Modeling Gas-Grain Chemistry in Dark Cloud Conditions

I first wrote a gas phase chemical code, which solves for the gas phase composition of an interstellar cloud as a function of time. We used this code to study the abundance ratios between the H3+ isotopologues, since in this case the interaction between processes in the gas phase and on the dust grain surface can be treated in a simplified way. Grain chemistry is necessary to explain the formation of many interstellar molecules. My first investigation on grain chemistry is from the mathematical side, by looking deep into the difficulties posed by its stochasticity and discreteness. After writing a Monte Carlo code to serve as a benchmark, I developed a new method called ``hybrid moment equation'' (HME) approach, which gives results that are more accurate than those obtained with the usual rate equation approach, and it runs much faster than the Monte Carlo method for a medium-to-large-sized reaction network. Improvements in this HME approach are needed if a very large surface network is to be used. Following the recent detection of hydrogen peroxide (H2O2) in the rho Ophiuchus~A cloud core, I modeled its formation with a gas-grain network. Its observed abundance, together with the abundances of other species detected in the same source can be reproduced in our model. These molecules are mainly driven into the gas phase from the dust grain surface by the heat released in chemical reactions. Our model predicted the presence of O2H molecule in the gas phase, which has indeed been detected recently. Further investigations are needed to answer whether H2O2 is widespread in the interstellar medium. I then studied the chemistry involving species containing one or more deuterium atoms with a gas-grain-mantle three-phase model, which takes into account recent experimental results on the key reactions. The observed fractionated deuterium enhancement in water, methanol, and formaldehyde is reproduced in our models. I demonstrated that the existence of abstraction reactions for methanol and formaldehyde is the main reason for these species to be more prone to deuterium enhancement than water. The observed low [D2O/H2O] ratio suggests that water is mainly formed through H2 + OH → H2O + H on the dust grain surface. Our model also gives a range of ice mantle compositions for the dust grains that agree with the observations in different sources

Volatile depletion in the TW Hydrae disk atmosphere

An abundance decrease in carbon- and oxygen-bearing species relative to dust has been frequently found in planet-forming disks, which can be attributed to an overall reduction of gas mass. However, in the case of TW Hya, the only disk with gas mass measured directly with HD rotational lines, the inferred gas mass ($\lesssim$0.005 solar mass) is significantly below the directly measured value ($\gtrsim$0.05 solar mass). We show that this apparent conflict can be resolved if the elemental abundances of carbon and oxygen are reduced in the upper layers of the outer disk but are normal elsewhere (except for a possible enhancement of their abundances in the inner disk). The implication is that in the outer disk, the main reservoir of the volatiles (CO, water, ...) resides close to the midplane, locked up inside solid bodies that are too heavy to be transported back to the atmosphere by turbulence. An enhancement in the carbon and oxygen abundances in the inner disk can be caused by inward migration of these solid bodies. This is consistent with estimates based on previous models of dust grain dynamics. Indirect measurements of the disk gas mass and disk structure from species such as CO will thus be intertwined with the evolution of dust grains, and possibly also with the formation of planetesimals.Comment: 8 pages, 4 figures; accepted by ApJL for publicatio

We Drink Good 4.5-Billion-Year-Old Water

Water is crucial for the emergence and evolution of life on Earth. Recent studies of the water content in early forming planetary systems similar to our own show that water is an abundant and ubiquitous molecule, initially synthesized on the surfaces of tiny interstellar dust grains by the hydrogenation of frozen oxygen. Water then enters a cycle of sublimation/freezing throughout the successive phases of planetary system formation, namely, hot corinos and protoplanetary disks, eventually to be incorporated into planets, asteroids, and comets. The amount of heavy water measured on Earth and in early forming planetary systems suggests that a substantial fraction of terrestrial water was inherited from the very first phases of the Solar System formation and is 4.5 billion years old

The ancient heritage of water ice in the solar system

Identifying the source of Earth's water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space. Water throughout the solar system exhibits deuterium-to-hydrogen enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebula protoplanetary disk. Utilizing a comprehensive treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient, curtailing the disk's deuterated water formation and its viability as the sole source for the solar system's water. This finding implies that if the solar system's formation was typical, abundant interstellar ices are available to all nascent planetary systems.Comment: 33 pages, 7 figures including main text and supplementary materials. Published in Scienc

Exploring the Origins of Deuterium Enrichments in Solar Nebular Organics

Deuterium-to-hydrogen (D/H) enrichments in molecular species provide clues about their original formation environment. The organic materials in primitive solar system bodies have generally higher D/H ratios and show greater D/H variation when compared to D/H in solar system water. We propose this difference arises at least in part due to 1) the availability of additional chemical fractionation pathways for organics beyond that for water, and 2) the higher volatility of key carbon reservoirs compared to oxygen. We test this hypothesis using detailed disk models, including a sophisticated, new disk ionization treatment with a low cosmic ray ionization rate, and find that disk chemistry leads to higher deuterium enrichment in organics compared to water, helped especially by fractionation via the precursors CH$_2$D$^+$/CH$_3^+$. We also find that the D/H ratio in individual species varies significantly depending on their particular formation pathways. For example, from $\sim20-40$ AU, CH$_4$ can reach $\rm{D/H\sim2\times10^{-3}}$, while D/H in CH$_3$OH remains locally unaltered. Finally, while the global organic D/H in our models can reproduce intermediately elevated D/H in the bulk hydrocarbon reservoir, our models are unable to reproduce the most deuterium-enriched organic materials in the solar system, and thus our model requires some inheritance from the cold interstellar medium from which the Sun formed.Comment: 11 pages, 7 figures, accepted for publication in Ap