Dissipative structures of diffuse molecular gas

Abstract

Aims. This paper belongs to a series of four, dedicated to the analysis of the dynamical, thermal and chemical properties of translucent molecular gas, with the perspective of characterizing the processes driving the dissipation of supersonic turbulence, an anticipated prerequisite of dense core formation. Methods. We analyze the small scale morphology and velocity structure of the parsec-scale environment of a low mass dense core (1 M� ). Our work is based on large maps made with the IRAM-30 m telescope in the two lowest rotational transitions of 12 CO and 13 CO with high angular (20 �� or 0.015 pc at 115 GHz) and spectral (0.055 km s −1 ) resolutions. The field is translucent, hence providing strong constraints on the column density and physical conditions in the gas. Results. More than one third of the field mass (6.5 M� ) lies in an elongated tail of dense and cold gas, possibly extending beyond the edge of the map and connected to the core in space and velocity. This core tail is highly turbulent and sub-structured into narrow filaments of aspect ratio up to 20. These are pure velocity structures with velocity shears in the range 2–10 km s −1 pc −1 . Another third of the mass, according to the weak extinction of the field, lies in more dilute molecular and atomic gas. Its molecular fraction, largely traced by optically thick 12 CO lines, is even more turbulent than the dense core tail. The gas emitting in the broad wings of the 12 CO lines is organized into a conspicuous network of narrow criss-crossed filaments, whose pattern at the parsec scale is seen for the first time. The gas there is optically thin in the 12 CO(1–0) line (τ12 < 0.2), warmer than 25 K and more dilute than 1000 cm −3 . These optically thin 12 CO-filaments, though contributing to about 10% of the mass of the environment, have a CO cooling rate a few times larger than that of the whole field on average. Whether dense or dilute, all the filamentary structures in the field (with transverse sizes 0.015–0.03 pc), are preferentially oriented along the direction of the magnetic fields, as measured a few parsecs away. Using the Chandrasekhar-Fermi method, we estimate the intensity of the magnetic fields intensity in the dilute molecular gas to be Bpos = 15 µG. We infer that the turbulent motions in the dilute gas are in the trans-Alfvenic range. –––