Non-equilibrium chemistry in the dissipative structures of interstellar turbulence
K. Joulain, E. Falgarone, G. P. Des Forets, D. Flower;
AaA, 1998, 340, 241
ABSTRACT:We study the chemical evolution of low density gas trapped in a vortex, representative of the dissipative structures of turbulence in the diffuse medium.
A
magnetic field is present and is close to being aligned locally with the
vorticity.
The chemical evolution is concentrated within tiny regions (~ 10 AU) and
short timescales (<~ a few 100 yr).
It is controlled by the sharp temperature
rise following the passage through layers where viscous dissipation is
intense, and by the ion-neutral drift in layers where the tangential velocity of
the neutrals is large.
The facts that these two processes are closely
associated in space and time, and that the amount of energy available in the
dissipative structures is large, could explain, without fine-tuning the
parameters of the model, the salient features of the observations of molecular
species in diffuse gas: the large column densities of CH^+, OH and HCO^+, the
remarkable proportionality of the OH and HCO^+ column densities, the similarity
of the OH and HCO^+ (resp.
CH and CH^+) line centroids, and the fact that
the OH-rich gas seen in absorption is not always detected in emission.
A
large number of such vortices must be intercepted at any time on any line of
sight to reproduce the observed column densities but we show that less than
one percent of the gas column density need to be in those chemically active
regions.
The turbulent energy dissipated in all these structures is, on average,
smaller than that available in the turbulent cascade of the diffuse
medium.
Last, the dependence of our results on the gas density confirms that this
'hot' chemistry must arise in low density gas in order to meet the
requirements of the observations.
KEYWORDS: ism: evolution, ism: kinematics and dynamics, ism: molecules, turbulence
PERSOKEY:turbulence, h2, ,
CODE: joulain98