A Corona Australis cloud filament seen in NIR scattered light

Abstract

Context. Using state-of-art near-infrared (NIR) instrumentation the near-infrared light scattered from interstellar clouds can be mapped over large areas. Measurement of the surface brightness provides information on the line-of-sight dust column density. Scattered light therefore provides an important tool to study the mass distribution of quiescent, interstellar clouds at high, even subarcsecond resolution. Aims. We test the assumption that light scattering is the dominant contributor to the surface brightness in all NIR bands. Furthermore, we want to show that scattered light can be used for an accurate estimation of dust column densities in clouds with extinction in the range A V = 1-15 m . Methods. We have obtained NIR images of a quiescent filament in the Corona Australis molecular cloud. The observations provide maps of diffuse surface brightness in the J, H, and Ks photometric bands. Assuming that the main contributor is indeed scattered light, we convert surface brightness data into a map of dust column density. The same observations provide colour excesses for a large number of background stars. These data are used to derive an extinction map of the cloud. The two, largely independent tracers of the cloud structure are compared. Results. In regions where the extinction is below A V ∼ 15 m , both diffuse surface brightness and background stars lead to similar column density estimates. The existing differences can be explained as a result of normal observational errors and bias in the sampling of extinctions provided by the background stars. There is no indication that thermal dust-emission would have a significant contribution even in the Ks band. The results show that, below A V ∼ 15 mag , scattered light provides a reliable way to map cloud structure. Compared with the use of background stars, it can provide data of a significantly higher spatial resolution.

Mining the VVV: star formation and embedded clusters

“…Because of multiple scattering no scattered Galactic near-IR radiation field is expected (cf. Lehtinen & Mattila 1996;Juvela et al 2008) and the surface brightness must have a local source; for example objects 21 and 23 in Table 3 are small angular size localised spots seen in the H and K s bands. Reflected light from the general ambient Galactic interstellar radiation field would instead cause a broadly distributed surface brightness at the boundaries of the densest region.…”

Profiling filaments: comparing near-infrared extinction and submillimetre data in TMC-1

“…Padoan et al 2006;Juvela et al 2006Juvela et al , 2008. However, the observations of NIR surface brightness can be quite difficult.…”

“…studying mass distributions (see e.g. Padoan et al 2006;Juvela et al 2006Juvela et al , 2008. However, all techniques have their own drawbacks (see, e.g., Goodman et al 2009, for a comparison of several methods).…”

The degeneracy between dust colour temperature and spectral index

“…Many other tracers suffer from some serious problems, especially in the case of dense prestellar and protostellar cores. The interpretation of molecular line data is complicated by the abundance variations and the optical depth effects and very high-resolution maps of dust extinction and scattering are difficult to obtain (Lombardi et al 2006;Goodman et al 2009;Juvela et al 2008Juvela et al , 2009). …”