interstellar chemical models with molecular anions eric herbst, osu t. millar, m. cordiner, c. walsh...
TRANSCRIPT
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?)