Context : L1630 in the Orion B molecular cloud , which includes the iconic Horsehead Nebula , illuminated by the star system \sigma Ori , is an example of a photodissociation region ( PDR ) .
In PDRs , stellar radiation impinges on the surface of dense material , often a molecular cloud , thereby inducing a complex network of chemical reactions and physical processes . Aims : Observations toward L1630 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 .
Our goal is to relate the [ C ii ] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1 . Methods : The [ C ii ] 158 \mu \mathrm { m } line emission of L1630 around the Horsehead Nebula , an area of 12 \arcmin \times 17 \arcmin , was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies ( upGREAT ) onboard the Stratospheric Observatory for Infrared Astronomy ( SOFIA ) . Results : Of the [ C ii ] emission from the mapped area 95 % , 13 L _ { \sun } , originates from the molecular cloud ; the adjacent H ii region contributes only 5 % , that is , 1 L _ { \sun } .
From comparison with other data ( CO ( 1 \mathchar 45 \relax 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 [ C ii ] 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 [ C ii ] is only 8 \% .
Our PDR models are able to reproduce the FIR- [ C ii ] correlations and also the CO ( 1 \mathchar 45 \relax 0 ) - [ C ii ] 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 . Conclusions : In L1630 only a small fraction of the gas mass is traced by [ C ii ] .
Most of the [ C ii ] emission in the mapped area stems from PDR surfaces .
The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions .
From our study , we conclude that the relation between [ C ii ] emission and physical conditions is likely to be more complicated than often assumed .
The theoretical heating efficiency is higher than the one we calculate from the observed [ C ii ] emission in the L1630 molecular cloud .