The Physical Structure of a Prestellar Core in IC 5146

Abstract

We observed an evenly sampled map of the ³ P ₁-³ P ₀ line of neutral atomic carbon in a 0.5 × 0.5 pc² quiescent, dark core region of IC 5146 immersed in the interstellar radiation field. For comparison at the same spatial resolution of 0.067 pc, we mapped in addition the emission of ¹³CO 2-1, and, previously, of C¹⁸O 2-1 and A _V . The evenly sampled map of A _V was created by Lada et al. [14] from NIR pencil beam observations towards background stars. We find that Ci and ¹³CO 2-1 line intensities are well correlated reflecting their similar line shapes. In contrast to C¹⁸O, their integrated intensities stay almost constant with extinctions and are poorly correlated. We argue that this is indicative of unresolved small scale structure and use a spherical symmetric model of photon dominated regions (PDRs) to study the emergent line intensities. The observations are consistent with a large number of small clumps having radii of e.g. 760 AU, masses of 10⁻³ M_⊙ and surface densities of 10⁵ cm⁻³. The volume filling factor is ~ 10%.