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Interstellar Chemical Models with
Molecular Anions
Eric Herbst, OSU
T. Millar, M. Cordiner, C. Walsh
Queen’s Univ. Belfast
R. Ni Chiumin,
U. Manchester
Reported Interstellar and Circumstellar Molecules N=2 N=2 N=3 N=3 N=4 N = 5 N = 6 N = 7 N = 8 N = 9 N = 10
H2 AlCl H3+ C2S NH3 CH4 CH3OH CH3NH2 HCOOCH3 (CH3)2O (CH3)2CO
CH PN CH2 OCS H3O+ SiH4 CH3SH CH3CCH CH3C2CN C2H5OH CH3C4CN
CH+ SiN NH2 MgCN H2CO CH2NH C2H4 CH3CHO C6H2 C2H5CN ?glycine?
NH SiO H2O MgNC H2CS H2C3 CH3CNc-
CH2OCH2
C7H CH3C4H CH3CH2CHO
OH SiS H2S NaCN l-C3H l-C3H2 CH3NC CH2CHCN HOCH2CHO C8H (CH2OH)2
HF CO+ C2H SO2 c-C3H c-C3H2 H2CCHO HC4CN CH3COOH HC6CN
C2 SO+ HCN N2O HCCH H2CCN NH2CHO C6H H2CCCHCN CH3CONH2
CN PO HNC SiCN HCNH+ H2NCN HC3NH+ H2CCHOH H2C6
CO SH HCO CO2 H2CN CH2CO H2C4 CH2CHCHO N = 11
CS AlF HCO+ c-SiC2 c-C3H HCOOH C5H C6H- C8H- HC8CN
CP FeO HOC+ SiNC HCCN C4H C5N CH3C6H
NO SiC HN2+ AlNC HNCO HC2CN C5O N = 12
NS CF+ HNO HCP HOCO+ HC2NC C5S C6H6
SO ? N2 ? HCS+ HNCS C4Si c-C3H2O
HCl C3 C2CN C5 CH2CNH N = 13
NaCl C2O C3O C4N HC10CN
KCl C3S H2COH+
SiC3 C4H-
ANIONS AT LAST
• All in family CnH-
• TMC-1, a cold interstellar core: n=6, 8 (McCarthy et al.; Bruenken et al.)
• L1527, a protostar: n=6 (Sakai et al.)
• IRC+10216, an extended circumstellar envelope: CnH-; n = 4,6,8 (McCarthy et al.; Cernicharo et al.; Remijan et al.; Kasai et al.)
10 K
10(4) cm-3
H2 dominant
sites of star formation
Dense Interstellar Cloud CoresGas + dust
Ion-molecule chemistry leads to many positive ions and other exotic species.
L1527: continuum map from protostar
IRC+10216
• >50 molecules detected: CO, C2H2, HC9N ...
• Newly discovered anions C6H-, C4H-, C8H-
Figures from Mauron & Huggins (2000) and Guelin et al. (1999)
The Horsehead Nebula, a PDR
Negative Ion Production
• Herbst (1981) considered the possible abundance of anions in cold regions of the ISM based on radiative attachment:
• A + e → A- + h• and estimated their maximum abundance
to be app.1% of the neutral counterparts. See Petrie (1996) for other mechanisms such as dissociative attachment:
• e + BC B- + C (normally endoergic)
Theory of Radiative Attachment
• Cn H + e ↔ CnH-* → CnH- + h• (originally done for carbon clusters by Terzieva
& Herbst 2000)• Competition occurs between the re-emission of
the electron and stabilization of the complex.• Phase-space theory shows that the efficiency is
much enhanced by large binding energies (electron affinities) of 3-4 eV and large sizes if phase space approach used. Other possibility: resonance into dipole-bound excited state.
Results for CnH-
• No. of C atoms
• 1-3
• 4
• 5
• 6
• 7
• katt (cm3 s-1)(300 K)
• tiny
• 2 10(-9)
• 9 10(-10)
• 6 10(-8)
• 2 10(-7)
High electron affinities near 4 eV!!!
Estimated rates; better ones in progress
Destruction of Anions
• 1) photodetachment: large cross section starting at relatively low energies in the visible. (E (photon) > E.A.)
• 2) reactions with atoms (associative detachment); e.g.,
• CnH- + H → CnH2 + e
• 3) normal ion-molecule reactions
• 4) ion-ion recombination (A+ - A-)
Millar et al. (2007)
C6H- observation
C6H observation
TMC-1 Abundance Ratios
Anion/Neutral Observed*
• C4H <0.00014
• C6H 0.016(3)
• C8H 0.05(1)
• C10H
Anion/Neutral Calculated#
• 0.0013• 0.052• 0.042• 0.041
* Bruenken et al. (2007); # Millar et al. (2007); calculations at early-time.
C4H-:C6H-:C8H- ratio:
Model: 1:17:6
Observation: 1:12:3
IRC+10216 results• Model:
– N(C4H-) = 1.0x1013 cm-2
– N(C4H) = 1.3x1015 cm-2
– Ratio = 0.008
– N(C6H-) = 1.7x1014 cm-2
– N(C6H) = 5.7x1014 cm-2
– Ratio = 0.30
– N(C8H-) = 5.8x1013 cm-2
– N(C8H) = 2.1x1014 cm-2
– Ratio = 0.28
• Observation:– N(C4H-) = 5.8x1011 cm-2
– N(C4H) = 2.4x1015 cm-2
– Ratio = 0.00025
– N(C6H-) = 6.9x1012 cm-2
– N(C6H) = 8.0x1013 cm-2
– Ratio = 0.09
– N(C8H-) = 2x1012 cm-2
– N(C8H) = 8x1012 cm-2
– Ratio = 0.25
• Prediction:– N(C10H-) = 2.3x1013 cm-2
Horsehead PDR results
• Model:– n(C4H-) = 8.4x10-11 n(H2)– n(C4H) = 2.4x10-9 n(H2)– Ratio = 0.035
– n(C6H-) = 4.5x10-11 n(H2)– n(C6H) = 9.6x10-12 n(H2)– Ratio = 4.7
• Observation:– n(C4H) = 3x10-9 n(H2)– n(C6H) = 10-10 n(H2)
• Prediction:– n(C8H-) = 9.3x10-11 n(H2)– n(C10H-) = 5.5x10-11 n(H2)
Summary
• High observed anion abundances are reproduced by our models– Modelled interstellar anion-to neutral ratios are ~ 0.01 to 5– Dependent primarily upon electron density, radiation field
strength, gas-phase H, H+, C+ abundances• TMC-1 model fits observations reasonably well• IRC+10216 model over-predicts abundances• Observed relative anion abundances support electron
attachment theory (phase space)• We predict observable abundances of C4H-, C6H-, C8H- in
CSEs, PDRs and dense clouds. C10H- at the limit of detectability
• Some anion reaction rates are currently uncertain:– Radiative electron attachment (resonances?)– Photodetachment (resonances?)