Determination of the far-infrared dust opacity in a prestellar core^⋆

¹ Department of Physics and AstronomyThe Open University, Walton Hall Milton Keynes, MK7 6AA, UK
e-mail: aleksi.suutarinen@gmail.com
² Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
³ Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁴ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁵ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁶ Institut d’Astrophysique Spatiale, UMR 8617, CNRS/Université Paris-Sud 11, 91405 Orsay, France
⁷ RAL Space, STFC Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire OX11 0QX, UK

Received: 23 February 2012
Accepted: 11 June 2013

Abstract

Context. Mass estimates of interstellar clouds from far-infrared and submillimetre mappings depend on the assumed dust absorption cross-section for radiation at those wavelengths.

Aims. The aim is to determine the far-IR dust absorption cross-section in a starless, dense core located in Corona Australis. The value is needed for determining of the core mass and other physical properties. It can also have a bearing on the evolutionary stage of the core.

Methods. We correlated near-infrared stellar H − K_s colour excesses of background stars from NTT/SOFI with the far-IR optical depth map, τ_FIR, derived from Herschel 160, 250, 350, and 500 μm data. The Herschel maps were also used to construct a model for the cloud to examine the effect of temperature gradients on the estimated optical depths and dust absorption cross-sections.

Results. A linear correlation is seen between the colour H − K_s and τ_FIR up to high extinctions (A_V ~ 25). The correlation translates to the average extinction ratio A_250  μm/A_J = 0.0014 ± 0.0002, assuming a standard near-infrared extinction law and a dust emissivity index β = 2. Using an empirical N_H/A_J ratio we obtain an average absorption cross-section per H nucleus of σ^H_{250 μm} = (1.8 ± 0.3) × 10^-25  cm H-atom, corresponding to a cross-section per unit mass of gas κ_{250 μm}^g = 0.08 ± 0.01  cm g. The cloud model, however, suggests that owing to the bias caused by temperature changes along the line-of-sight, these values underestimate the true cross-sections by up to 40% near the centre of the core. Assuming that the model describes the effect of the temperature variation on τ_FIR correctly, we find that the relationship between H − K_s and τ_FIR agrees with the recently determined relationship between σ^H and N_H in Orion A.

Conclusions. The derived far-IR cross-section agrees with previous determinations in molecular clouds with moderate column densities, and is not particularly large compared with some other cold cores. We suggest that this is connected to the core not being very dense (the central density is likely to be  ~10⁵ cm), and judging from previous molecular line data, it appears to be at an early stage of chemical evolution.