Study of Photon Dominated Regions in Cepheus B

Abstract

Aim: The aim of the paper is to understand the emission from the photon dominated regions in Cepheus B, estimate the column densities of neutral carbon in bulk of the gas in Cepheus B and to derive constraints on the factors which determine the abundance of neutral carbon relative to CO. Methods: This paper presents 15'x15' fully sampled maps of CI at 492 GHz and 12CO 4-3 observed with KOSMA at 1' resolution. The new observations have been combined with the FCRAO 12CO 1-0, IRAM-30m 13CO 2-1 and C18O 1-0 data, and far-infrared continuum data from HIRES/IRAS. The KOSMA-tau spherical PDR model has been used to understand the CI and CO emission from the PDRs in Cepheus B and to explain the observed variation of the relative abundances of both C^0 and CO. Results: The emission from the PDR associated with Cepheus B is primarily at V_LSR between -14 and -11 km s^-1. We estimate about 23% of the observed CII emission from the molecular hotspot is due to the ionized gas in the HII region. Over bulk of the material the C^0 column density does not change significantly, (2.0+-1.4)x10^17 cm^-2, although the CO column density changes by an order of magnitude. The observed \cbyco abundance ratio varies between 0.06 and 4 in Cepheus B. We find an anti-correlation of the observed C/CO abundance ratio with the observed hydrogen column density, which holds even when all previous observations providing C/CO ratios are included. Here we show that this observed variation of C/CO abundance with total column density can be explained only by clumpy PDRs consisting of an ensemble of clumps. At high H2 column densities high mass clumps, which exhibit low C/CO abundance, dominate, while at low column densities, low mass clumps with high C/CO abundance dominate.

12 pages, 10 figures, Accepted for publication in A&A

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• The Influence of Far-Ultraviolet Radiation on the Properties of Molecular Clouds in the 30 Dor Region of the Large Magellanic Cloud
• Submillimeter Line Emission from LMC N159W: a Dense, Clumpy PDR in a Low Metallicity Environment
• Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields
• 12CO 4-3 and [CI] 1-0 at the centers of NGC 4945 and Circinus
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• The Photon Dominated Region in the IC 348 molecular cloud
• The structure of hot gas in Cepheus B
• Searching for spatial variations of alpha^2/mu in the Milky Way
• Photon Dominated Regions in NGC 3603
• Distribution of Water Vapor in Molecular Clouds. II
• Spectroscopic [CI] mapping of the infrared dark cloud G48.65-0.29
• Radiation-driven Implosion in the Cepheus B Molecular Cloud