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Project Progress

1. Overview
The interstellar medium (ISM) is a dynamic ecosystem in which matter cycles between tenuous intercloud gas and dense molecular clouds in which stars and planetary systems are born. Known interstellar molecules range from simple diatomics to species of potential astrobiological significance such as water, methane, hydrogen cyanide, formaldehyde, formic acid, and glycolaldehyde. These and other compounds contribute to the reservoir of raw materials from which planets and life may subsequently arise. Our research focuses on clarifying key steps in the evolution of this reservoir. Observations of environments ranging from quiescent molecular cores to protostellar envelopes of young stars over a range of mass and evolutionary state enable us to build a robust picture of the chemical pathways that control the interstellar input to planet-forming disks. Our long-term goal is to determine the inventories of the disks around young sun-like stars, leading to a clear understanding of the processes that led to our origins and insight into the probability of life-supporting environments emerging around other stars.

2. The oxygen inventory of the interstellar medium
After hydrogen, oxygen is the most ubiquitous of the elements essential to life, yet its abundance and distribution in the environments that give birth to new stars and planetary systems are poorly constrained (and hotly debated) quantities. Data obtained by NASA’s Submillimeter Wave Astronomy Satellite showed that gas phase molecules such as H2 O and O2 are not major reservoirs of interstellar oxygen. Spitzer and ground-based observations by our team (Whittet et al. 2007, 2009) provide a robust inventory of O uptake into ices in molecular clouds. A detailed analysis was recently completed (Whittet 2009) that links, for the first time, results for molecular clouds to observations of more tenuous phases of the ISM, where astronomers measure the “depletion” of atomic O and other elements from gas into dust (Jenkins 2009). A major discrepancy is found (fig. 1), extending over a wide range of interstellar environments, between the uptake of atomic O into known primary reservoirs (silicates, ices, gaseous CO) and the observed rate of depletion with respect to density, implying that O is being sequestered into another form. The most plausible candidate is carbonaceous matter similar in general composition to the Obearing organics found in cometary particles returned by the Stardust mission. This conclusion motivates future research to conduct a more stringent observational test for the presence of such material in the ISM than has previously been possible.

3. 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. A detailed study is in progress of the solid CO₂ vibrational bending-mode absorption at 15µm, using the Spitzer Space Telescope in coordination with ground-based observations of other major constituents. The 15µm feature is a valuable diagnostic of the physics and the chemistry of the ices, being sensitive to the temperature, crystallinity, composition and structure of the matrix that contains the CO₂. Pristine interstellar ices form in an amorphous state, displaying broad, smooth absorption profiles; when the ices are heated to crystallization temperatures in proximity to a young star, structure appears that provides a definitive signature of this process.

Our observations demonstrate that the ices contain three primary constituents: H₂O, CO and CO₂. Ternary plots are especially useful analytical tools for studying systems with three primary variables (see fig. 2). Background field stars (triangles) that sample matter in quiescent regions of molecular clouds follow a distribution consistent with adsorption and sublimation of CO, the most volatile constituent (trend toward and away from the CO apex of the plot; grey arrows). Young stellar objects (YSOs) with evidence of crystallization (red circles) are segregated away from the CO apex, consistent with thermal processing. The vertical spread in the points indicates variations in CO₂ concentration, which may arise in several ways (e.g., they may reflect CO₂ production efficiency in the parent molecular cloud or photochemical processes that produce additional CO₂ in the environments of YSOs). The trend represented by the magenta arrows illustrates the effect of differing branching ratios for the reaction pathways O+2H →H₂O and O+CO→CO₂. Interpretation of ternary plots in this context is still under development (Cook et al. 2009; Cook 2010). We expect it to become a valuable analytical tool for future work as the observational database is refined and extended to enhance both accuracy and sky coverage.

4. Organic molecules in disks around young stars

Our observations of organic molecules are designed to test chemical models that predict high abundances of molecules of prebiotic importance in preplanetary disks (Herbst & van Dishoeck 2009). Models of disks around T Tauri stars predict a flared structure, which leads to a variety of environments in which molecules may exist: (1) the cold midplane where volatiles freeze onto dust grains in the region beyond the snow line (i.e., the stellar distance where water is in solid form, ∼ 5 AU in our Solar System), (2) the disk surface, which is exposed to stellar and interstellar radiation giving rise to a photon-dominated region characterized by ionization and dissociation products, and (3) a warm molecular layer sandwiched between (1) and (2), where a rich molecular chemistry is predicted to occur. Previous observations by our group and others provided a tantalizing glimpse of the chemical profusion and structure of these regions. For example, Gibb et al. (2007) used Keck to detect HCN, C₂ H₂ and CO (and place limits on CH₄ ) in absorption toward GV Tau N, observations that are thought to sample the warm molecular layer of the disk around this star. The relative abundances of these species are generally consistent with predictions of chemical models and similar to those found among the comet population, but CH₄ appears to be under-abundant in the young stars sampled to date. Keck time awarded in January 2009 provided new spectra of these and other organic species that are currently being analyzed.

5. Astrochemical models of interstellar clouds and protoplanetary disks

The Ohio State gasgrain network of reactions is being used to construct models in which gasphase and grainsurface chemistry occur simultaneously and are coupled by both desorption and accretion processes. Rate equations are currently used, in which the time derivatives of the concentrations of individual species are related to the products of the concentrations of reactants and the rates of desorption and accretion. This approach is satisfactory for gasphase chemistry, but less so for grainsurface processes because of the small number of important reactants on grains. Under such conditions, stochastic treatments of the rate processes are preferable, but they require enormously larger amounts of computer time. Other inadequacies of the current approach are (i) the limitation to one size of interstellar grains rather than a distribution of sizes, and (ii) the lack of local structure, so that the only variables are the total numbers of species per grain rather than their location in specific monolayers. Our research during the reporting period thus consisted of two activities: use of the current gas-grain code in assorted interstellar regions, and a program of improvements to the code, especially the portion having to do with surface chemistry.

Current Uses: The chemistry of cold, dense, interstellar cores is treated by a pseudo-time- dependent approach, in which the chemistry occurs under fixed and homogeneous physicalconditions. The chemistry that occurs during the formation and evolution of a cold core following the passage of a shock wave through diffuse material (Bergin et al. 2004) was investigated. Initial results show that gaseous CO and the major ices (CO, CO₂ , H₂O) form early in the evolution and that complex molecules form much later. This result will have important ramifications regarding the chemistry of prebiotic species.

Improvements: Of several stochastic methods available, the master equations method offers the most efficient computational approach: it follows the stochastic nature of granular chemistry with deterministic equations akin to rate equations, but determines probabilities of specific populations rather than concentrations. Our previous work showed that approximations to the complete master equation are needed to make the method practical (Stantcheva & Herbst 2004). We are working on coding one such approximation, the method of moments, developed by Barzel & Biham (2007). Currently we are able to use the method with moderate numbers of different molecules on grains; our goal is to increase the number of species to the large numbers used in our standard rate equation treatment. Another improvement to our chemical model is the addition of negative molecular ions (anions), which have been recently detected in a variety of sources including protostars (Harada & Herbst 2008). The model’s predictions for the abundances of anions are generally in excellent agreement with observation.

References
Barzel, B., & Biham, O. 2007, J. Chem. Phys., 127, 144703
Bergin, E. A., et al. 2004, Astrophys. J., 612, 921
Cook, A. M., 2010, Ph.D. thesis, Rensselaer Polytechnic Institute, in preparation
Cook, A. M., et al. 2009, Astrophys. J., in preparation
Gibb, E. L., et al. 2007, Astrophys. J., 660, 1572
Harada, N., & Herbst, E. 2008, Astrophys. J., 685, 272
Herbst, E., & van Dishoeck, E. F. 2009, Annual Reviews Astron. Astrophys., 47, 427
Stantcheva, T., & Herbst, E. 2004, Astron. Astrophys., 423, 241
Whittet, D. C. B. 2009, Astrophys. J., in press
Whittet, D. C. B., et al. 2007, Astrophys. J., 655, 332
Whittet, D. C. B., et al. 2009, Astrophys. J., 695, 94