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This presentation explores the role of electron collisions in molecular astrophysics, focusing on harsh environments such as planetary atmospheres, the interstellar medium, and photon-dominated regions. Key topics include electron impact excitation, dissociative recombination, and molecule formation in early universe conditions. We discuss electron cooling mechanisms by H2 and the significance of electron fractions in various regions. The impact of these collisions on chemistry and excitation of reactive molecular ions is highlighted, with particular attention to molecules like HCO+, H3+, and their implications for astrophysical processes.
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Electron-molecule collisions in harshastronomicalenvironments Alexandre Faure1 & Jonathan Tennyson2 1Université de Grenoble / CNRS, France 2University College London, UK CRISM 2011, Montpellier,june 2011
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
This talk Electron-impact (de)excitation@ Ecol< 0.1 eV e-(v) + + e-(v’>v)
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+
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 (?)
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
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 !
Dipolar (Coulomb)-Born approximation predicts transitions with J=1 only
R-matrixstudies • Long-rangetheories are not reliable, exceptfor dipolar transitions in strongly polar species (> 2D) • J>1 significant and dominated by short-range effects
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]
Theoryversusexperiment e-H2O e-HD+ [Zhang et al. Phys. Scrip. 2009] [Shafir et al. PRL 2009] [Schwalm et al. J PhysConf, submitted]
Electron densityenhancementin shocks Seealso Robert et al. A&A 2010
Excitation of H13CO+ Physical conditions: >> Tkin=25K >> Trad=2.73K >> n(H2)=104cm-3 >> N(H13CO+)=1012cm-2
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
Excitation of metastable H3+ (3, 3) (2, 2) (1, 1) [Oka & EppApJ 2005] [Faure et al. Phil. Trans. R. Soc. A 2006] [Black 2007]
Conclusions • Electron collisions can drive bothchemistry and excitation of molecules • Impact excitation crucial whenxe >10-5 • Moleculartracers of xe: Strongdipoles !
List of studiedspecies • Ions • H2+ • HeH+ • CH+ • CO+ • NO+ • HCO+, HOC+ • H3+, H3O+ • Neutrals • H2O • HCN, HNC • CS • SiO
Excitation vs. DR H3+ HCO+ • Abovethresholds, electron collisions provide a source of rotationalheating [Faure et al. J PhysConf 2009]
