Variation of the extinction law in the Trifid nebula
  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.
In this paper we seek variations of the extinction law within the Trifid nebula
region. Such variations would demonstrate a local dust evolution linked to
variation of the environment parameters such as the density or the interstellar
radiation field. The extinction values, A_lambda/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. We measured the extinction law at
3.6, 4.5, and 5.8 um 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 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. Our results
suggest a dust transition at large extinction which has not been reported so
far and dust emissivity variations.
