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Project Progress

The goal of this task is to determine the chemical composition of icy bodies and establish their potential for delivering pre-biotic organic materials and water to the young Earth and other planets. This is being performed through detailed chemical modeling, coupled with physical evolution, of the interstellar precursor material and of the protosolar nebula. Related observational and laboratory studies are also undertaken.

Progress in Theoretical Models for Chemical Evolution:

We made significant progress in two theoretical sub-projects. Primitive solar system materials (including cometary volatiles) display distinctive, enhanced, and apparently homogeneous, isotopic fractionation ratios (D/H, ¹⁵N/¹⁴N, ¹³C/¹²C, 17,¹⁸O/¹⁶O). Molecular ortho/para spin ratios are also constant amongst the comets observed thus far. The facts that the ratios are different from those generally measured in cold molecular clouds, and that each requires different physical conditions to be reproduced, has led us to consider a comprehensive new explanation.

In this new scenario, the pre-stellar clump that eventually formed the protosun evolves into a 'depletion core’, similar to those observed throughout the Galaxy (e.g., B68, L1544). Initially, there are strong radial gradients in the fractionation with the D/H ratios largest near the center, where CO is completely frozen out, and the 15N/14N ratios enriched in a shell where there is selective CO/N₂ depletion. We further assume that this core is located in/near a cluster of OB stars such that the outer shells experience strong UV irradiation with the isotope-selective photodissociation of CO producing enhanced ^17,18O/¹⁶O ratios in them. We follow the gravitational collapse of this system and find that the physical and chemical structures of the envelope can account quite generally for the isotopic reservoir of primitive materials. For example, the observed co-existence of key temperature windows of 25-35 K for the D/H and ortho/para ratios, and of 10K or less for the nitrogen fractionation. This work is to be published in Space Science Reviews (Charnley & Rodgers 2007) and a paper is being prepared for submission to the Astrophysical Journal Letters.

Development of our disk chemistry code is continuing, using a modular approach. First, an existing model of gas-grain chemistry in a static disk is being modified to include all the relevant microphysics. A detailed treatment of the UV photochemistry is now included and X-ray chemistry will be added next. Ultimately, this new chemical model will be included in the model of a dynamically-evolving disk. This is being developed as two additional modules. The problem of accurately computing the chemistry under advection and diffusion is being addressed separately, using a simplified reaction network. Second, a physical model of an evolving disk will be constructed (following Ruden & Pollack 1991). The final modular model will combine all three elements.

Progress in observational programs related to the connections between interstellar and cometary chemistries :

Three studies of chemistry in star-forming regions were completed. We published the first detection of interstellar heavy water (D₂O) (in IRAS 16293-2422, Butner et al. 2007). From the observed H₂O:HDO:D₂O fractionation ratios, we conclude that the D2O molecules must have formed in the cool pre-stellar gas and condensed as ices, being subsequently evaporated once the protostars heated the gas.

We also initiated a series of laboratory experiments carried out at Warsaw University. These involved measuring the rotational spectra of pyruvic acid and glycolic acid. The pyruvic acid data are now published (Kisiel et al. 2007) and an astronomical search is planned for the next funding period.

Ruiterkamp et al. (2007) surveyed several star-forming cores for ketene emission, concluding that CH2CO is quite common in these environments and most likely is produced in grain-surface reactions.

We also performed (May 2007) Submillimeter Array observations of the organic-rich hot gas in the inner (protoplanetary) disk of the Class I source IRS 46. This complements our earlier ARO SMT observations of strong formaldehyde and methanol emission from IRS46 (see also the report by GCA co-Investigator Geoffrey Blake). Data reduction is being carried out by our collaborators at ASIAA (group of Prof. Y-J. Kuan).

We completed the analysis of data from our HCN observations of comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) (Charnley et al. 2007). When these data are combined with those from CN optical observations, we can follow the heliocentric evolution of the HCN/CN ratio. While the HCN/CN ratio is consistent with CN being the photodissociation daughter of HCN in T7, in Q4 there must be another unknown molecule that is producing most of the observed CN. Finally, with G. Villanueva (NPP – GSFC), we recently participated in radio observations of comets Encke, Machholtz and Lovejoy at ARO (May 2007); these data are currently being reduced.