Evolution of the Volatile Inventory During Planet Formation

[摘要] Today, with the wealth of data provided by the Atacama Large Millimeter/ submillimeter Array (ALMA), we are beginning to characterizing the chemistry associated with the early stages of planet formation. Planets are born within disks of gas, primarily in molecular form, and dust. ALMA enables us to, for the first time, resolve these disks down to the radii of giant planet formation, and in some instances even into the zone where Earth-like planets are born. In this dissertation I explore one of the major results from ALMA regarding the disposition of the primary carriers of carbon and nitrogen within protoplanetary disks. The state of carbon and nitrogen has important implications for the composition of planets. Knowing the abundance of gas phase species in the disk provides the starting composition for the atmospheres of gaseous giant planets while the composition of ices influence the composition of solid bodies, such as terrestrial planets.Using both models and observations, this dissertation explores the evolution of volatile molecules in protoplanetary disks. Using chemical models, I have shown that volatile nitrogen in protoplanetary disks is likely found mainly in the form of molecular nitrogen, a molecule which remains in the gas phase throughout much of the disk (Chapter 2).The rest of this dissertation focuses on the chemistry of carbon, as the main carbon carriers are more readily accessible to observational characterization. My analysis of CO isotopologue emission in the protoplanetary disk TW Hydrae, in conjunction with emission from the molecular hydrogen isotopologue HD, reveals that CO gas, the primary carrier of volatile carbon, is under-abundant relative to the total gas mass throughout the disk (Chapter 3). I thus demonstrate that it is CO, and not the total gas, which is missing in this one system. To explore the potential cause of this depletion I then ran a large grid of chemical models for disks with a wide range of physical conditions in order to analyze how effective chemical reactions are at removing volatile molecules from the gas. I found that in both the upper layers of the disk (Chapter 4) and in the midplane (Chapter 5), an ISM level cosmic ray ionization rate, one unattenuated by disk winds, is needed to reduce the CO gas abundance by greater than an order of magnitude during the typical disk lifetime. In the absence of cosmic rays, chemical processes involving ultraviolet or X-ray photons can also reprocess CO on timescales of several million years, though not to the extent seen in the high cosmic ray rate models. I conclude that chemistry is unlikely to be the only cause of volatile depletion, given that many young, 1-3 million year old, protoplanetary disks have measured CO abundances one to two orders of magnitude below expectations. Other processes, such as vertical mixing of the gas and grain growth, must also contribute. The results of my chemical modeling suggest that, under certain circumstances, gas giants which form after a million years of chemical evolution may accrete envelopes under-abundant in volatile elements such as carbon, nitrogen, and oxygen. To conclude, a summary of the findings and future directions are discussed in Chapter 6.

[发布机构] University of Michigan

[效力级别] protoplanetary disks

[关键词] astrochemistry;protoplanetary disks;planet formation;star formation;molecular astrophysics;Astronomy;Science;Astronomy and Astrophysics