Dissipative structures of diffuse molecular gas

I. Broad HCO⁺ (J = 1–0) emission

¹ LERMA/LRA, CNRS - UMR 8112, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France e-mail: edith.falgarone@lra.ens.fr
² IAS, CNRS - UMR 8617, Université Paris-Sud, 91405 Orsay, France
³ LUTH, CNRS - UMR 8102, Observatoire de Paris, 92195 Meudon, France
⁴ IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
⁵ MPIfR, Auf den Hügel 69, 53121 Bonn, Germany

Received: 22 August 2005
Accepted: 23 January 2006

Abstract

Aims.Specific chemical signatures of the intermittent dissipation of turbulence were sought in diffuse molecular clouds.

Methods.We observed (1-0) lines and the two lowest rotational transitions of  and  with an exceptional signal-to-noise ratio in the translucent environment of low-mass dense cores, where turbulence dissipation is expected to take place. Some of the observed positions belong to a new kind of small-scale structure identified in CO(1-0) maps of these environments as the locus of non-Gaussian velocity shears in the statistics of their turbulent velocity field, i.e. singular regions generated by the intermittent dissipation of turbulence.

Results.We report the detection of broad (1-0) lines ( K). We achieve the interpretation of ten  velocity components by conducting it in conjunction with that of the associated optically thin  emission. The derived  column densities span a broad range, () /km s^-1, and the inferred  abundances, () , are more than one order of magnitude above those produced by steady-state chemistry in gas that is weakly shielded from UV photons, even at large densities. We compare our results with predictions of non-equilibrium chemistry, swiftly triggered in bursts of turbulence dissipation and followed by a slow thermal and chemical relaxation phase, assumed to be isobaric. The set of values derived from observations, i.e. large  abundances, temperatures in the range of 100-200 K, and densities in the range 100–10³ , unambiguously belongs to the relaxation phase. In contrast, the kinematic properties of the gas suggest that the observed  line emission results from a space-time average in the beam of the whole cycle followed by the gas and that the chemical enrichment is made at the expense of the non-thermal energy. Last, we show that the “warm chemistry” signature (i.e. large abundances of , , , and OH) acquired by the gas within a few hundred years, which is the duration of the impulsive chemical enrichment, is kept over more than a thousand years. During the relaxation phase, the /OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/ ratio increases by more than one order of magnitude.