Determination of the far-infrared dust opacity in a prestellar core
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 − Ks colour excesses of background stars
from NTT/SOFI with the far-IR optical depth map, τFIR, 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 − Ks and τFIR
up to high extinctions (AV ~ 25). The
correlation translates to the average extinction ratio
A250 μm/AJ = 0.0014 ± 0.0002,
assuming a standard near-infrared extinction law and a dust emissivity
index β = 2. Using an empirical
NH/AJ
ratio we obtain an average absorption cross-section per H nucleus
of σH250 μm = (1.8 ± 0.3) × 10-25
cm
H-atom, corresponding to a cross-section per unit mass of gas κ250 μmg = 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 τFIR correctly, we find that the relationship
between H − Ks and
τFIR agrees with the recently determined relationship
between σH and NH 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.
Domains
Astrophysics [astro-ph]
Origin : Publication funded by an institution
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