Variation of the extinction law in the Trifid nebula

TL;DR

This study investigates how the infrared extinction law varies within the Trifid nebula, revealing a transition at high extinction levels and suggesting local dust evolution linked to environmental factors.

Contribution

It provides the first detailed mapping of extinction law variations within the Trifid nebula, demonstrating a transition at A_V ~ 20 mag and linking it to dust evolution.

Findings

Extinction law is flatter above A_V ~ 20 mag.

A dust transition occurs at high extinction levels.

Millimeter emissivity is enhanced in dense cloud regions.

Abstract

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.