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Project Progress

The long-term goal of this project is to develop comprehensive, realistic models for the evolution of interstellar organic matter and volatiles, from initial formation in interstellar clouds to eventual inclusion in protoplanetary disks. In the past year we have made important progress with several subprojects, described below. We have also been successful in a bid for observing time on the Stratospheric Observatory for Infrared Astronomy (SOFIA), results from which are expected to be a major driver of our research in future years.

The interplay of gas and dust in a prestellar core
Chemical evolution in prestellar molecular clouds is dependent on interactions between atomic and molecular gas and solid-phase dust and ices: the dust provides a substrate for many important chemical surface reactions and also constrains gas-phase chemistry by removing species from the gas. We have observed the abundance and distribution of molecular gas in the cold (10 – 14 K), starless core DC 000.4-19.5 (SL42) in Corona Australis (figure 1) using data from the Swedish-ESO Submillimeter Telescope and the Herschel Space Telescope, made available to us through a collaboration with the University of Helsinki. We have mapped the distribution of two key species, namely CO (which tends to readily freeze onto dust at these temperatures) and N₂H⁺ (which typically remains in the gas). Herschel data for the same region allow direct comparison with the dust component of the cloud core: results confirm and quantify the depletion of gaseous CO at the highest extinctions. We tested two chemical models and found that a steady-state depletion model best fits the observations. Kinetic and potential energy estimates of SL42, in addition to its density profile, suggest that it is collapsing to become a site of star formation. This project is a major component of the thesis research of NAI-supported graduate student Emily Hardegree-Ullman, who is lead author of a paper on this work submitted to Astrophysical Journal (currently under review).

The evolution of dust in an isolated dark globule subjected to local UV irradiation
The dark globule DC314.8-5.1 provides a unique opportunity to study the interaction of an isolated, starless core with a UV radiation field generated by the serendipitous presence of a nearby hot field star. By studying this interaction we aim to better understand the role of photochemistry in the evolution of preplanetary matter. UV photochemistry is expected to play an important role in organic synthesis in the circumstellar envelopes of young stars, but the radiation field is often poorly constrained due to uncertainties in their intrinsic spectra and the amount of absorption in their circumstellar disks. In the case of DC314.8-5.1, the radiation field is well constrained because the illuminating source lacks a circumstellar disk and its spectral type and intrinsic temperature are accurately determined. We are analyzing spatially extended spectral observations obtained with the Spitzer Space Telescope to study changes in the intensity of the emission spectrum of polycyclic aromatic hydrocarbons (PAHs) in the globule as a function of the distance from the illuminating source. The spectra are being interpreted with reference to the NASA Ames Polycyclic Aromatic Hydrocarbon Infrared Spectroscopic Database. Results to date indicate that (a) PAHs are well mixed with other forms of small dust in the globule; (b) the strongest PAH emission features are seen on the edges of the globule because the exciting UV photons do not penetrated deep inside due to shielding by the dust; and© the ratio of ionized to neutral PAH is almost constant. A paper detailing our results is in preparation.

3. Molecular abundances and gas/dust stratification in protoplanetary envelopes
We were awarded time on the Keck II telescope to study the abundance and evolution of gas phase organic molecules in protoplanetary disks around solar-mass stars. We confirmed the presence of water in the disk surrounding DR Tau and are currently modeling the profile to constrain the location of the water in its protoplanetary disk. We detected strong HCN absorption features and weaker features due to C₂H₂ in the spectrum of GV Tau, and have found evidence for real temporal variation in the strengths of the features in observations taken 4 years apart. The results of a detailed investigation of the gas to dust ratio in several T Tauri stars have been published (D. Horne et al, 2012, ApJ, 754, 64): the results confirm those of a previous study that found evidence for a correlation between gas/dust ratio and disk inclination – evidence for stratification in the disks.