[C II] emission from L1630 in the Orion B molecular cloud

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, Netherlands
e-mail: pabst@strw.leidenuniv.nl
² ICMM-CSIC, Calle Sor Juana Ines de la Cruz 3, 28049 Cantoblanco, Madrid, Spain
³ Herschel Science Center, ESA/ESAC, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁴ CNRS, IRAP, 9 avenue Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁵ Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁶ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁷ I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
¹⁰ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France

Received: 28 March 2017
Accepted: 9 June 2017

Abstract

Context. L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ 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 μm line emission of L1630 around the Horsehead Nebula, an area of 12′ × 17′, 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_⊙, originates from the molecular cloud; the adjacent H ii region contributes only 5%, that is, 1 L_⊙. 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_H ~ 3 × 10³ cm^-3, with surface layers, including the Horsehead Nebula, having a density of up to n_H ~ 4 × 10⁴ cm^-3. The temperature of the surface gas is T ~ 100 K. The average [C ii] cooling efficiency within the molecular cloud is 1.3 × 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 − 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.