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

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 [CII] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods-The[CII] 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 [CII] emission from the mapped area 95%, 13 L ⊙ , originates from the molecular cloud; the adjacent HII 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 · 10 3 cm −3 , with surface layers, including the Horsehead Nebula, having a density of up to n H ∼ 4 · 10 4 cm −3 . The temperature of the surface gas is T ∼ 100 K. The average [CII] cooling efficiency within the pabst@strw.leidenuniv.nl. Europe PMC Funders GroupAuthor Manuscript Astron Astrophys. Author manuscript; available in PMC 2017 October 06. Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts molecular cloud is 1.3 · 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.Conclusions-In L1630 only a small fraction of the gas mass is traced by [CII]. Most of the [CII] 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 [CII] 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 [CII] emission in...

Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A

“…This improves on previous comparisons that used velocity unresolved observations of the 158 µm-[CII] line (Natta et al 1994). The velocity resolution of the 158 µm-[CII] line ob-servations in this study was of ≈ 50 km s −1 , while in the observations of Pabst et al (2019) this is 0.2 km s −1 . Additionally, we use models that describe the level population of carbon atoms including the effect of dielectronic capture (Salgado et al 2017a).…”

“…Here we compare the latest 158 µm-[CII] line maps of Pabst et al (2019) with the CRRLs observed in absorption. The cube of Pabst et al (2019) presents the 158 µm-[CII] line resolved in velocity and samples a region larger than that studied in CRRLs. With this we were able to perform a direct comparison between the lines over the same regions without making assumptions about their velocity structure.…”

“…These cover an area of roughly 20 × 5 (Dark Bay plus northern streamer), 3 .5 × 3 .5 (M43) and 10 × 8 (limb brightened Veil) on the sky. If we correct the column density for the effect of beam dilution we arrive at a value of (2.3 ± 0.4) × 10 18 cm −2 , between the value towards the Dark Bay (1.5×10 18 cm −2 ; Goicoechea et al 2015) and the limb brightened Veil (3.5 × 10 18 ; Pabst et al 2019).…”

“…This is similar to the cooling rate found through UV absorption studies towards diffuse clouds (Pottasch et al 1979;Gry et al 1992); however, the Veil is exposed to a radiation field ∼ 100 higher than the average ISRF. Given the geometry of the Veil, a large fraction of the 158 µm-[CII] emission comes from regions that are optically thick towards the observer (Pabst et al 2019, Figure 14). Thus, the cooling rate we derive is likely a lower limit.…”

“…The goal of this work is to re-evaluate the relation between the 158 µm- [CII] line and CRRLs at radio frequencies in the light of new models and observations of Orion A. We take advantage of new large-scale maps (≈ 1 deg 2 ) of the 158 µm-[CII] line in the FIR (Pabst et al 2019). This improves on previous comparisons that used velocity unresolved observations of the 158 µm-[CII] line (Natta et al 1994).…”