2006A&A...446..551J

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 (A₇) 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 A₇/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 A₁₅ is around 0.40A_Ks.