Electron-molecule collisions in harsh astronomical environments

1. Electron-molecule collisions in harshastronomicalenvironments Alexandre Faure1 & Jonathan Tennyson2 1Université de Grenoble / CNRS, France 2University College London, UK CRISM 2011, Montpellier,june 2011

2. Electron collisions in molecularastrophysics • Planetaryatmospheres: drivers of aurorae • Interstellar medium: dissociative recombination • PDRs, comets:rovibrationalexcitation • Molecule formation in the earlyuniverse • X-rayirradiatedclouds:e.g. impact dissociation of H2

3. This talk Electron-impact (de)excitation@ Ecol< 0.1 eV e-(v) + + e-(v’>v)

4. Free electrons in the ISM ? • Electrons are injectedatEkin ~ 30 eV fromionization of H2 by cosmic rays[e.g. Cravens& Dalgarno1978] • Electrons are cooled by H2 down to ~ 0.1 eV in typically 1 year[Field et al. 2007] • Additionalcooling by strongly polar speciessuch as H2O and HCO+

5. Electron fraction • Darkmolecularclouds • xe =n(e-)/nH≤ 10-7, TK ~ 10 K • Photon-dominated regions (PDR) • xe~ 10-4, TK ~ 100 K • X-ray dominated regions (XDR) • xe~ 10-4 - 10-3, TK ~ 100-1000 K • Cosmic-ray dominated regions(CRDR) • xe~ 10-4 - 10-3, TK ~ 100-1000 K (?)

6. Electron-impact rates • Electron-impactrotational (de)excitation of polar ions isfast: • k(e)~ 10-7 - 10-5 cm3s-1 • By comparison: • k(H, H2) ~ 10-12 - 10-10 cm3s-1 • Electrons are important as soon as: • xe>10-5

7. Non-LTEeffects ? • In interstellarregionswherexe>10-5, the electrondensityistypically0.1 cm-3 • For a polarion like HCO+, the criticalelectrondensity for rotationallevelsisncr~1 cm-3 • n < ncr non-LTE populations !

8. Dipolar (Coulomb)-Born approximation predicts transitions with J=1 only

9. R-matrixstudies • Long-rangetheories are not reliable, exceptfor dipolar transitions in strongly polar species (> 2D) • J>1 significant and dominated by short-range effects

10. Near-thresholdexcitation of ions • Excitation cross sections are large and finiteatthreshold, in agreement withWigner’slaw. • Large Rydberg resonancesattached to the first closed-channel e-H3+ [Faure et al. J Phys B 2006] [Kokooulineet al.MNRAS 2010]

11. Theoryversusexperiment e-H2O e-HD+ [Zhang et al. Phys. Scrip. 2009] [Shafir et al. PRL 2009] [Schwalm et al. J PhysConf, submitted]

12. Electron densityenhancementin shocks Seealso Robert et al. A&A 2010

13. Excitation of H13CO+ Physical conditions: >> Tkin=25K >> Trad=2.73K >> n(H2)=104cm-3 >> N(H13CO+)=1012cm-2

14. Reactivemolecular ions • Reactivespecies(CH+, H2O+, etc.) are destroyed on almostevery collisions with H, H2, e- • Their excitation isstronglycoupled to theirchemistrywhenx(e)>10-5: tion ~ tcol < 1 year

15. Coupling excitationwithchemistry

16. Excitation of metastable H3+ (3, 3) (2, 2) (1, 1) [Oka & EppApJ 2005] [Faure et al. Phil. Trans. R. Soc. A 2006] [Black 2007]

17. Conclusions • Electron collisions can drive bothchemistry and excitation of molecules • Impact excitation crucial whenxe >10-5 • Moleculartracers of xe: Strongdipoles !

18. List of studiedspecies • Ions • H2+ • HeH+ • CH+ • CO+ • NO+ • HCO+, HOC+ • H3+, H3O+ • Neutrals • H2O • HCN, HNC • CS • SiO

19. Excitation vs. DR H3+ HCO+ • Abovethresholds, electron collisions provide a source of rotationalheating [Faure et al. J PhysConf 2009]

20. Ions versus neutrals