IDMC, 2014, Tezpur APOD Archive

H2 Formation on Interstellar Grains

Kinsuk Acharyya

Department of Chemistry, University of Virginia, Charlottesville, VA, USA

2/18

n ~ 10 – 102 cm-3

T ~ 30 – 80 K

Low-density clouds in the ISM that can easily be penetrated by UV radiation.

T ~ 10 K

n ~ 103 - 106 cm-3

Opaque to UV photons. Almost no ionisation. Material is mostly molecular, dominant species is H2.

T ~ 10-50 K,

n ~ 104 - 107 cm-3

Material is mostly molecular.

Diffuse clouds Dark clouds GM clouds

Various regions of ISM

APOD Archive

IRAS 4A

Star Forming Regions

Accretion disc of AGN (Harada et al., 2010)

4/18

Most abundant molecule

Promotes interstellar chemistry

H2 + Cosmic ray H2+ + e

H2+ + H2 H3

+ + H

H3+ + O HO+ + H2

Controls the mass budget in the star formation site

Important coolant

But it’s formation route is unclear.

Molecular Hydrogen

APOD Archive

The Molecular Hydrogen Problem

• Gas-Phase formation: H + H → H2 + hν

• Very slow, insignificant in ISM

• 3-body reaction is rare in ISM due to low density

• Gould and Salpeter (1963) proposed the idea of H2 formation on the grain surface.

• Interstellar dust grains will acts as a catalyst.

• Hollenbach & Salpeter (1970, 1971) developed a semi-classical model in which the mobility of the atoms on a grain surface is treated quantum mechanically. They found that even at 10 K, tunneling could provide enough mobility to form H2.

Basic Physical Process on Grain Surfaces

There are four major physical processes involved during a

gas-grain interaction.

),/exp(

)/exp(

)/exp(

2

1

0

2kTEW

kTEW

kTEa

H

H

H

Chemisorption Physisorption Rough

Surface

H2 Formation at Various Temperatures

< 15K

> 50K

Langmuir-Hinshelwood Mechanism

Eley-Rideal Mechanism

Formation Mechanisms

Surface H-diffusion

Energy

(meV)

H-desorption

Energy

(meV)

H2-desorption

Energy

(meV)

Efficiency

Range

(K)

Olivine

(Polycryst.)

24.7 32.1 27.1 6 – 9

Silicate

(Amorphous)

35 44 35, 53 8 – 13

Carbon

(Amorphous)

44 56.7 46.7 9 – 14

Ice

(Amorphous)

44.5 (LD)

55(HD)

52.3

62

46.5 – 61.2

68.7

9 – 14

10 – 15

H2 formation is efficient only on low temperatures (< 15 K)

H2 formation on interstellar ice analogs

Various methods to study gas-grain interaction

1. Rate Equation: Rate equations depend on an average being a physically meaningful quantity – ok for gas but not for grains

4 grains + 2 H atoms – average = 0.5 H atoms per grain

2. Monte Carlo Method : o Reaction on the surface can only occur if a particle

arrives while one is already on the surface.

o This methods attach probabilities to arrival of individual particles and fire randomly at surface according to these probabilities.

3. Master Equation: Reaction depends on the probabilities of a particular number of species being on the grains e.g. PH(0), PH(1), PH(2), … PH(N), PO(0), PO(1), …

10/18

H

Physical processes on the grain surface

H

H H2

We used a Continuous Time Random Walk (CTRW) Monte Carlo Method to simulate these processes.

12/18

Efficiency on silicate Grains

Recombination efficiency of H2 as a function of the grain temperature and the number density nh in the ambient gas of Tgas = 90 K

(Cuppen 2005-MNRAS)

H2 formation at intermediate temperature

Introduction of rough or amorphous surfaces, extend H2

formation up to 30 – 50 K upon whether the surface is a

silicate or carbonaceous (Chang et al. 2005), Cuppen &

Herbst (2005 ).

Olivine Carbon

H2 formation at high temperature

H2 formation using only physisorption sites is inefficient at high temperature because it can not keep H atoms long enough to H2 formation to take place.

Therefore One need to consider chemisorption sites.

It could be via physisorption, i.e, adsorption occurs into a physisorbed site, then it can go to a physisorbed site or to a chemisorbed site.

it could be direct chemisorption (ER mechanism)

In chemisorption sites H is more strongly bound can stay up to much higher temperature.

Maximum temperature for which one can good efficient H2 formation using chemisorption depends on depth of the potential well and barrier height.

[Iqbal et al, ApJ, (2012),751, 58]

With ER Mechanism

No H2 Formation No H2 Formation

17/18

Diffuse Clouds

Conclusions

• H2 formation on grains is temperature sensitive

• In presence of only physisorption site it is efficient only in a narrow temperature window.

• In presence of rough surface, H2 formation could be efficient up to 50 K

• H2 formation to occur over 50 K, chemisorbed sites are required.

• Presence of Deeper chemisorption well, can extend the temperature window up to 800 K.

• Even for diffuse clouds, chemisorption sites are needed for efficient H2 formation.

Thank You