Extinction at 7  m and 15  m from the ISOGAL survey

¹ Department of Astronomy, Beijing Normal University, Beijing 100875, PR China e-mail: bjiang@bnu.edu.cn
² Institut d'Astrophysique de Paris, CNRS, 98 bis Bd Arago, 75014 Paris, France
³ GEPI, Observatoire de Paris, France

Received: 24 May 2005
Accepted: 14 September 2005

Abstract

The extinction laws at 7 m and 15 m 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 which are RGB tip stars or early AGB stars with moderate mass loss. They have well-defined intrinsic color indices (s)₀, (Ks)₀ and (Ks)₀. By a linear fitting of the observed color indices Ks and Ks to the observed s, we obtain the ratio between the E(Ks) and E(Ks) color excesses and E(s). We infer the selective extinctions at 7 and 15 m 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 0.47 and ranging from 0.33 to 0.55. There is some evidence that A₇/ may vary significantly depending on the line of sight. The derived selective extinction at 15 m suffers uncertainty mainly from the dispersion in the intrinsic color index (Ks)₀ which is affected by dust emission from mass-losing AGB stars. The peak value of is around 0.40.