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Project Progress

Overview
This research focuses on clarifying key steps in the evolution of preplanetary matter from simple molecules in the interstellar medium to prebiotic compounds found in primitive bodies in the Solar System. It requires a synergy of observational astronomy and astrochemical modeling that builds upon the expertise within our team. The past year has seen much progress and substantial growth in the scope of our activities. New collaborations have been forged with groups at NASA Ames (Yvonne Pendleton), JPL (Paul Goldsmith) and the University of Helsinki (Kalevi Mattila et al) that have provided access to new databases as well as new expertise. Our team also co-organized and actively contributed to the NAI Workshop Without Walls – The Organic Continuum from the Interstellar Medium to the Early Solar System. Another highlight was Amanda Cook’s successful completion of her doctoral degree at RPI and award of a NASA Postdoctoral Fellowship at Ames Research Center to work on the O/OREOS mission.

The evolution of interstellar and protostellar ices
Spectral features of ices yield both quantitative information on their abundances and insight into the environments in which they form and evolve. Our work on solid CO₂ discussed in last year’s report has now been submitted for publication (Cook et al, 2010). Our current work focuses on methanol (CH₃OH) as a diagnostic of photochemistry in protostellar envelopes.
A key question in interstellar chemistry is the relative efficiency of oxidation vs. hydrogenation of CO: whereas oxidation produces the relatively unreactive CO₂, hydrogenation leads to CH₃OH, a species easily synthesized into more complex organic molecules. A catalog of ice-phase abundances for both CO₂, and CH₃OH has been assembled from the literature and from unpublished archival data. Figure 1 shows a histogram of the abundance ratio CH₃OH/CO₂ for field stars and young stellar objects (YSOs) in our catalog. All field stars and the majority of YSOs lie in a distribution centered on CH₃OH/CO₂ ~ 0.2, indicating a strong preference for CO oxidation in molecular clouds. However, enhanced methanol (EM) is evident in a subset of YSOs characterized by CH₃OH/CO₂ > 0.8. This result provides strong support for the hypothesis that energy from the young star itself drives methanol formation in favorable situations, thus opening new pathways to the production of complex organics in protostellar environments.

Gas/Grain interactions in interstellar clouds
Chemical evolution in the interstellar medium depends on both gas-phase reactions and surface reactions on dust particles: the extent to which key molecules such as CO “deplete” from the gas onto the dust in dense clouds is fundamental to our models, yet very poorly constrained by previous studies. We have combined radio observations of gas phase CO with infrared observations of solid CO toward background stars that probe dense cores within the Taurus molecular cloud. We find that the depletion increases rapidly with the total quantity of dust, and exceeds 60% toward the densest cores where new stars are likely to be born. We show that it is plausible for such high levels of depletion to be reached in the cores on timescales of about 600,000 years, comparable with the timescale for collapse to form new stars. The results of this work were recently published by Whittet et al. (2010).

Using data from the SEST (Swedish European Submillimetre Telescope) provided by our collaborators at the University of Helsinki, we have mapped the intensity of gaseous C¹⁸O (J=1-0 and 2-1), ¹³CO (J=1-0 & 2-1) and N²H⁺ (J=1-0) emission lines toward a dark globule in the vicinity of Rossano cloud B (Corona Australis). We have derived accurate column densities and excitation temperatures for these species. We detect a distinct difference in the location of the peak abundance of CO and N₂H⁺, indicating depletion of the former from the gas to the solid phase. To complement our SEST observations, we mapped the visual extinction (A_v) toward the target cloud utilizing infrared photometry from the 2MASS catalog. The level of CO depletion correlates well with visual extinction, suggesting that this volatile is freezing onto grains at higher densities and lower temperatures. Analysis of results and their implications for grain models is continuing.

Astrochemical models of interstellar clouds and protoplanetary disks
Progress has been made in our quest to simulate the chemistry of regions of the interstellar medium along the evolutionary trail to the formation of stars and planets. We have developed a new network of chemical reactions operative in the temperature range 10 – 800 K (Harada et al. 2010). Our previous network of reactions was designed mainly for the low temperature interstellar medium, with a maximum temperature of 100 K. With the new network, we are now able to study portions of protoplanetary disks near their young central star, and so simulate the chemistry in the interesting region of 1 – 10 AU from the star. We have also further developed a sensitivity analysis for determining to which parameters the results of simulations are most sensitive (Wakelam et al. 2010). Among the most important parameters are the rates of specific chemical reactions, and so the sensitivity analysis can be utilized to determine which reactions should be studied in the laboratory, either for the first time, or to determine more accurate results than with previous studies.

We have also studied the chemistry of unusual isomers as a tool towards understanding the physical conditions of star-forming regions. The most recent isomers studied are HNCS (iso-thiocyanic acid) and HSCN (thiocyanic acid); these species are found in both the cold core TMC-1 and the hot core Sgr B2 (Adande et al. 2010). The fact that, unlike the analogous oxygen-containing CHNO isomers, HNCS and HSCN are of nearly equal abundance in Sgr B2 indicates that they are out of equilibrium, and that both molecules are likely produced at surprisingly low temperature from a common precursor, HNCSH⁺.

References
Adande, G. R., Halfen, D. T., Ziurys, L. M., Quan, D., & Herbst, E. 2010, Astrophys. J., in press
Cook, A. M., Whittet, D. C. B., Shenoy, S. S., Gerakines, P. A., White, D. W., & Chiar, J. E. 2010, Astrophys. J., submitted
Harada, N., Herbst, E., & Wakelam, V. 2010, Astrophys. J., 721, 1570
Wakelam, V., Herbst, E., Le Bourlot, J., Hersant, F., Selsis, F., & Guilloteau, S. 2010, Astron. Astrophys., 517, A21
Whittet, D. C. B., Goldsmith, P. F., & Pineda, J. L. 2010, Astrophys. J., 720, 259