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=3 N=4 N = 5N = 6N = 7N = 8N = 9N = 10 H2H2 AlClH3+H3+C2SC2SNH 3 CH 4 CH 3 OHCH 3 NH 2 HCOOCH 3 (CH 3 ) 2 O(CH 3 ) 2 CO CHPNCH 2 OCSH3O+H3O+ SiH 4 CH 3 SHCH 3 CCHCH 3 C 2 CNC 2 H 5 OHCH 3 C 4 CN CH + SiNNH 2 MgCNH 2 COCH 2 NHC2H4C2H4 CH 3 CHOC6H2C6H2 C 2 H 5 CN ?glycine? NHSiOH2OH2OMgNCH 2 CSH2C3H2C3 CH 3 CN c- CH 2 OCH 2 C7HC7HCH 3 C 4 H CH 3 CH 2 CHO OHSiSH2SH2SNaCNl-C 3 Hl-C 3 H 2 CH 3 NCCH 2 CHCN HOCH 2 CHO C8HC8H (CH 2 OH) 2 HFCO + C2HC2HSO 2 c-C 3 Hc-C 3 H 2 H 2 CCHOHC 4 CNCH 3 COOHHC 6 CN C2C2 SO + HCNN2ON2OHCCHH 2 CCNNH 2 CHOC6HC6H H 2 CCCHCN CH 3 CONH 2 CNPOHNCSiCNHCNH + H 2 NCNHC 3 NH + H 2 CCHOH H2C6H2C6 COSHHCOCO 2 H 2 CNCH 2 COH2C4H2C4 CH 2 CHCHO N = 11 CSAlFHCO + c-SiC 2 c-C 3 HHCOOHC5HC5HC6H-C6H- C8H-C8H- HC 8 CN CP FeOHOC + SiNCHCCNC4HC4HC5NC5NCH 3 C 6 H NOSiC HN 2 + AlNC HNCOHC 2 CNC5OC5O N = 12 NSCF + HNO HCPHOCO + HC 2 NCC5SC5S C6H6C6H6 SO? N 2 ? HCS + HNCSC 4 Sic-C 3 H 2 O HCl C3C3 C 2 CNC5C5 CH 2 CNH N = 13 NaCl C2OC2OC3OC3OC4NC4NHC 10 CN KCl C3SC3SH 2 COH + SiC 3 C4H-C4H-

ANIONS AT LAST All in family C n H - 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: C n H - ; 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 Cores Gas + dust Ion-molecule chemistry leads to many positive ions and other exotic species.

L1527: continuum map from protostar

IRC >50 molecules detected: CO, C 2 H 2, HC 9 N... Newly discovered anions C 6 H -, C 4 H -, C 8 H - 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 C n H + e ↔ C n H - * → C n H - + 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 C n H - No. of C atoms k att (cm 3 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., C n H - + H → C n H 2 + e 3) normal ion-molecule reactions 4) ion-ion recombination (A + - A - )

Millar et al. (2007) C 6 H - observation C 6 H observation

TMC-1 Abundance Ratios Anion/Neutral Observed* C 4 H < C 6 H 0.016(3) C 8 H 0.05(1) C 10 H Anion/Neutral Calculated# * Bruenken et al. (2007); # Millar et al. (2007); calculations at early-time.

C 4 H - :C 6 H - :C 8 H - ratio: Model: 1:17:6 Observation: 1:12:3 IRC results Model: –N(C 4 H - ) = 1.0x10 13 cm -2 –N(C 4 H) = 1.3x10 15 cm -2 –Ratio = –N(C 6 H - ) = 1.7x10 14 cm -2 –N(C 6 H) = 5.7x10 14 cm -2 –Ratio = 0.30 –N(C 8 H - ) = 5.8x10 13 cm -2 –N(C 8 H) = 2.1x10 14 cm -2 –Ratio = 0.28 Observation: –N(C 4 H - ) = 5.8x10 11 cm -2 –N(C 4 H) = 2.4x10 15 cm -2 –Ratio = –N(C 6 H - ) = 6.9x10 12 cm -2 –N(C 6 H) = 8.0x10 13 cm -2 –Ratio = 0.09 –N(C 8 H - ) = 2x10 12 cm -2 –N(C 8 H) = 8x10 12 cm -2 –Ratio = 0.25 Prediction: –N(C 10 H - ) = 2.3x10 13 cm -2

Horsehead PDR results Model: –n(C 4 H - ) = 8.4x n(H 2 ) –n(C 4 H) = 2.4x10 -9 n(H 2 ) –Ratio = –n(C 6 H - ) = 4.5x n(H 2 ) –n(C 6 H) = 9.6x n(H 2 ) –Ratio = 4.7 Observation: –n(C 4 H) = 3x10 -9 n(H 2 ) –n(C 6 H) = n(H 2 ) Prediction: –n(C 8 H - ) = 9.3x n(H 2 ) –n(C 10 H - ) = 5.5x n(H 2 )

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 model over-predicts abundances Observed relative anion abundances support electron attachment theory (phase space) We predict observable abundances of C 4 H -, C 6 H -, C 8 H - in CSEs, PDRs and dense clouds. C 10 H - at the limit of detectability Some anion reaction rates are currently uncertain: –Radiative electron attachment (resonances?) –Photodetachment (resonances?)