Variation of the extinction law in the Trifid nebula

¹ Observatoire astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg France
e-mail: cambresy@astro.unistra.fr
² Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
³ Institut de Recherche en Astrophysique et Planétologie de Toulouse (IRAP), Université Paul Sabatier, CNRS, UMR 7095, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
e-mail: Douglas.Marshall@cesr.fr
⁴ Stratospheric Observatory for Infrared Astronomy, Universities Space Research Association, Mail Stop 211-3, Moffett Field, CA 94035, USA
e-mail: [jrho;wreach]@sofia.usra.edu

Received: 4 October 2010
Accepted: 21 December 2010

Abstract

Context. In the past few years, the extinction law has been measured in the infrared wavelengths for various molecular clouds and different laws have been obtained.

Aims. In this paper we seek variations of the extinction law within the Trifid nebula region. Such variations would demonstrate local dust evolution linked to variation of the environment parameters such as the density or the interstellar radiation field.

Methods. The extinction values, A_λ/A_V, are obtained using the 2MASS, UKIDSS and Spitzer/GLIMPSE surveys. The technique is to inter-calibrate color-excess maps from different wavelengths to derive the extinction law and to map the extinction in the Trifid region.

Results. We measured the extinction law at 3.6, 4.5, and 5.8 μm and we found a transition at A_V ≈ 20 mag. Below this threshold the extinction law is as expected from models for R_V = 5.5 whereas above 20 mag of visual extinction, it is flatter. Using these results the color-excess maps are converted into a composite extinction map of the Trifid nebula at a spatial resolution of 1 arcmin. A tridimensional analysis along the line-of-sight allowed us to estimate a distance of 2.7 ± 0.5 kpc for the Trifid. The comparison of the extinction with the 1.25 mm emission suggests the millimeter emissivity is enhanced in the dense condensations of the cloud.

Conclusions. Our results suggest a dust transition at large extinction which has not been reported so far as well as dust emissivity variations.