CB 17: Inferring the Dynamical History of a Prestellar Core with Chemodynamical Models

Abstract

We present a detailed theoretical study of the isolated Bok globule CB 17 (L1389) based on spectral maps of CS, HCO+, C18O, C34S, and H13CO+ lines. The intensity of the external UV field, the probability for molecules to stick onto dust grains, the core age, the infall, and rotation velocity all significantly affect the molecular line spectra. We demonstrate that these parameters are well constrained when results of the modeling are compared to observations in multiple lines of sight through the core. We use a detailed chemical model to compute the time-dependent abundances in a number of locations within the core. Both static and dynamically evolving cloud configurations are considered. These abundances are then used to simulate the spectral maps. We developed a general criterion that allows us to quantify the difference between observed and simulated spectral maps. By minimizing this difference, we isolate the model that represents a good approximation to the core chemical and kinematic structure. The chemical age of the core is about 2 Myr, while the most probable effective sticking probability value is 0.3-0.5. The spatial distribution of intensities and self-absorption features of optically thick lines is indicative of attenuated UV radiation of the core. The line asymmetry pattern in CB 17 is reproduced by a combination of infall, rotation, and turbulent motions with velocities of ~0.05, ~0.1, and ~0.1 km s-1, respectively. These parameters correspond to energy ratios Erot/Egrav ≈ 0.03, Etherm/Egrav ≈ 0.8, and Eturb/Egrav ≈ 0.05 (the rotation parameters are determined for i = 90°). Based on the angular momentum value, we argue that the core is going to fragment, i.e., to form a binary (multiple) system.