[CII] emission from L1630 in the Orion B molecular cloud
  Observations towards L1630 in the Orion B molecular cloud, comprising the
iconic Horsehead Nebula, allow us to study the interplay between stellar
radiation and a molecular cloud under relatively benign conditions, that is,
intermediate densities and an intermediate UV radiation field. Contrary to the
well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher
conditions, L1630 has little star formation. We aim to relate the [CII]
fine-structure line emission to the physical conditions predominant in L1630
and compare it to studies of OMC1. The [CII] $158\,\mu\mathrm{m}$ emission from
an area of $12' \times 17'$ in L1630 was observed using the upGREAT instrument
onboard SOFIA. Of the [CII] emission from the mapped area 95%, $13\,L_{\odot}$,
originates from the molecular cloud; the adjacent HII region contributes only
5%, that is, $1\,L_{\odot}$. From comparison with other data (CO (1-0)-line
emission, far-infrared (FIR) continuum studies, emission from polycyclic
aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of
$n_{\mathrm{H}}\sim 3\cdot 10^3\,\mathrm{cm^{-3}}$, with surface layers,
including the Horsehead Nebula, having a density of up to $n_{\mathrm{H}}\sim
4\cdot 10^4\,\mathrm{cm^{-3}}$. The temperature of the surface gas is $T\sim
100\,\mathrm{K}$. The average [CII] cooling efficiency within the molecular
cloud is $1.3\cdot 10^{-2}$. The fraction of the mass of the molecular cloud
within the studied area that is traced by [CII] is only $8\%$. Our PDR models
are able to reproduce the FIR-[CII] correlations and also the CO (1-0)-[CII]
correlations. Finally, we compare our results on the heating efficiency of the
gas with theoretical studies of photoelectric heating by PAHs, clusters of
PAHs, and very small grains, and find the heating efficiency to be lower than
theoretically predicted, a continuation of the trend set by other observations.
