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?)