Determination of the far-infrared dust opacity in a prestellar core
  (abridged) We correlated near-infrared stellar H-Ks colour excesses of
background stars from NTT/SOFI with the far-IR optical depth map, tauFIR,
derived from Herschel 160, 250, 350, and 500 um 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. A linear correlation is seen between the colour H-Ks and tauFIR
up to high extinctions (AV ~ 25). The correlation translates to the average
extinction ratio A250um/AJ = 0.0014 +/- 0.0002, assuming a standard
near-infrared extinction law and a dust emissivity index beta=2. Using an
empirical NH/AJ ratio we obtain an average absorption cross-section per H
nucleus of sigmaH(250um) = (1.8 +/- 0.3) * 10^(-25) cm^2 / H-atom,
corresponding to a cross-section per unit mass of gas kappaG(250 um) = 0.08 +/-
0.01 cm^2 / 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 tauFIR correctly, we
find that the relationship between H-Ks and tauFIR agrees with the recently
determined relationship between sigmaH and NH in Orion A. 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 beng very
dense (the central density is likely to be ~10^5 cm^-3) and judging from
previous molecular line data, it appears to be at an early stage of chemical
evolution.
