electron-molecule collisions in harsh astronomical environments alexandre faure 1 & jonathan...
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+ e - ( v’>v ) e - ( v ) + This talk Electron-impact E col < 0.1 eVTRANSCRIPT
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Electron-molecule collisions in harsh astronomical
environmentsAlexandre Faure1 & Jonathan Tennyson2
1Université de Grenoble / CNRS, France2University College London, UK
CRISM 2011, Montpellier, june 2011
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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
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+ e-(v’>v)e-(v) +
This talk Electron-impact (de)excitation
@ Ecol < 0.1 eV
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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+
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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 (?)
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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
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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 !
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Dipolar (Coulomb)-Born approximation
predicts transitions with J=1 only
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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
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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]
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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+
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See also Robert et al. A&A 2010
Electron density enhancement in shocks
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Excitation of H13CO+
Physical conditions:
>> Tkin=25K
>> Trad=2.73K
>> n(H2)=104cm-3
>> N(H13CO+)=1012cm-2
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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
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Coupling excitationwith chemistry
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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)
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Conclusions
• Electron collisions can drive both chemistry and excitation of molecules
• Impact excitation crucial when xe > 10-5
• Molecular tracers of xe: Strong dipoles !
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List of studied species
• Ions– H2
+
– HeH+
– CH+
– CO+
– NO+
– HCO+, HOC+
– H3+, H3O+
• Neutrals– H2O– HCN, HNC– CS– SiO
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Excitation vs. DR
• Above thresholds, electron collisions provide a source of rotational heating
H3+ HCO+
[Faure et al. J Phys Conf 2009]
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Ions versus neutrals