Electron-molecule collisions in harsh astronomical

environmentsAlexandre Faure1 & Jonathan Tennyson2

1Université de Grenoble / CNRS, France2University 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 H2

+ e-(v’>v)e-(v) +

This talk Electron-impact (de)excitation

@ Ecol < 0.1 eV

Free electrons in the ISM ?

• Electrons are injected at Ekin ~ 30 eV from ionization of H2 by cosmic rays [e.g. Cravens & Dalgarno 1978]

• Electrons are cooled by H2 down to ~ 0.1 eV in typically 1 year [Field et al. 2007]

• Additional cooling by strongly polar species such as H2O and HCO+

Electron fraction• Dark molecular clouds

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-impact rotational (de)excitation of polar ions is fast:

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-LTE effects ?

• In interstellar regions where xe>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 ncr~ 1 cm-3

n < ncr 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-H3+

[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-H2O e-HD+

See also Robert et al. A&A 2010

Electron density enhancement in shocks

Excitation of H13CO+

Physical conditions:

>> Tkin=25K

>> Trad=2.73K

>> n(H2)=104cm-3

>> N(H13CO+)=1012cm-2

Reactive molecular ions

• Reactive species (CH+, H2O+, etc.) are destroyed on almost every collisions with H, H2, e-

• Their excitation is strongly coupled to their chemistry when x(e)>10-5:

tion ~ tcol < 1 year

Coupling excitationwith chemistry

Excitation of metastable H3+

[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 xe > 10-5

• Molecular tracers of xe: Strong dipoles !

List of studied species

• Ions– H2

+

– HeH+

– CH+

– CO+

– NO+

– HCO+, HOC+

– H3+, H3O+

• Neutrals– H2O– HCN, HNC– CS– SiO

Excitation vs. DR

• Above thresholds, electron collisions provide a source of rotational heating

H3+ HCO+

[Faure et al. J Phys Conf 2009]

Ions versus neutrals