The photon dominated region in the IC 348 molecular cloud
I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: firstname.lastname@example.org
2 SRON Netherlands Institut for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
3 Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India
4 Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
Accepted: 17 July 2008
Aims. In this paper we discuss the physical conditions of clumpy nature in the IC 348 molecular cloud. We show that the millimeter and sub-millimeter line emission from the IC 348 molecular cloud can be modelled as originating from a photon dominated region (PDR).
Methods. We combine new observations of fully sampled maps in [C I] at 492 GHz and 12CO 4–3, taken with the KOSMA 3 m telescope at about 1´ resolution, with FCRAO data of 12CO 1–0, 13CO 1–0 and far-infrared continuum data observed by HIRES/IRAS. To derive the physical parameters of the region we analyze the line ratios of [C I] 3P1–3PCO 4–3, [C I]3P1–3PCO 1–0, and 12CO 4–3/12CO 1–0. A first rough estimate of abundance is obtained from an LTE analysis. To understand the [C I] and CO emission from the PDRs in IC 348, we use a clumpy PDR model. With an ensemble of identical clumps, we constrain the total mass from the observed absolute intensities. Then we apply a more realistic clump distribution model with a power law index of 1.8 for clump-mass spectrum and a power law index of 2.3 for mass-size relation.
Results. We provide detailed fits to observations at seven representative positions in the cloud, revealing clump densities between 4 104 cm-3 and 4 105 cm-3 and C/CO column density ratios between 0.02 and 0.26. The derived FUV flux from the model fit is consistent with the field calculated from FIR continuum data, varying between 2 and 100 Draine units across the cloud. We find that both an ensemble of identical clumps and an ensemble with a power law clump mass distribution produce line intensities which are in good agreement (within a factor ~ 2) with the observed intensities. The models confirm the anti-correlation between the C/CO abundance ratio and the hydrogen column density found in many regions.
Key words: ISM: clouds / ISM: dust, extinction / ISM: structure / ISM: individual objects: IC 348
© ESO, 2008