nai

Project Progress

The long-term goal of this project is to develop comprehensive, realistic astrochemical models for the evolution of interstellar organic matter and volatiles, from initial formation in interstellar clouds to subsequent infusion into protoplanetary disks. The models are constrained by observations of preplanetary material in both solid (dust, ices) and gaseous states.

1. Preplanetary dust and ices in interstellar clouds at the lowest temperatures
Dust grains are nucleation centers and catalysts for the growth of icy mantles in quiescent interstellar clouds, the products of which accumulate into preplanetary matter when new stars and solar systems form within the clouds. In a paper completed and published this year, Whittet et al (2013) present the first spectroscopic detections of silicate dust and molecular ices (H₂O, CO, CO₂) within the dark prestellar core L183, one of the coldest known interstellar clouds in the solar neighborhood of our Galaxy. A new spectral decomposition technique was developed to optimize extraction of absorption features of these materials in spectra from the NASA Spitzer Space Telescope and the ground-based Infrared Telescope Facility. Results indicate the occurrence of a transition from bare silicate grains in the outer layers of the cloud to ice-mantled grains within, providing information on the physical conditions necessary for initiation of the surface chemistry that produces these volatiles. The ices are amorphous in structure and have been maintained at low temperature (below 15 Kelvin) since formation. The spectral decomposition technique used in this work is now being applied to studies of ice and dust in stellar birth regions, where energy from the newly formed stars effects both the physical state of the material (via annealing and sublimation) and the range of possible chemical reactions (e.g. endothermic and photochemical reactions in addition to exothermic reactions).

2. Organic molecules and water in the circumstellar disks of newly formed stars
Two papers were completed and published in the reporting period.
(a) We completed a spectro-astrometric study of a solar-type young stellar object (DR Tau; Brown et al. 2013). This technique enables us to determine the location of gas in a disk to accuracy limits set only by atmospheric seeing. We reported the first spectro-astrometric signals due to water in the disk of DR Tau and determined the inner and outer radii of emitting regions to be 0.056 and 0.38 AU. The inner radius is close to the expected dust sublimation radius, suggesting that it is set by the environment in which dust shields volatiles from photodissociation.
(b) Our analysis of methane absorption in the solar-type young stellar object GV Tau N was completed and published (Gibb & Horne 2013). We detected many lines (up to J = 20) and determined a gas temperature of about 750 K. The abundance of CH₄ relative to HCN is
consistent with some disk models and comet abundances, assuming that both molecules absorb from the same column of gas.

3. Astrochemical modeling of preplanetary matter
We have made significant progress in the last year toward our goal of understanding the production of complex organic molecules in relevant environments. Until recently, complex organic molecules (COMs), a term usually meaning gaseous organic molecules of six or more atoms that are not exotic but instead normal terrestrial species, were found only in star formation regions at temperatures of up to 300 K, known as hot cores. However, in the past 2 years it has been shown that COMs such as methyl formate and dimethyl ether exist in cold dense cores of the interstellar medium, an observation not predicted by our theory of COM production, which relies heavily on the chemistry that occurs on warming dust grains. We were able to reproduce the abundances of the observed cold COMs by adding some gas-phase reactions to our gas-grain network used in simulations but also adding a more efficient non-thermal mechanism of ejecting molecules on the ice mantles of dust grains into the gas phase, where they act as precursors for the gas-phase production of COMs (Vasyunin & Herbst 2013). We are now focused on determining whether COMs can be found in protoplanetary disks, the direct precursors to stellar and planetary systems (Vasyunina et al 2013). Given the low columns expected from the results of our chemical simulations, we have applied for time on ALMA to look for the simplest COM – gaseous methanol.

Work in progress includes a gas-grain calculation in which we are able to simulate the chemistry in both phases, for the first time, with a so-called stochastic method that is much more accurate, especially for the granular chemistry, than kinetic equations, the more standard approach. We are also starting a program of coupling gas-grain chemistry with 3D-hydrodynamics to follow the collapse of a cold core into a budding protoplanetary disk. The chemical network will include deuterium fractionation, and how it is affected by the ortho and para states of important species.

Finally, a review article on interstellar chemistry that emphasizes reactions on grain surfaces has been completed (Herbst 2013).