Atomic to molecular hydrogen transition in interstellar clouds

Federman, S. R.; Glassgold, A. E.; Kwan, John

doi:10.1086/156753

Cited by 159 publications

(116 citation statements)

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“…Usually run with homogeneous gas densities, the code can also accept input of density profiles via a text file. At each point along the line of sight, radiative transfer in the UV is treated to solve for the H/H 2 transition, using either the approximations by Federman et al (1979), or an exact method based on a spherical harmonics expansion of the specific intensity (Goicoechea & Le Bourlot 2007). A number of heating (photoelectric effect on grains, cosmic rays) and cooling (infrared and millimeter emission lines) processes contribute to the computation of thermal balance.…”

Section: The Meudon Pdr Codementioning

confidence: 99%

UV-driven chemistry in simulations of the interstellar medium

Levrier

Petit

Hennebelle

et al. 2012

A&A

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Context.Observations have long demonstrated the molecular diversity of the diffuse interstellar medium (ISM). Only now, with the advent of high-performance computing, does it become possible for numerical simulations of astrophysical fluids to include a treatment of chemistry, to faithfully reproduce the abundances of the many observed species, and especially that of CO, which is used as a proxy for molecular hydrogen. When applying photon-dominated region (PDR) codes to describe the UV-driven chemistry of uniform density cloud models, it is found that the observed abundances of CO are not well reproduced. Aims. Our main purpose is to estimate the effect of assuming uniform density on the line-of-sight in PDR chemistry models, compared to a more realistic distribution for which total gas densities may well vary by several orders of magnitude. A secondary goal of this paper is to estimate the amount of molecular hydrogen that is not properly traced by the CO (J = 1 → 0) line, the so-called "dark molecular gas". Methods. We used results from a magnetohydrodynamical (MHD) simulation as a model for the density structures found in a turbulent diffuse ISM with no star-formation activity. The Meudon PDR code was then applied to several lines of sight through this model, to derive their chemical structures.Results. We found that compared to the uniform density assumption, maximal chemical abundances for H 2 , CO, CH and CN are increased by a factor ∼2-4 when taking into account density fluctuations on the line of sight. The correlations between column densities of CO, CH and CN with respect to those of H 2 are also found to be in better overall agreement with observations. For instance, at N

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Section: The Meudon Pdr Codementioning

confidence: 99%

UV-driven chemistry in simulations of the interstellar medium

Levrier

Petit

Hennebelle

et al. 2012

A&A

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“…The energy of H 2 formation is assumed to be equally distributed between kinetic energy, internal energy of H 2 and heating of the grain. The radiative transfer in the absorption lines of H 2 and CO is treated in detail and the individual line profiles are described with the prescription of Federman et al (1979). For the dust extinction properties, we use the analytical fit of Fitzpatrick & Masse (1988) to the HD147889 star extinction curve.…”

Section: Pdr Modelmentioning

confidence: 99%

H₂infrared line emission across the bright side of theρOphiuchi main cloud

Habart¹,

Boulanger²,

Verstraete³

et al. 2002

A&A

100

143

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Abstract. We present imaging and spectroscopic observations of dust and gas (H 2 ) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ ∼ 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 ± 15 pc from the stellar parallax) and the layer of UV light penetration and of H 2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H 2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H 2 emitting layer, in which the ortho-to-para H 2 ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ([C/H] PAH 5 × 10 −5 ), we find that the H 2 formation rate R f must be high in warm gas (∼6 times the rate derived by Jura, 1975), in order to account for the observed H 2 emission. This result and the spatial coincidence between the PAHs and H 2 emission suggest that H 2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in R f may also result from an increased PAH abundance: assuming that R f scales with the PAH abundance, the observed H 2 excitation is well explained with R f 1 × 10 −16 cm 3 s −1 at T gas = 330 K (∼3 times the rate derived by Jura 1975) and [C/H] PAH 7.5 × 10 −5 .

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“…The radiative transfer is solved in decoupling the continuum extinction due to dust and gas and the absorption in lines of H 2 , HD and CO. In addition, we use the approximation introduced by Federman et al (1979) to estimate the self-absorption in the dissociating lines of H 2 , HD and CO, and we neglect the overlap between the lines of H 2 and other molecules. This approximation reproduces the main physical properties as shown by Abgrall et al (1992) and allows rapid computations.…”

Section: General Featuresmentioning

confidence: 99%

D/HD transition in Photon Dominated Regions (PDR)

Petit¹,

Roueff²,

Bourlot³

2002

A&A

128

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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...

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Atomic to molecular hydrogen transition in interstellar clouds

Cited by 159 publications

References 0 publications

UV-driven chemistry in simulations of the interstellar medium

UV-driven chemistry in simulations of the interstellar medium

H₂infrared line emission across the bright side of theρOphiuchi main cloud

D/HD transition in Photon Dominated Regions (PDR)

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Atomic to molecular hydrogen transition in interstellar clouds

Cited by 159 publications

References 0 publications

UV-driven chemistry in simulations of the interstellar medium

UV-driven chemistry in simulations of the interstellar medium

H2infrared line emission across the bright side of theρOphiuchi main cloud

D/HD transition in Photon Dominated Regions (PDR)

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H₂infrared line emission across the bright side of theρOphiuchi main cloud