Dissipative structures of diffuse molecular gas

Abstract

Aims. Specific chemical signatures of the intermittent dissipation of turbulence were sought in diffuse molecular clouds. Methods. We observed HCO + (1−0) lines and the two lowest rotational transitions of 12 CO and 13 CO with an exceptional signal-tonoise 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 HCO + (1−0) lines (10 mK < T * A < 0.5 K). We achieve the interpretation of ten HCO + velocity components by conducting it in conjunction with that of the associated optically thin 13 CO emission. The derived HCO + column densities span a broad range, 10 11 < N(HCO + )/∆v < 4 × 10 12 cm −2 /km s −1 , and the inferred HCO + abundances, 2 × 10 −10 < X(HCO + ) < 10 −8 , 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 HCO + abundances, temperatures in the range of 100−200 K, and densities in the range 100-10 3 cm −3 , unambiguously belongs to the relaxation phase. In contrast, the kinematic properties of the gas suggest that the observed HCO + 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 HCO + , CH + , H 2 O, 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 H 2 O/OH abundance ratio stays close to the value measured in diffuse gas by the SWAS satellite, while the OH/HCO + ratio increases by more than one order of magnitude.

Time-dependent H2 formation and protonation in diffuse clouds

“…2) Dark clouds have haloes (Wannier et al 1993;Bensch 2006) and it has been suggested that the molecules supposedly seen in diffuse clouds are actually located around and exchanging material with dark clouds (Federman & Allen 1991); this may be true in some cases, but complex trace molecules are also seen along sightlines which are quite well separated from the nearest dark material . 3) Perhaps most promisingly, turnover within diffuse neutral gas due to turbulence, rapidly cycling material through a very dense phase, may provide for much faster formation of H 2 and trace species (Glover & Mac Low 2005;Falgarone et al 2005), establishing high molecular fractions from scratch on short timescales. The small fraction of neutral material which is observed to exist at very high pressure within diffuse clouds (Jenkins & Tripp 2001) and other aspects of small-scale structure in diffuse gas (Deshpande 2000) may perhaps also be understood in such terms.…”

Intermittency of interstellar turbulence: parsec-scale coherent structure of intense, velocity shear

“…These pure velocity-structures do not follow those of dense gas: they tend to be parallel to the magnetic field orientation, they are associated with gas warmer (T kin > 25 K) than the bulk of the gas, and they bear chemical signatures of a warm chemistry that is not driven by UV photons (Falgarone et al 2006;Godard et al 2009). In one of these E-CVI structures, Plateau de Bure Interferometre (PdBI) observations disclose several substructures of intense velocity-shear at scales as small as 6 milliparsec (mpc) (Falgarone et al 2009, hereafter FPH09).…”

First Detection of 13 CH + ( J = 1-0)

“…We thus conclude that the line arises in a gas phase In several respects, the existence of in the diffuse inter-ϩ CH stellar medium has remained for several decades among the most intractable puzzles in astrophysics (e.g., Bates & Spitzer 1951;Black & Dalgarno 1973;Black et al 1975;Falgarone et al 1995;Gredel et al 2002): needs H 2 to form but, once formed, it ϩ CH recombines rapidly with electrons or reacts with H 2 to form . However, the radiative association process…”

“…The alternative ϩ CH formation route, , is highly endothermic (Crane et al 1995;Gredel 1997), and their line widths, of only a few kilometers per second, unambiguously ascribe them to the CNM. The pivotal role of H 2 in the chemistry leading to its formation, but also to its destruction, confines to the layers of the CNM where ϩ CH the visual extinction is less than a few tenths, i.e., where the H 2 abundance is nonnegligible but not dominant (Falgarone et al 1995). Moreover, there would have to be some fraction of this medium that has gas temperatures much greater than the typical 100 K of the CNM in order to provide a sufficient production rate of , implying a nonthermal energy source of some kind.…”

“…n 0 a few thousand years after the blowup of the dissipative and heating event (Falgarone et al 2005a Vladilo & Centurion 1990;Hawkins et al 1993). The sample of stars is by nature different (lower extinction) from the sample of radio continuum sources against which and OH (or from space) are observed, so that the…”