Steven B. Charnley

The End of Interstellar Chemistry as the Origin of Nitrogen in Comets and Meteorites

We describe a mechanism for enhanced nitrogen isotope fractionation in dense molecular gas where most of the molecules containing carbon and oxygen have condensed on grains but where N2 remains in the gas. The lack of hydroxl molecules prevents the recycling of N atoms into N2, and the nitrogen eventually becomes atomic. Ammonia is formed efficiently under these conditions and rapidly accretes as ice. We find that a significant fraction of the total nitrogen is ultimately present as solid NH3. This interstellar ammonia is enhanced in 15N with 15NH3/14NH3 almost 80% higher than the cosmic 15N/14N ratio. It is possible that a large part of the nitrogen available to the early solar system was highly fractionated ammonia ice and hence that the 15N enhancements of primitive solar system material and the depletion of N2 in comets are concomitant. Other implications of this theory for observations of dense molecular material and the nitrogen inventory available to the protosolar nebula are briefly discussed.

Chemical Differentiation in Regions of Massive Star Formation

We have reexamined the origin of the apparent differentiation between nitrogen-bearing molecules and complex oxygen-bearing molecules that is observed in hot molecular cores associated with massive protostars. Observations show that methanol is an ubiquitous and abundant component of protostellar ices. Recent observations suggest that ammonia may constitute an appreciable fraction of the ices toward some sources. In contrast to previous theories that suggested that N/O differentiation was caused by an anticorrelation between methanol and ammonia in the precursor grain mantles, we show that the presence of ammonia in mantles and the core temperature are key quantities in determining N/O differentiation. Calculations are presented which show that when large amounts of ammonia are evaporated alkyl cation transfer reactions are suppressed and the abundances of complex O-bearing organic molecules greatly reduced. Cooler cores (100 K) eventually evolve to an oxygen-rich chemical state similar to that attained when no ammonia was injected, but on a timescale that is an order of magnitude longer (~10(5) yr). Hotter cores (300 K) never evolve an O-rich chemistry unless ammonia is almost absent from the mantles. In this latter case, a complex O-rich chemistry develops on a timescale of ~10(4) yr, as in previous models, but disappears in about 2 x 10(5) yr, after which time the core is rich in NH3, HCN, and other N-bearing molecules. There are thus two ways in which N-rich cores can occur. We briefly discuss the implications for the determination of hot-core ages and for explaining N/O differentiation in several well-studied sources.

CHEMICAL EVOLUTION IN PROTOSTELLAR ENVELOPES: COCOON CHEMISTRY

We have modeled the chemistry that occurs in the envelopes surrounding newborn stars as they are gradually heated by the embedded protostar and the ice mantles of dust grains evaporate, resulting in a hot molecular core. We consider two dynamical scenarios: (1) a cloud undergoing the ‘‘ inside-out ’’ gravitational collapse calculated by Shu and (2) a quasi-stationary envelope. The radial distribution of dust temperature means that differences in surface binding energies result in distinct spatial zones with specific chemistries, as more volatile species (e.g., H2S) are evaporated before more tightly bound species (e.g., H2O). We use our results to identify chemical features that depend on the nature of the collapse and so determine observational tests that may be able to distinguish between different dynamical models of the star formation process. We show that the observed molecular abundances in massive hot cores can be explained only if these objects are supported against collapse. Subject headings: astrochemistry — ISM: abundances — ISM: clouds — molecular processes — stars: formation

Organic Molecules in Low-Mass Protostellar Hot Cores: Submillimeter Imaging of IRAS 16293?2422

Arcsecond-resolution spectral observations toward the protobinary system IRAS 16293-2422 at 344 and 354 GHz were conducted using the Submillimeter Array. Several complex organic molecules, such as CH3OH and HCOOCH3, were detected and mapped. Together with the rich organic inventory revealed, it clearly indicates the existence of two, rather than one, compact hot molecular cores (400 AU in radius) associated with each of the protobinary components identified by their dust continuum emission in the inner star-forming core.

Spectroscopic diagnostics of organic chemistry in the protostellar environment

A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.

Stochastic Theory of Molecule Formation on Dust

The formation of interstellar molecules on the surfaces of dust grains is calculated in the framework of stochastic reaction kinetics. The master equation and the state transition probabilities are defined, and the growth of grain mantles following accretion and reaction of gas-phase species is computed. The results are compared to the observed composition and structure of interstellar ices. The differences between this approach to gas-grain kinetics and previous work is discussed, and possible extensions of the theory are outlined.

The Astrobiology of Nucleobases

Nucleobases are nitrogen heterocycles (N-heterocycles) that are essential components of the genetic material in all living organisms. Extraterrestrial nucleobases have been found in several carbonaceous chondrites, but only in traces. No astronomical data on these complex molecules are currently available. A large fraction of the cosmic carbon is known to be incorporated into aromatic material, and given the relatively high abundance of cosmic nitrogen, the presence of N-heterocycles can be expected. We present infrared spectroscopic laboratory data of adenine and uracil under simulated space conditions. At the same time we tested the stability of these nucleobases against ultraviolet (UV) irradiation at 12 K. Our experimental results indicate that gas-phase adenine and uracil will be destroyed within hours in the Earths vicinity. In dense interstellar clouds exposed to UV radiation, only adenine could be expected to survive for a few million years. We discuss possible formation routes to purines and pyrimidines in circumstellar environments and in meteorite parent bodies.

Multilayer Formation and Evaporation of Deuterated Ices in Prestellar and Protostellar Cores

Extremely large deuteration of several molecules has been observed toward prestellar cores and low-mass protostars for a decade. New observations performed toward low-mass protostars suggest that water presents a lower deuteration in the warm inner gas than in the cold external envelope. We coupled a gas-grain astrochemical model with a one-dimensional model of a collapsing core to properly follow the formation and the deuteration of interstellar ices as well as their subsequent evaporation in the low-mass protostellar envelopes with the aim of interpreting the spatial and temporal evolutions of their deuteration. The astrochemical model follows the formation and the evaporation of ices with a multilayer approach and also includes a state-of-the-art deuterated chemical network by taking the spin states of H2 and light ions into account. Because of their slow formation, interstellar ices are chemically heterogeneous and show an increase of their deuterium fractionation toward the surface. The differentiation of the deuteration in ices induces an evolution of the deuteration within protostellar envelopes. The warm inner region is poorly deuterated because it includes the whole molecular content of ices, while the deuteration predicted in the cold external envelope scales with the highly deuterated surface of ices. We are able to reproduce the observed evolution of water deuteration within protostellar envelopes, but we are still unable to predict the super-high deuteration observed for formaldehyde and methanol. Finally, the extension of this study to the deuteration of complex organics, important for the prebiotic chemistry, shows good agreement with the observations, suggesting that we can use the deuteration to retrace their mechanisms and their moments of formation.

Formation and photostability of N-heterocycles in space I. The effect of nitrogen on the photostability of small aromatic molecules

Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for exam- ple as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UV stabil- ity. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10-100 years, while in the Solar System at 1 AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UV flux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astro- nomical environments.

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block—the aromatic benzene molecule—has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C2H + H2CCHCHCH2 → C6H6 + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.