Atmospheric Chemistry in Brown Dwarfs and Low Mass Stars

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Chemistry is the foundation for understanding the evolution and spectral appearance of any given astronomical object. Spectroscopic observations probe the chemistry, and in order to interpret such observations, it is crucial to know how the chemistry operates. In the cool atmospheres of low-mass stars and brown dwarfs, molecular and cloud condensation chemistry is of particular importance as it determines the major outcome of photometric and spectroscopic observations. In this project, the investigators are determining the gas and cloud condensation chemistry in cool dwarf star atmospheres as a function of temperature, total pressure, and metallicity. Chemical equilibrium depends only on pressure, temperature, and composition. Thus a grid of equilibrium abundances can be calculated independently from any particular atmospheric model. On the other hand, atmospheric and spectral models require reliable input chemistry. Hence the second major objective is to provide a reliable abundance grid of gaseous, liquid, and solid compounds derived from thermochemical equilibrium, and, where necessary, thermochemical kinetic computations. The work is essential to address key questions about low-mass objects, including the following: What are the effective temperature scales of L and T dwarfs? What are their atmospheric compositions? How common are departures from chemical equilibrium due to vertical transport processes? How do clouds and 'weather' affect the spectra of brown dwarfs? A natural extension is to apply the chemistry to generate new atmospheric models and high-resolution synthetic spectra of low-mass objects to determine spectral diagnostics of effective temperature, gravity, and composition as functions of mass and age. Broader Impacts. The work is complementary to observational studies of low-mass objects and provides a guide to interpret future observations from a wide variety of instruments from ground-based observatories. Future results will be described in scientific papers, in talks at professional meetings, and in public lectures. The long standing experience of the investigators with thermochemical and kinetic atmospheric modeling in planetary science benefits the existing interdisciplinary collaboration with other research groups working on spectral models. Undergraduate student participation, already ongoing, is being extended to the current work.