The Physical Structure of a Prestellar Core in IC 5146

We observed an evenly sampled map of the 3 P 1-3 P 0 line of neutral atomic carbon in a 0.5 x 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 C I 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%.