A Corona Australis cloud filament seen in NIR scattered light

II. Comparison with sub-millimeter data

Helsinki University Observatory, 00014, University of Helsinki, Finland e-mail: mika.juvela@helsinki.fi

Received: 6 June 2009
Accepted: 21 July 2009

Abstract

Context. Dust emission and scattering provide tools for the study of interstellar clouds. Detailed knowledge of grain properties is essential for correct analysis of these observations. However, dust properties are suspected to change from diffuse to dense regions. The changes could affect our interpretation of observations from large scales (e.g., clump mass spectra) down to the structure of individual cores.

Aims. We study a northern part of the Corona Australis molecular cloud that consists of a filament and a dense sub-millimetre core inside the filament. Our aim is to measure dust temperature and sub-mm emissivity within the region. We also look for confirmation that near-infrared (NIR) surface brightness can be used to study the structure of even very dense clouds.

Methods. We extend our previous NIR mapping south of the filament. The dust colour temperatures are estimated using Spitzer 160 μm and APEX/Laboca 870 μm maps. The column densities derived based on the reddening of background stars, NIR surface brightness, and thermal sub-mm dust emission are compared. A three dimensional toy model of the filament is used to study the effect of anisotropic illumination on near-infrared surface brightness and the reliability of dust temperature determination.

Results. Relative to visual extinction, the estimated emissivity at 870 μm is   10^-5 . This is similar to the values found in diffuse medium. A significant increase in the sub-millimetre emissivity seems to be excluded. In spite of saturation, NIR surface brightness was able to accurately pinpoint, and better than measurements of the colour excesses of background stars, the exact location of the column density maximum. Both near- and far-infrared data show that the intensity of the radiation field is higher south of the filament.