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Modeling chemical abundance distributions of Local Group dwarf galaxies

(Figures from Escala et al. 2018)

As a junior graduate student at Caltech, I investigated the stellar metallicity distribution functions (MDFs), including iron and alpha element abundances, in dwarf galaxies from the Feedback in Realistic Environments (FIRE) project. I examined both isolated dwarf galaxies and those that are satellites of a Milky Way mass galaxy. In particular, I studied the effects of a sub-grid turbulent model for the diffusion of metals in gas. Compared to simulations without diffusion, we observe a narrowing of both the metallicity distribution function (left) and abundance ratio distributions, which results from individual particles being driven toward the average metallicity. This effect results in better agreement with observations of Local Group dwarf galaxies (right), e.g., in terms of the metallicity distribution function shape and width. We also find agreement between the small intrinsic scatter in the alpha-to-iron ratio in simulations and observations of dwarf galaxies. This implies that the interstellar medium in dwarf galaxies is well-mixed at nearly all cosmic times. Lastly, via comparison between isolated and satellite dwarf galaxies, I found that the similarly between their chemical abundance distributions suggests that environmental effects play a minor role compared to internal chemical evolution in the FIRE simulations.

Determining the physical characteristics of a set of unusually magnetically active brown dwarfs

See Table 4 and Section 6.2 of Kao et al. 2016. Figure from Escala et al., in prep.

As an undergraduate, I worked on developing a semi-empirical method to determine the physical parameters of a set of late L-type and T-type brown dwarfs from their low-resolution, near-infrared spectra. I calibrated empirical H20-J and K-H spectral indicides, which disentangle effective temperature and surface gravity, to spectral indicies generated from brown dwarf atmospheric models, using the benchmark brown dwarfs Gliese 570D (T7.5) and HN Peg B (T2.5) as fiducials. We improved on prior work by Burgasser et al. by including a probabilistic treatment of spectral indicies and integrating the uncertainties in the adopted benchmark parameteres and the measured indicies via Monte Carlo methods. I compared the results to those from atmospheric model fitting, and checked that the results were consistent between calibrations. The set of brown dwarfs exhibits unusual magnetic activity, thus their parameters may provide insight into dynamo theory in the substellar objects regime.