Electron-molecule collisions in harsh astronomical environments Alexandre Faure 1 & Jonathan Tennyson 2 1 Université de Grenoble / CNRS, France 2 University College London, UK CRISM 2011, Montpellier, june 2011

Electron collisions in molecular astrophysics Planetary atmospheres: drivers of aurorae Interstellar medium: dissociative recombination PDRs, comets: rovibrational excitation Molecule formation in the early universe X-ray irradiated clouds: e.g. impact dissociation of H 2

+ e - ( v’>v ) e - ( v ) + This talk Electron-impact E col < 0.1 eV

Free electrons in the ISM ? Electrons are injected at E kin ~ 30 eV from ionization of H 2 by cosmic rays [e.g. Cravens & Dalgarno 1978] Electrons are cooled by H 2 down to ~ 0.1 eV in typically 1 year [Field et al. 2007] Additional cooling by strongly polar species such as H 2 O and HCO +

Electron fraction Dark molecular clouds x e =n(e - )/n H ≤ 10 -7, T K ~ 10 K Photon-dominated regions (PDR) x e ~ 10 -4, T K ~ 100 K X-ray dominated regions (XDR) x e ~ , T K ~ K Cosmic-ray dominated regions (CRDR) x e ~ , T K ~ K (?)

Electron-impact rates Electron-impact rotational (de)excitation of polar ions is fast: k(e) ~ cm 3 s -1 By comparison: k(H, H 2 ) ~ cm 3 s -1 Electrons are important as soon as: x e > 10 -5

Non-LTE effects ? In interstellar regions where x e >10 -5, the electron density is typically 0.1 cm -3 For a polar ion like HCO +, the critical electron density for rotational levels is n cr ~ 1 cm -3 n < n cr non-LTE populations !

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

R-matrix studies Long-range theories are not reliable, except for dipolar transitions in strongly polar species (  > 2D)  J >1 significant and dominated by short- range effects

Near-threshold excitation of ions Excitation cross sections are large and finite at threshold, in agreement with Wigner’s law. Large Rydberg resonances attached to the first closed-channel [Faure et al. J Phys B 2006] e-H 3 + [Kokoouline et al. MNRAS 2010]

Theory versus experiment [Shafir et al. PRL 2009] [Schwalm et al. J Phys Conf, submitted] [Zhang et al. Phys. Scrip. 2009] e-H 2 O e-HD +

See also Robert et al. A&A 2010 Electron density enhancement in shocks

Excitation of H 13 CO + Physical conditions: >> T kin =25K >> T rad =2.73K >> n( H 2 )=10 4 cm -3 >> N (H 13 CO + )=10 12 cm -2

Reactive molecular ions Reactive species (CH +, H 2 O +, etc.) are destroyed on almost every collisions with H, H 2, e - Their excitation is strongly coupled to their chemistry when x(e) >10 -5 : t ion ~ t col < 1 year

Coupling excitation with chemistry

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

Conclusions Electron collisions can drive both chemistry and excitation of molecules Impact excitation crucial when x e > Molecular tracers of x e : Strong dipoles !

List of studied species Ions – H 2 + – HeH + – CH + – CO + – NO + – HCO +, HOC + – H 3 +, H 3 O + Neutrals – H 2 O – HCN, HNC – CS – SiO

Excitation vs. DR Above thresholds, electron collisions provide a source of rotational heating H3+H3+ HCO + [Faure et al. J Phys Conf 2009]

Ions versus neutrals