Variation of the extinction law in the Trifid nebula

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.

B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid nebula, one of the youngest star forming regions

“…Rho et al (2008) reviewed literature values between 1.68 and 2.84 kpc and adopted a distance of about 1.7 kpc. Cambrésy et al (2011) found 2.7 ± 0.5 kpc in their analysis of new near-and mid-infrared data. Torii et al (2011) used both 1.7 and 2.7 kpc in their discussion.…”

“…On the other hand, there are almost no close binaries amongst the magnetic intermediate-mass main-sequence Ap-type stars (e.g. Carrier et al 2002). Since this is expected if they were merger products, this argues for the binary hypothesis of the field origin.…”

Dust coagulation and fragmentation in molecular clouds

“…Recently, large scale surveys at nearand mid-IR wavelength (2MASS, UKIDSS and GLIMPSE) have provided new insights into the extinction behavior of dust deep inside dense clouds by probing the colors of background stars. These studies have revealed that the near-IR extinction law shows a flattening in dense cores, typically at depths corresponding to an A V of ∼20 mag (Indebetouw et al 2005;Chapman et al 2009;Cambrésy et al 2011) indicative of grain growth A&A 532, A43 (2011) (Weingartner & Draine 2001). Recently, too, near-IR scattered light has been detected from dense cloud cores and this is indicative of grain growth to micron-sizes Steinacker et al 2010).…”

Understanding star formation in molecular clouds

“…On the other hand, widefield extinction maps obtained by near-IR colorexcess techniques cover a wider range of column densities, typically from N(H + H 2 ) ∼ 0.1−40 × 10 21 cm −2 (Lada et al 1994;Lombardi & Alves 2001;Lombardi 2009;Cambrésy et al 2011Cambrésy et al , 2013Rowles & Froebrich 2009;Schneider et al 2011). Some major results obtained from these studies are, for example, that there is a relation between the rate of star formation and the amount of dense gas in molecular clouds , that a universal threshold in visual extinction of A v ≈ 6 mag for the formation of stars could exist (Froebrich & Rowles 2010), and that there are characteristic size scales in molecular clouds, indicating the dissipation and injection scales of turbulence (Schneider et al 2011).…”