGB stars with moder- ate mass loss. They have well-defined intrinsic color indices (J-Ks)0, (Ks-(7))0 and (Ks-(15))0. By a linear fitting of the observed color indices Ks-(7) and Ks-(15) to the observed J-Ks, we obtain the ratio between the E(Ks-(7)) and E(Ks-(15)) color excesses and E(J-Ks). We infer the selective extinctions at 7 and 15µm in terms of the near-infrared extinction in the Ks band. The distribution of the derived extinc- tions around 7 micron (A7) is well represented by a Gaussian function, with the peak at about 0.47AKS and ranging from 0.33 to 0.55AKS (using the near-infrared extinctions of Rieke & Lebovsky 1985). There is some evidence that A7/AKs may vary significantly depending on the line of sight. The derived selective extinction at 15µm suffers uncertainty mainly from the dispersion in the intrinsic color in- dex (Ks-(15))0 which is affected by dust emission from mass-losing AGB stars. The peak value of A15 is around 0.40AKS.