Extinction at 7 μm and 15 μm 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 moderate 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 extinctions around 7 micron (A 7 ) is well represented by a Gaussian function, with the peak at about 0.47A KS and ranging from 0.33 to 0.55A KS . There is some evidence that A 7 /A KS 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 index (Ks-[15]) 0 which is affected by dust emission from mass-losing AGB stars. The peak value of A 15 is around 0.40A KS .