S. D. Rodgers

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

NITROGEN ISOTOPIC FRACTIONATION OF INTERSTELLAR NITRILES

In light of recent measurement of nitrogen isotope ratios in CN and HCN in several comets, and the correlation between 15N excess and the presence of nitrile (-CN) functional groups in meteoritic samples, we have reassessed the potential of interstellar chemistry to directly fractionate nitriles. We focus in particular on the 15N chemistry in selective depletion cores where O-bearing molecules are depleted yet N- and C-bearing species remain in the gas, as revealed by the recent detection of CN in dense CO-depleted cores. We show that large HC15N/HC14N ratios can be generated if the reaction of CN with N has a barrier, and suggest that cometary HCN and CN may trace material originally formed in dense interstellar clouds.

Gas-Phase Production of NHD2 in L134N

We show analytically that large abundances of NH2D and NHD2 can be produced by gas-phase chemistry in the interiors of cold dense clouds. The calculated fractionation ratios are in good agreement with the values that have been previously determined in L134N and suggest that triply deuterated ammonia could be detectable in dark clouds. Grain-surface reactions may lead to similar NH2D and NHD2 enhancements but, we argue, are unlikely to contribute to the deuteration observed in L134N.

Astrochemistry of dimethyl ether

Dimethyl ether (DME, CH 3 OCH 3 ) is one of the largest organic molecules detected in the interstellar gas and shows high abundances in star-forming regions, known as hot molecular cores. The observed DME might be formed on grains or by secondary gas phase reactions from a precursor molecule, which in turn was sublimed into the gas phase from the grain surface. Studies on the stability and degradation pathways of DME therefore provide important constraints on the evolutionary cycle of large organic molecules and chemical pathways in the interstellar medium. We studied the UV photodestruction rate of DME in a solid argon matrix. DME was destroyed with a half-life of 54 seconds under laboratory conditions, which corresponds to

Biomolecules in the interstellar medium and in comets

8.3\times10 ^{5}

The HNC/HCN ratio in comets: Observations of C/2002 C1 (Ikeya-Zhang)

years in a dense cloud. We discuss the UV photochemistry of DME in the context of two issues: its formation mechanism and its chemistry in hot cores. Chemical models of shielded hot core regions indicate that UV photodestruction is relatively unimportant for DME, even by the internally-generated radiation field. These models clearly show that gas phase processes are almost certainly responsible for the formation of interstellar DME.

CHEMICAL PROCESSES IN COMETARY COMAE

Abstract We review recent studies of organic molecule formation in dense molecular clouds and in comets. We summarise the known organic inventories of molecular clouds and recent comets, particularly Hale-Bopp. The principal chemical formation pathways involving gas phase reactions, as well as formation by catalytic reactions on grain surfaces or through dust fragmentation, are identified for both dense clouds and cometary comae. The processes leading to organic molecules with known biological function, carbon chains, deuterium fractionation, HNC and S-bearing compounds are described. Observational searches for new interstellar organics are outlined and the connection between observed interstellar organics and those detected in comets Hale-Bopp and Hyakutake are discussed.

Chemical chronology of the Southern Coalsack

Abstract We have observed HNC and HCN in the coma of comet C/2002 C1 (Ikeya-Zhang). We derive HNC/HCN ratios of 23 per cent and 3 percent at heliocentric distances of 0.73 and 0.96 AU respectively. These amounts of HNC cannot be synthesised in the coma via bimolecular chemical reactions, and so these observations appear to confirm that the dominant source of HNC in cometary comae is the degradation of complex organic material.