On Simulating the Proton-Irradiation of O2_2 and H2_2O Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity

Many astrochemical models today explicitly consider the species that comprise the bulk of interstellar dust grain ice-mantles separately from those in the top few monolayers. Bombardment of these ices by ionizing radiation - whether in the form of cosmic rays, stellar winds, or radionuclide emission - represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice-mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O$_2$ ice at 5 K and, (b) pure H$_2$O ice at 16 K and 77 K, were bombarded by keV H$^+$ ions. Our aims are twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine in such a well-constrained system how bulk chemistry is best handled using the same gas-grain codes that are used to model the interstellar medium (ISM). We find that our modified astrochemical model is able to reproduce both the abundance of O$_3$ in the 5 K pure O$_2$ ice, as well as both the abundance of H$_2$O$_2$ in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice.Comment: ApJ, accepted. 30 pages, 5 figure

The Case of H2_2C3_3O Isomers, Revisited: Solving the Mystery of the Missing Propadienone

To date, two isomers of H$_2$C$_3$O have been detected, namely, propynal (HCCCHO) and cylclopropenone (c-H$_2$C$_3$O). A third, propadienone (CH$_2$CCO), has thus far eluded observers despite the fact that it is the lowest in energy of the three. This previously noted result is in contradiction of the minimum energy principle, which posits that the abundances of isomers in interstellar environments can be predicted based on their relative stabilities - and suggests, rather, the importance of kinetic over thermodynamic effects in explaining the role of such species. Here, we report results of \textit{ab initio} quantum chemical calculations of the reaction between H and (a) HC$_3$O, (b) H$_2$C$_3$O (both propynal and propadienone), and (c) CH$_2$CHCO. We have found that, among all possible reactions between atomic hydrogen and either propadienone or propynal, only the destruction of propadienone is barrierless and exothermic. That this destruction pathway is indeed behind the non-detection of CH$_2$CCO is further suggested by our finding that the product of this process, the radical CH$_2$CHCO, can subsequently react barrierlessly with H to form propenal (CH$_2$CHCHO) which has, in fact, been detected in regions where the other two H$_2$C$_3$O isomers are observed. Thus, these results not only shed light on a previously unresolved astrochemical mystery, but also further highlight the importance of kinetics in understanding the abundances of interstellar molecules.Comment: ApJ, accepted: 14 pages, 2 figure

COSMIC RAY-DRIVEN RADIATION CHEMISTRY IN COLD INTERSTELLAR ENVIRONMENTS

The physiochemical impact of cosmic rays on interstellar regions is widely known to be significant \footnote{Indriolo, N. \& McCall, B. J.,\textit{Chem. Soc. Rev.}, 42, 7763-7773, 2013}. Indeed, the cosmic ray-driven formation of H$_3^+$ via the ionization of H$_2$ was shown to be of key importance in even the first astrochemical models \footnote{Herbst, E. \& Klemperer, W., \textit{Ap.J.}, 185, 505-534, 1973}. Later, cosmic rays were implicated in the collisional excitation of H$_2$, which leads to the production of internally produced UV photons that also have profound effects on the chemistry of molecular clouds \footnote{Prasad, S. S. \& Tarafdar, S. P.,\textit{Ap.J.}, 267, 603-609, 1983}. Despite these key findings, though, attempts at a more complete consideration of interstellar radiation chemistry have been stymied by the lack of a general method suitable for use in astrochemical models and capable of preserving the salient macroscopic phenomena that emerge from a large number of discrete microscopic events. Recently, we have developed a theoretical framework which meets these criteria and allows for the estimation of the decomposition pathways, yields, and rate coefficients of radiation-chemical reactions \footnote{Shingledecker, C. N. \& Herbst, E., \textit{Phys. Chem. Chem. Phys.}, 20, 5359-5367, 2018}. In this talk, we present preliminary results illustrating the effect of solid-phase radiation chemistry on models of TMC-1 in which we consider the radiolysis of the primary ice-mantle constituents of dust grains. We further discuss how the inclusion of this non-thermal chemistry can lead to the formation of complex organic molecules from simpler ice-mantle constituents, even under cold core conditions

A NEW MODEL OF THE CHEMISTRY OF IONIZING RADIATION IN SOLIDS

Cosmic rays are a form of high energy radiation found throughout the galaxy that can cause significant physio-chemical changes in solids, such as interstellar dust grain ice-mantles. These particles consist mostly of protons and can initiate a solid-state irradiation chemistry of significant astrochemical interest. In order to better understand the chemical effects of long-term exposure to ionizing radiation, we have written a new Monte Carlo model, CIRIS: the Chemistry of Ionizing Radiation in Solids, which is, to the best of our knowledge, the first successful program of its kind to follow the damage and subsequent chemistry of an irradiated material over time. In our code, two distinct regimes are considered. One is dominated by the atomic physics of track calculations in which both the irradiating proton and the subsequently generated secondary electrons are followed on a collision by collision basis. The other regime occurs after the ion-target collision, in which mobile species are free to randomly hop throughout the bulk of the ice and react via a diffusive mechanism. Here, we will present an initial test of our code in which we have successfully modeled previous experimental work. In these simulations, we are able to reproduce the measured abundances and predict the approximate ice thickness used in that study

Science with an ngVLA: Observing the Effects of Chemistry on Exoplanets and Planet Formation

One of the primary mechanisms for inferring the dynamical history of planets in our Solar System and in exoplanetary systems is through observation of elemental ratios (i.e. C/O). The ability to effectively use these observations relies critically on a robust understanding of the chemistry and evolutionary history of the observed abundances. Significant efforts have been devoted to this area from within astrochemistry circles, and these efforts should be supported going forward by the larger exoplanetary science community. In addition, the construction of a next-generation radio interferometer will be required to test many of these predictive models in situ, while simultaneously providing the resolution necessary to pinpoint the location of planets in formation.Comment: To be published in the ASP Monograph Series, "Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA

CSO and CARMA Observations of L1157. I. A Deep Search for Hydroxylamine (NH2_2OH)

A deep search for the potential glycine precursor hydroxylamine (NH$_2$OH) using the Caltech Submillimeter Observatory (CSO) at $\lambda = 1.3$ mm and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at $\lambda = 3$ mm is presented toward the molecular outflow L1157, targeting the B1 and B2 shocked regions. We report non-detections of NH$_2$OH in both sources. We a perform non-LTE analysis of CH$_3$OH observed in our CSO spectra to derive kinetic temperatures and densities in the shocked regions. Using these parameters, we derive upper limit column densities of NH$_2$OH of $\leq1.4 \times 10^{13}$~cm$^{-2}$ and $\leq1.5 \times 10^{13}$~cm$^{-2}$ toward the B1 and B2 shocks, respectively, and upper limit relative abundances of $N_{NH_2OH}/N_{H_2} \leq1.4 \times 10^{-8}$ and $\leq1.5 \times 10^{-8}$, respectively.Comment: Accepted in the Astrophysical Journa