Dissipative structures of diffuse molecular gas^*

II. The translucent environment of a dense core

¹ LRA-LERMA, 24 rue Lhomond, École normale supérieure (Observatoire de Paris), 75231 Paris Cedex 05, France e-mail: hilyblan@iram.fr
² IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France

Received: 22 November 2006
Accepted: 21 February 2007

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 ). Our work is based on large maps made with the IRAM-30 m telescope in the two lowest rotational transitions of ¹²CO and ¹³CO with high angular (20'' or 0.015 pc at 115 GHz) and spectral (0.055 kms 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 ) 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 kms 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 ¹²CO lines, is even more turbulent than the dense core tail. The gas emitting in the broad wings of the ¹²CO 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 ¹²CO(1–0) line , warmer than 25 K and more dilute than 1000 cm^-3. These optically thin ¹²CO-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 B_pos = 15 μg . We infer that the turbulent motions in the dilute gas are in the trans-Alfvénic range.

Conclusions.The 1  dense core is surrounded by a translucent and highly turbulent environment whose gas dynamics are not super-Alfvénic. The low mass dense core is not isolated but still connected to a massive reservoir of dense gas. Filaments of optically thin ¹²CO are found to radiate more efficiently in the CO lines than the whole field on average. These are the structures that we tentatively identify with the locus of intermittent dissipation of turbulence, and for which there is no observational evidence that they are shocks.