ERIC HERBST DEPARTMENTS OF PHYSICS AND ASTRONOMY THE OHIO STATE UNIVERSITY Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions

Dust particles contain 1% of interstellar matter.

Cosmic rays produce ions

Radical-Neutral Reactions Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by K: k  10(-10) cm 3 s -1

(diffusion)

FORMATION OF GASEOUS WATER H 2 + COSMIC RAYS  H e Elemental abundances: C,O,N = 10(-4); C<O H H 2  H H H O  OH + + H 2 OH n + + H 2  OH n H H 3 O + + e  H 2 O + H; OH + 2H, etc

FORMATION OF O 2,N 2 CO OH + O  O 2 + H OH + N  NO + H NO + N  N 2 + O CO, N 2 + He +  C +, N + +… Precursor to ammonia, hydrocarbons CH + O  CO + H

NEUTRAL-NEUTRAL RX (CONT) CN + C 2 H 2  HCCCN + H CCH + HCN  HCCCN + H YES NO CCH + C 2 H 2  C 4 H 2 + H YES O + CCH  CO + CH MAYBE (E a = 250K?) k  (-11) cm 3 s -1

Successes for quiescent cores: (1)Reproduces 80% of abundances including ions, radicals, isomers (2)Predicts strong deuterium fractionation

CURRENT APPROACH TO NEUTRAL-NEUTRAL RX DO NOT EXTRAPOLATE NEW LOW TEMPERATURE RESULTS UNLESS IN FAMILIES (e.g. C + HCCCN NO) DO NOT EXTRAPOLATE NEW LOW TEMPERATURE RESULTS UNLESS IN FAMILIES (e.g. C + HCCCN NO) MAINTAIN PAST ESTIMATES UNLESS CONSENSUS THAT THEY ARE INCORRECT. (E.G. O + C n, C n H) INCLUDES LOW RATE CONSTANT. MAINTAIN PAST ESTIMATES UNLESS CONSENSUS THAT THEY ARE INCORRECT. (E.G. O + C n, C n H) INCLUDES LOW RATE CONSTANT. ESTIMATES IMPROVED WITH IAN; WORK IN PROGRESS ESTIMATES IMPROVED WITH IAN; WORK IN PROGRESS

IRC10216: An AGB (Old) Star Molecules and dust here C>O N,T similar to cloud UV radiation + cosmic rays LTE CO, C 2 H 2, HCN

Actual Distributions

GROWTH OF MOLECULES Occurs via neutral and ionic (+ and -) reactions. Modified network necessary to account for acetylenic chemistry. Photochemistry important in the production of radicals such as CN and CCH C 2 H 2 + h  CCH + H

GROWTH OF MOLECULES. II CCH + C 2n H 2  C 2n+2 H 2 + H CN + C 2n H 2  HC 2n+1 N + H C 2n H reactions with hydrocarbons, HCN(?), HNC(?) as well as cyanoacetylenes. Benzene formed via ion-molecule rx.

CYANOPOLYYNES in IRC Radius 

CRL618: A Protoplanetary Nebula empty Dense Thin shell 250 K Detection of benzene Photons, X-rays from central star! 100 x normal ionization rate

Fractional Abundance 

SUMMARY Thanks to Ian Smith and others (Bertrand Rowe, Ian Sims, David Clary etc.), we now know that neutral-neutral reactions are competitive in both oxygen-rich and carbon-rich chemistries in interstellar and circumstellar sources!!!!!

SYNTHESIS OF BENZENE C 2 H C 2 H 2  C 4 H H C 4 H C 2 H 2  c-C 6 H h c-C 6 H H 2  c-C 6 H h c-C 6 H e  c-C 6 H 6 + H

INITIAL ABUNDANCES w.r.t. H 2 CO CO C 2 H 2 C 2 H 2 HCN HCN CH 4 CH 4 NH 3 NH 3 6(-4) 6(-4) 5(-5) 5(-5) 8(-6) 8(-6) 2(-6) 2(-6)

ACTUAL CLOUD CORES Model TMC-1 L134N NSM/+grains 80% 80% NNM 50% 70% NOTES: (1) C=0.42 x O (2) N-N reactions involving C and O most important. (3) Larger molecules more strongly affected.

TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B  AB association H + H  H 2 H + X  XH (X = O, C, N, CO, etc.) WHICH CONVERTS O  OH  H 2 O C  CH  CH 2  CH 3  CH 4 N  NH  NH 2  NH 3 CO  HCO  H 2 CO  H 3 CO  CH 3 OH X + Y  XY ?????????? H + HX  H 2 + X abstraction