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, tau(FIR), derived from Herschel 160, 250, 350, and 500 mu m 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 tau(FIR) up to high extinctions (A(V) similar to 25). The correlation translates to the average extinction ratio A(250 mu m)/A(J) = 0.0014 +/- 0.0002, assuming a standard near-infrared extinction law and a dust emissivity index beta = 2. Using an empirical N-H/A(J) ratio we obtain an average absorption cross-section per H nucleus of sigma(H)(250 mu m) = (1.8 +/- 0.3)x10(-25) cm(2) H-atom(-1), corresponding to a cross-section per unit mass of gas kappa(g)(250 mu m) = 0.08 +/- 0.01 cm(2) g(-1). 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 tau(FIR) correctly, we find that the relationship between H - K-s and tau(FIR) agrees with the recently determined relationship between sigma(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 similar to 10(5) cm(-3)), and judging from previous molecular line data, it appears to be at an early stage of chemical evolution.