Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

Abstract

Aims. In interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around until it finds another one with which it combines. This necessarily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our aim is to discover this dependency. Methods. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. Results. We find that the steady state results of our simulation match very well with those obtained from a simpler analytical considerations provided the "effective" grain surface area is written as ∼ S α , where S is the actual physical grain area and α is a function of the flux of atomic hydrogen, which is determined from our simulation. We carry out the simulation for various astrophysically relevant accretion rates. For high accretion rates, small grains tend to become partly saturated with H and H 2 , and the subsequent accretion will be partly inhibited. For very low accretion rates, the number of sites to be swept before a molecular hydrogen can form is too large compared to the actual number of sites on the grain, implying that α is greater than unity.

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

“…The dust provides a surface for hydrogen atoms to meet and react and also removes enough of the reaction exothermicity so as to stabilize H 2 formation. In the last few decades, the formation of H 2 in the ISM has been studied in extensive detail, both theoretically (Gould & Salpeter 1963;Hollenbach & Salpeter 1970Smoluchowski 1981Smoluchowski , 1983Aronowitz & Chang 1985;Duley & Williams 1986;Farebrother et al 1999;Biham et al 2001;Green et al 2001;Chang et al 2005Chang et al , 2006Acharyya et al 2005;Chakrabarti et al 2006) and experimentally (Brackmann & Fite 1961;Schutte et al 1976;Pirronello & Averna 1988;Pirronello et al 1997aPirronello et al , 1997bPirronello et al , 1999Manicò et al 2001;Hornekaer et al 2003;Roser et al 2002Roser et al , 2003.…”

“…The technique has been used to look at the development of ice mantles (Cuppen & Herbst 2007;Cuppen et al 2009) and probe details of H 2 formation (Cazaux et al 2005;Cuppen et al 2006;. Chakrabarti et al (2006) used a similar method that keeps track of each individual reactant and their movements and calculated the effective grain surface area involved in the formation of molecular hydrogen in interstellar clouds. Most recently, the kinetic Monte Carlo method has been used to study the very complex formation of H 2 on graphite, which involves preliminary dimer formation (Cuppen & Hornekaer 2008;Gavardi et al 2009).…”

Composition and evolution of interstellar grain mantle under the effects of photodissociation

“…Similarly, the chemical and physical processes occurring on the grain surfaces are also very poorly known till date. A number of theoretical attempts have been made to verify the observed abundances of the gas‐phase and condensed‐phase species (Hollenbach & Salpeter 1970; Watson & Salpeter 1972; Allen & Robinson 1975; Tielens & Hagen 1982; Hasegawa, Herbst & Leung 1992; Charnley 2001; Stantcheva, Shematovich & Herbst 2002; Chakrabarti et al 2006a,b; Cuppen & Herbst 2007; Das et al 2008a,b; Das, Acharyya & Chakrabarti 2010). Both the deterministic and stochastic approaches have been tested in these studies.…”

Effects of initial condition and cloud density on the composition of the grain mantle

“…Hollenbach & Salpeter (1970) were the first to introduce the grain‐surface chemistry to explain the formation of molecular hydrogen. Since then it has been widely used by a number authors (Watson & Salpeter 1972a,b; Allen & Robinson 1975, 1976, 1977; Tielens & Hagen 1982; Hasegawa, Herbst & Leung 1992; Charnley 2001; Stantcheva, Shematovich & Herbst 2002; Acharyya, Chakrabarti & Chakrabarti 2005; Biham et al 2001; Chakrabarti et al 2006a; Green et al 2001; Das et al 2008a,b). These studies mainly belong to two distinct categories, namely the deterministic approach and the stochastic approach.…”

“…They used a continuous random‐walk is a computational technique to study the formation of molecular hydrogen. Chakrabarti et al (2006a,b) used a similar method, which keeps track of each individual reactant and their movements, and calculated the effective grain‐surface area involved in the formation of molecular hydrogen in the interstellar clouds. Furthermore, they defined an important parameter called ‘catalytic capacity’, which measures the efficiency of the formation of H 2 on a grain surface for a given pair of H atoms residing on it.…”