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Issue A&A
Volume 446, Number 2, February I 2006
Page(s) 551 - 560
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20053501

A&A 446, 551-560 (2006)
DOI: 10.1051/0004-6361:20053501

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

B. W. Jiang1, J. Gao1, A. Omont2, F. Schuller2 and G. Simon3

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 ${\rm\mu}$m and 15 ${\rm\mu}$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 [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 15 ${\rm\mu}$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 $A_{\rm K_{S}}$ and ranging from 0.33 to 0.55AKS. There is some evidence that $A_{\rm 7}$/ $A_{K{\rm s}}$ may vary significantly depending on the line of sight. The derived selective extinction at 15 ${\rm\mu}$m 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 $A_{\rm 15}$ is around 0.40 $A_{K_{\rm S}}$.


Key words: stars: AGB and post-AGB -- stars: late-type -- stars: mass loss -- ISM: dust, extinction -- infrared: ISM -- infrared: stars

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