Interstellar Medium Physics & Chemistry

1. Interstellar Medium Physics & Chemistry Gianfranco Vidali

2. ISM {atoms, molecules, dust} • How do we know? • Where do they come from? • What is their role in the ISM? Over all the sky – the sky! far, far out of reach, studded, breaking out, the eternal stars W.Whitman

3. Tools • From ISO (Infrared Space Observatory)

4. The ISM A tumultuous cloud Instinct with fire and nitre

5. Atoms • H, He • Source:Big Bang • Metals (O, C, N, Si, Fe, …) • Source: interior of stars, supernovae

6. Molecules • A+B C dN(C)/dt=k N(A) N(B) • Ion-molecule (10-9 cm3 s-1) • Neutral-neutral (<10-11 cm3 s-1) H2, CO, OH, CS CO2, H20, HCN H2CO, NH3, C2H2 CH4, CH3OH, HCOOH, OCS PAH 21 cm rotations vibrations electronic

7. Dust • Origin • Novae • Supernovae • Stellar outflows • Characteristics • a~0.1 mm N~a-3.5 • Silicates, carbonaceous material (dep. on C/O)

8. Dust • Extinction of starlight by dust (Mie scattering)

9. Carbon in Space Source: Charnley and Ehrenfreund, Ann. Rev. Astron. Astroph.

10. Olivine (a silicate) (Mg, Fe)2 SiO4

11. Role of ISM • Star formation • Early universe H+H+H2+ + e H2+ + H H2 + H+ • Current Cooling by H2, CO ..the Almighty Maker them ordain His dark materials to create more Worlds

12. Chemical balance regulates abundance of atoms, molecules, dust • Diffuse clouds, dense clouds, circumstellar envelopes Stellar UV sources Cosmic rays shocks Thermal IR sources Dynamical shocks

13. Molecular Hydrogen • Coolant – promotes star formation • Tracer of warm gas • Promotes interstellar chemistry H2 + cr  H2+ + e H2+ + H2 H3+ + H H3+ + O  OH+ + H2 • Weak quadrupolar transitions; CO is a tracer

14. H+HH2 not in the gas phase Other routes: H+eH-+hn H-+HH2+e requires ionized medium The Molecular Hydrogen Problem b3Su+ X1Sg+ • H+HH2 on dust grains Salpeter, Hollenbach ~1970 dn(H2)/dt=R n n(H)-b n(H2)

15. 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)

16. Application of surface science techniques to astrophysical problems • Measurement of hydrogen recombination and hydrogenation/oxidation reactions on surfaces of dust grain analogues • Experimental Conditions • Low kinetic energy of H atoms (gas phase atoms): ~ 200-300 K • Low flux of H atoms <1012 atoms/cm2/sec • Low sample temperature (5-40 K) • Low background pressure (10-10 torr) • Experiment: Schutte et al. (1976), King and Wise (1963) • Not in astrophysically relevant conditions

17. Our Research Program • Experiments of molecular synthesis on interstellar dust grain analogues • H+H  H2 • Measure H2 formation on: • Silicates (olivine) Ap.J. 475, L69 (1997); Ap.J.483, L131 (1997) – first experiments to study H2 formation on dust grains analogues in astrophysically relevant conditions • Carbonaceous Materials (amorphous carbon) A&A 344, 681 (1999) • Amorphous Water Ice: Ap.J. 548, L253 (2001); Ap.J. accepted (2002) • CO+OCO2 • Measure CO2 formation due to oxidation of CO-ice by atomic oxygen

18. The Team • Joe Roser • Bob D’Agostino • Chris Nagele, Emily Watkins, Sam Palermo • Sol Swords • Ofer Biham • Valerio Pirronello, Giulio Manico’

19. Experimental Apparatus

20. Apparatus to study molecule formation on dust grain analogues

22. Measurement Methods • Irradiation of sample with thermal energy H atoms • Measurement of hydrogen recombination events • Measurement of H2 formation due to fast processes, due to: • Eley-Rideal ("prompt") reaction • Fast diffusion on surface of grain analogue • Thermal Programmed Desorption, to: • Desorb molecules that have already formed on the surface • Acceleratethe diffusion of H atoms and favour the recombination process

23. H adsorption and measurement of H2 due to the fast reaction process Tbeam~150-200 K Mass discriminating detector T ~ 5 - 20 K samples: olivine ((Fe,Mg)2 SiO4), amorphous carbon, ice, etc.

24. Temperature Programmed Desorption To desorb molecules already formed on surface or to set atoms in motion detector temperature time heat Temperature ramp

25. Hydrogen recombination reaction • Thermal desorption trace: HD from olivine (a silicate) as a function of exposure (sub-monolayer coverage) • Ap.J. 1997 • Learn about reaction kinetics and rates

26. Analysis of Temperature Programmed Desorption results • Desorption rate (Polanyi-Wigner): R (t) ~ nb n(t)b exp (-Ed/kT) Order of desorption b=0 desorption from multilayer b=1 direct or molecular desorption b=2 associative desorption Order of desorption Adatom density Desorption energy barrier

27. Rate equations d nH/dt = F (1- nH - nH2)- pH nH - 2 a nH2 d nH2/dt = a m nH2- pH2 nH2 pH = n exp(-EH/kT) - desorption rate a= n exp(-Ed/kT) - diffusion+recomb. rate • RH2 (t) = (1- m) a nH2 + pH2 nH2 Attempt frequency Recombination rate

28. Analysis of rate equations • Steady state conditions (dnH, H2/dt=0): • RH2 = 1/2 (nH vHsx) ng • Indep. of H coverage - linear in flux • Applicable when mobility is high (a>>pH/F; pH = 1/tH) • RH2= 1/2 (nH vHsx tH)2 ngag` • Quadratic in H coverage - quadratic in flux • Applicable when mobility is slow or coverage is low (a<<pH/F) • 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 H fluxes

29. Hydrogen recombination reaction • Molecular hydrogen recombination efficiency on different dust grain analogues amorphous carbon water ice olivine

30. Desorption of HD from amorphous ice (Roser et al., ApJ ‘02): high density, low density, gas phase deposited Recombination efficiency on amorphous ice surfaces (Roser et al., ApJ ‘02): high density low density gas-phase deposited Influence of ice morphology on Hsurface+DsurfaceHD reaction

31. Experiment-ISM connections • Theoretical and computational methods connecting laboratory data to actual processes in the ISM • Model hydrogen recombination reactions in the ISM using laboratory results: • Ap.J. 553, 595 (2001); Ap.J. 522, 305 (1999); MNRAS 296, 869 (1998) Amorphous carbon

32. The Problem: Solid CO2 more abundant than explained by gas-phase reactions Solid CO2 can be made by UV in CO- and O2–rich ices However, solid CO2 is seen in quiescent regions – no UV Can solid CO2 be made by: COice + Ogas CO2 ice ? Example II: Oxidation reaction of CO Whittet et al, A&A, 1998 Spectrum towards Elias16

33. Roser et al., Ap.J. 2001 Oxidation of CO ice by atomic O O CO ~100 layers CO +O CO/O ~5.6-21 ~100 layers H2O substrate CO2 CO O heat

34. Current Research: Study of the energetics of H2 formation • Goal: • Measurement of excitation state of molecular hydrogen formed on dust grain analogues • Techniques: • Time-of-flight detection to measure the translationalenergy of molecules • (2+1) REMPI (Resonance Enhance MultiPhoton Ionization) to measure the roto-vibrational state of molecules leaving the dust grain analogue

35. Time-of-flight measurements • In the time-of-flight experiment, the desorbing flux is chopped by a rotating mechanical wheel, see adjacent sketch. The time that a pulse of molecules takes to go from the chopper to the detector is measured and the kinetic energy calculated. • Of the 4.5 eV energy released in the recombination reaction, it is not known quantitatively the partition of the in roto-vibration vs. translation of the molecule. Guess estimates of the amount of translational energy range from thermal energy (~20 K) to 1 eV. • The challenge is to measure the velocity distribution of the molecules exiting the surface during the brief time (~ a few tens of sec.) of the TPD run. This imposes stringent requirements on abating the residual gas background pressure. • Such experiment has not been done before under these conditions.

36. Measurement of the roto-vibrational energy of H2 • Of the 4.5 eV energy released in the recombination reaction, some is available to the molecule as roto-vibrational energy. Estimates of this energy vary greatly. • The experiment consists in probing the quantum state of the desorbing hydrogen molecules. Because vibrationalstates of H2 lie in the UV, the measurement of the roto-vibrational state is challenging. • We use the (2+1) REMPI (Resonance Enhanced MultiPhoton Ionization). The molecule is taken to an electronically excited state by the absorption of two photons. Here the molecule absorbs another photon that removes an electron. The molecular ion is then collected by a detector (a channel-plate), see adjacent diagrams.

37. Specifics of the detection of roto-vibrational energy levels • The light from a Nd:YAG laser (1089 nm) is doubled and sent to a dye laser for tuning. The ~600 nm light is then sent to a non-linear crystal that convert visible light into a 200 nm and a 300 nm beams. The molecule absorbs a 200 nm photon that takes it to a virtual state. If the molecule absorbs another photon, then it can go in an electronically excited state, see diagram. From there, the absorption of a 300 nm photon ionizes the molecule. That’s the explanation for the (2+1) nomenclature. • The challenge is to have a beam of photons intense enough so the molecule can absorb two photons virtually simultaneously. Furthermore, because the generation of tunable laser light at 200 nm requires the use of the non-linearity of special crystals, the process is inherently inefficient and the experiment needs powerful lasers.

38. Molecular hydrogen formation on dust grain analogues in ISM conditions • Study of molecular hydrogen formation on amorphous ices found in various interstellar environments. • Study of the role of ice morphology and UV processing on H2 formation. • Comparison of recombination efficiency due to surface or near-surface processes with competing mechanisms, such as cosmic rays and UV photons. See:Ap.J. 548, L243 (2001). • Study of evolution of morphology of icy grains through astrophysical environments (Ap.J., accepted - 2002)

39. Summary of accomplishments and future directions • We showed that: • Measurement of hydrogen recombination and CO oxidation reactions on dust grain analogues can explain processes occurring in the ISM • Challenges: • Composition, morphology of dust poorly known • Partition of reaction energy between new-born molecule and solid • Excitation of molecule ejected into the gas phase; theoretical estimates vary greatly • Role of energy deposited in the ISM

40. Analysis of experimental results • Second order kinetics (b=2): • R (t) ~ n2 n(t)2 exp (-Ed/kT) • Ed = effective activation energy barrier for formation of H2 and desorption • Examples: • E ~ 26 meV (olivine); ~45 meV (amorphous carbon) n2 ~ 10-3 cm2/s • Derivation of Ed dR(t)/dt=0 (max of desorption rate), T=T0+at Ed/kTmax=ln(Tmax2/a) – ln (Ed/kn) b=1 Ed/kTmax=ln(n Tmax2/a) – ln (Ed/2kn) b=2

41. Details of calculations • Numerical integration of rate equations • Fit to ALL TPD curves for each surface with 4 parameters: • activation energy for H desorption: E1 • activation energy for H2desorption: E2 • activation energy for H diffusion: E0 • fraction of H2 desorbing: m • Results: • E0, E1, E2 : tens of meV; higher for a-carbon • E0, E2 : well determined • Recombination efficiency = R (recombination rate)/ F/2 (desorption rate)

42. Results and Analysis • Second order kinetics • Rate equations • Numerical integration of rate equations • Fit to ALL TPD curves for each surface with 4 parameters • Fit to experimental desorption curves • Obtain physically relevant parameters • Construct plot of recombination efficiency as a function of T and for a range of H fluxes

43. Basic processes of atom -surface interaction applied to astrochemsitry1. Prompt reaction” / Eley-Rideal reaction Significantonly if most of the surface is covered with adsorbed atoms Direct hit

44. Basic processes of atom -surface interaction applied to astrochemsitry1. Indirect mechanism(Langmuir-Hinshelwood) • Sticking • Diffusion • Reaction • Desorption