Molecular Hydrogen Formation on Interstellar Dust Grain Analogues: Experiments and Simulations

1. Molecular Hydrogen Formation on Interstellar Dust Grain Analogues: Experiments and Simulations Gianfranco Vidali Syracuse University Valerio Pirronello (Univ.Catania) Joe Roser (Syracuse University) Ofer Biham (Hebrew Univ.) Supported by NASA

2. The Interstellar Medium • Diffuse Clouds (10 - 102 H/cm-3) • Dense Clouds (103 - 104 H/cm-3) • Gas: H, He, H2, CO • Dust: • r ~ 0.1 mm • silicates, amorphous carbon • T ~ 10 - 15 K • ng ~ 10-12 nH

3. H2 formation routes: in the gas phase V(r) • Gas-phase reactions • H + H H2 +hn • H- + H H2 + e 3Su+ r 3Sg+

4. H2 formation routes: on dust grains • H2formation on surfaces of dust grains • d n(H2)/dt = R n n(H) - b n(H2) • n = n(H) + 2 n(H2) • b ~1011 sec-1 • from observations: R ~ 3 10-17 cm3 sec-1 Rate constant

5. Models • Hollenbach and Salpeter (1970) • Semiclassical sticking, quantum mechanical tunneling • RH2 = 1/2 ( nH vHs x) g ng # density of grains prob. of recomb. sticking grain cross-section flux Recombination efficiency (# molecules/sec-1 cm-3)

6. Previous Experiments • Not under astrophysically relevant conditions: • Low surface temperature • Low flux of H atoms • Low kinetic energy of H atoms • Schutte et al. (1976), King and Wise (1963)

7. Adsorption of H on dust grain analogues Tbeam~150-200 K detector T ~ 5 - 20 K samples: olivine ((Fe,Mg)2 SiO4), amorphous carbon

8. Desorption experiment detector temperature time heat

9. Analysis of experimental results • Second order kinetics: desorption rate prop. to n2H • Propose: • RH2 = 1/2 (nH vHsx tH)2 ngag` diffusion term probability to form H2 upon meeting recombination efficiency coverage of H residence time

10. Rate equations • d nH/dt = F - pH nH - a nH2 • d nH2/dt = a nH2 • pH = n exp(-EH/kT) - desorption rate • a= n exp(-Ed/kT) - diffusion+recomb. rate Attempt frequency

11. Analysis of rate equations • Steady state conditions: • RH2 = 1/2 (nH vHsx) ng • Indep. of H coverage - linear in flux • Applicable when mobility is high • RH2= 1/2 (nH vHsx tH)2 ngag` • Quadratic in H coverage - quadratic in flux • Applicable when mobility is slow or coverage is low • Kinetics • Fit to experimental desorption curves • Obtain physically relevant parameters • Construct plot of recombination efficiency as a function of T and for a range of fluxes

12. Summary • First experiments to measure H recombination on dust grain analogues under astrophysically relevant conditions • At T ~ 10~15 K H recombination efficiency is higher on amorphous carbon than on olivine • Simulation with input from experiments gives recombination efficiency for a range of flux and temperature

13. Outlook • Experiments to study physical/chemical phenomena under astrophysically relevant conditions are feasible • Simulations using parameters obtained from best-fits to experimental data yield additional information on key physical quantities (such as the H recombination efficiency) under wide range of conditions

14. Pirronello, V., Biham, O., Liu, C., Shen, L. and Vidali, G. 1997 Astrophys. J. 483, L131. Pirronello, V., Liu, C., Shen, L. and Vidali, G. 1997 Astrophys. J. 475, L69. Vidali, G., Pirronello, V., Liu, C. and Shen, L. 1998 Astrophys. Letters and Comm. (review article) 35, 423. Biham, O., Furman, I., Katz, N., Pirronello, V., and Vidali, G., 1998, MNRAS 296, 869. physics.syr.edu/research/surface_physics