D/HD transition in Photon Dominated Regions (PDR)

Abstract

Abstract. We present the basic features of a steady state chemical model of Photon Dominated Regions (PDR), where the deuterium chemistry is explicitly introduced. The model is an extension of a previous PDR model (Abgrall et al. 1992;Le Bourlot et al. 1993; in which the microscopic processes relative to HD have been incorporated. The J-dependent photodissociation probabilities have been calculated and included in the statistical equilibrium of the rotational levels of HD where the latest collision molecular data are also introduced. The thermal balance is calculated from the equilibrium between the different heating and cooling processes. We introduce a standard model of density n H = 500 cm −3 embedded in the Interstellar Standard Radiation Field (ISRF) from which we derive the main properties of HD in PDR. The D/HD transition does not depend only on the density, radiation field but also on the chemical processes and especially on the dust formation efficiency. In standard radiation field conditions, the D/HD transition occurs in a narrow range of visual extinctions as long as density is less than 1000 cm −3 and HD is formed through the D + + H 2 reaction. At higher densities a logarithmic dependence of the location of the transition is derived. The model is applied both to ultraviolet absorption observations from the ground rotational state of HD performed in diffuse and translucent clouds and infrared emission detectable at high densities and for high ultraviolet radiation fields coming from the bright surrounding stars.Key words. astrochemistry -molecular processes -ISM: molecules IntroductionSince deuterium has been formed only at the early beginning of the universe, the elemental deuterium to hydrogen ratio is one pivotal parameter to understand the evolution of astrophysical media. In the interstellar medium (ISM), deuterium may be present in atomic but also in molecular form and over 20 single D-bearing molecules and two doubly deuterated ones have been found in cold molecular clouds and star forming regions Loinard et al. 2000). Last but not least, the triply deuterated ammonia has been detected towards two dense cold clouds (Lis et al. 2002;van der Tak et al. 2002). However, chemical fractionation processes and mantle desorption from grains take place in these molecular clouds and it is not straightforward to derive the elemental deuterium abundance from such observations (Roberts & Millar 2000a,b).Atomic deuterium has first been observed with the Copernicus satellite in the local interstellar medium and in diffuse clouds in absorption towards bright stars where HD has also been detected (see Lemoine et al. 1999 for a review). The successful launch of the FUSE (Far Ultraviolet Spectroscopy Explorer) mission allows to search for molecular HD towards fainter sources, in translucent clouds which are intermediate between diffuse and molecular clouds. FUSE detections Send offprint requests to: E. Roueff, e-mail: Evelyne.Roueff@obspm.fr of interstellar HD have been reported in a variety of galactic lines o...

Incorporation of stochastic chemistry on dust grains in the Meudon PDR code using moment equations

“…The resulting formulae are very similar to the pure molecular hydrogen case. Following previous considerations by both Watson (1974) and Dalgarno, Black, & Weisheit (1973), Le Petit et al (2002) found that HD is far more efficiently formed in the gas phase, in a succession of reactions initiated by cosmic ray ionization of atomic hydrogen. When protons are formed, the sequence of reactions involved is the following:…”

“…If the exponent of the power law is q = 3.5, analytical formulae are obtained as derived by Le Bourlot et al (1995). Le Petit et al (2002) specifically studied the D/HD transition occurring at the edge of a translucent cloud and in dense PDRS. They also studied the formation of HD molecules on the surfaces of dust grains, following the procedure of Le Bourlot et al (1995).…”

“…In this paper, we incorporate the moment equations into the Meudon PDR code (Le Petit et al 2002), a stationary model of Photon Dominated Regions (PDRs).…”

ALMA detection of [C ii] 158μm emission from a strongly lensedz= 2.013 star-forming galaxy

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Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions

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