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

Petit, Franck; Barzel, Baruch; Biham, Ofer; Roueff, E.; Bourlot, J. Le

doi:10.1051/0004-6361/200912269

Cited by 32 publications

(34 citation statements)

References 39 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…As shown by many authors, this formation mechanism can only take place in a small window of grain temperatures (Biham & Lipshtat 2002;Biham et al 2005;Le Petit et al 2009), namely 10-20 K for amorphous carbons. As a consequence, the LH mechanism becomes the major H 2 formation process at moderate visual extinctions.…”

Section: Discussionmentioning

confidence: 94%

“…Le Petit et al (2009) included the possibility of the rejection of the impinging hydrogen atoms, when they land on an already occupied site, and introduced these mechanisms into the Meudon PDR code together with the size distribution of grains, while assuming a uniform dust temperature for all grain sizes. The net result, for H 2 formation, is that the reaction is only efficient for a narrow window of dust temperatures, ranging typically from 11 K to 19 K, depending on the values of desorption and diffusion barriers, as also pointed out by Chang et al (2006).…”

Section: H 2 Formation Efficiencymentioning

confidence: 99%

“…We investigate the possible consequences of this result in our present study. Le Petit et al (2009) studied the efficiency of H 2 formation for different dust temperatures when H 2 formation takes place on amorphous carbon, assuming that the grains have the same temperature, independently of their size. This hypothesis is inadequate as discussed by different authors .…”

Section: H 2 Excitation In the Formation Processmentioning

confidence: 99%

See 2 more Smart Citations

Surface chemistry in the interstellar medium

Bourlot

Petit

Pinto

et al. 2012

A&A

Self Cite

154

183

View full text Add to dashboard Cite

Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H 2 , can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the observed formation rates of H 2 , which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims. We attempt to include as much experimental and theoretical information as possible to describe H 2 formation in astrophysical environments to solve this problem. Methods. We modified our "Meudon PDR code" to include a detailed treatment of H 2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results. We are able to form H 2 even in regions where the dust temperature is higher than 25 K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions. Our approach results in a higher H 2 formation rate than for the "standard" 3 × 10 −17 n H n(H) cm 3 s −1 expression.

show abstract

Section: Discussionmentioning

confidence: 94%

Section: H 2 Formation Efficiencymentioning

confidence: 99%

Section: H 2 Excitation In the Formation Processmentioning

confidence: 99%

See 1 more Smart Citation

Surface chemistry in the interstellar medium

Bourlot

Petit

Pinto

et al. 2012

A&A

Self Cite

154

183

View full text Add to dashboard Cite

show abstract

“…This mechanism has been studied in detail using the master equation approach (Biham & Lipshtat 2002), the moment equation approach (Lipshtat & Biham 2003;Le Petit et al 2009) and Monte Carlo simulations (Chang et al 2005;Chang et al 2006). Laboratory experiments have also been conduced and their results modeled (Pirronello et al 1997a(Pirronello et al ,b, 1999Katz et al 1999).…”

Section: Introductionmentioning

confidence: 99%

Surface chemistry in the interstellar medium

Bron

Bourlot

Petit

2014

A&A

118

View full text Add to dashboard Cite

Context. The H 2 formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption. Aims. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the H 2 formation rate under the full range of astrophysically relevant UV intensities and gas conditions. Methods. The master equation approach is generalized to coupled fluctuations in both the grain's temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed. Results. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under the most astrophysically relevant conditions. Typical differences are a factor of 2−3 on the intensities of the H 2 v = 0 lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency. Conclusions. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.

show abstract

“…Even so, approximations are needed to render the approach more tractable for large-scale calculations: these include using the master equation only for reactive surface species of small abundance (Stantcheva & Herbst 2004;Acharyya et al 2005) and using the method of moments (Barzel & Biham 2007). Although useful for H 2 formation (Le Petit et al 2009) and larger systems of surface equations, the latter approximation is still under development for systems coupled to gas-phase chemistry because of mathematical instabilities. Recently, so-called hybrid methods have been used to stabilize the moment equation approach (Du & Parise 2011).…”

Section: Introductionmentioning

confidence: 99%

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Iqbal

Acharyya

Herbst

2012

ApJ

View full text Add to dashboard Cite

We have used the continuous-time random-walk Monte Carlo technique to study the formation of H 2 from two hydrogen atoms on the surface of interstellar dust grains with both physisorption and chemisorption sites on olivine and carbonaceous material. In our standard approach, atoms must first enter the physisorption site before chemisorption can occur. We have considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. The temperature range between 5 K and 825 K has been explored for different incoming H fluxes representative of interstellar environments with atomic hydrogen number density ranging between 0.1 cm −3 and 100 cm −3 and dust grain sizes ranging from 100 sites to 10 6 sites, the latter corresponding roughly to olivine grains of radius 0.2 μm. In addition, we have also considered rough surfaces with multiple binding sites. Tunneling is found to dominate the surface chemistry at low temperature, but as the temperature increases, the scenario changes. The inclusion of chemisorption sites can provide a meaningful efficiency for H 2 production up to temperatures as high as 700 K depending upon the depth of the chemisorption well. We found that over virtually the entire temperature range studied, the use of rate equations overestimates the H 2 formation rate to some extent. This overestimate is large at high temperatures, due to very low surface residence times. We have also considered models in which chemisorption sites are entered directly and diffusion proceeds only to other chemisorption sites.

show abstract

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

Cited by 32 publications

References 39 publications

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Contact Info

Product

Resources

About

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

Cited by 32 publications

References 39 publications

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

KINETIC MONTE CARLO STUDIES OF H2FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Contact Info

Product

Resources

About

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE