Chemistry of Candidate Molecular Ion Carriers of the Diffuse Interstellar Bands Theodore P. Snow Nicholas Betts Meredith Drosback Veronica Bierbaum Outline of Talk * Overview of the Colorado program * The FA-SIFT apparatus * The DIBs problem * Studies of PAH cations * Studies of carbon chain anions * Future work

Boulder Ion Gang

Insram Flowing Afterglow – Selected Ion Flow Tube Reactions of Ions with Atoms

ISM

eous Pillars

DIB line spectrum

DIB (spectrum overlaid)

Recently Considered DIB Carriers * PAH cations Composition: Abundant elements, rich structural diversity Accommodate energy without fragmentation Expected to have strong visible absorptions Likely carriers of IR emissions bands * Carbon chain anions Studies of Maier and co-workers Striking coincidences with several DIBs

Why Are PAH Cations of interest? PAH rationale Why Are PAH Cations of interest? * PAHs may be common in the ISM (UIBs) * PAHs easily satisfy abundance requirements for the DIBs * PAH cations are known to have rich optical spectra * In the diffuse ISM where the DIBs form, PAHs are singly ionized * PAH cations of sufficient size are able to absorb photons without being destroyed

PAH+ generation X X X

PAH+ + H2/H

Reactivity trends1

Timescales

PAH modeling

Major processes

Parameters

Modeling Results * State of Ionization [PAH+] > [PAH] > [PAH–] * Astrophysical Journal Hydrogenation and Charge States of Polycyclic Aromatic Hydrocarbons in Diffuse Clouds, Papers I and II (ApJS, 132, 233; APJ, 584, 3160 V. Le Page, T. P. Snow, and V. M. Bierbaum * State of Ionization [PAH+] > [PAH] > [PAH–] * State of Hydrogenation Depends strongly on PAH size * See also: Ruiterkamp et al. (A&A, 432, 515, 2005 (presented at this conference by Nick Cox)

Size

Summary: PAH Cations in the Diffuse ISM * PAH cations in diffuse interstellar clouds rapidly add an H atom and become protonated, a trend that we expect will continue to larger PAH+ not yet measured. * PAH cations in the size range from 50 to 100 carbon atoms appear to be favored (LePage et al 2001, 2003; Ruiterkamp et al. 2005) * PAH cations are excellent candidates for the DIB carriers!

Carbon Chain rationale Why Are Carbon Chain Anions of interest? * Carbon chains are the most common molecules in interstellar space * Carbon chains easily satisfy abundance requirements for the DIBs * Carbon chain anions are known to have rich optical spectra * Maier et al. found tantalizing coincidences in wavelength with some DIBs * Carbon chain anions may lead chemically to larger species observed in molecular clouds

Carbon chain generation

Carbon Chain Anion Generation Cold Cathode Source

Summary: Carbon Chain Anions * Carbon chain anions are not viable carriers of the DIBs * Reactions of organic anions with H, N, O occur readily and reveal rich chemistry * These processes may contribute to formation of observed interstellar species in molecular clouds

* Probe reactions of larger PAH+ PAH oven and electrospray ionization Future Directions * Model the contribution of negative ion chemistry to molecular synthesis in clouds * Probe reactions of larger PAH+ PAH oven and electrospray ionization * Explore structures of product ions incorporating H, N, O

Chuck DePuy • Shuji Kato Acknowledgments Veronica Bierbaum Valery Le Page Momir Stepanovic Cindy Barckholtz Brian Eichelberger Nick Betts Meredith Drosback Chuck DePuy • Shuji Kato NASA