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Galactic Evolution Through the Far-Ultraviolet Lens


This thesis adds to the toolbox of techniques in estimating late-F, G and K type dwarf star fundamental parameters; specifically age. Stellar age cannot be directly measured, yet age determinations are fundamental to understanding the evolution of our universe's constituents. Stellar modeling techniques such as isochrone fitting are used to estimate ages and interpret the evolution of the Milky Way galaxy. However, the models have varying accuracy. Through model-independent estimates of age and metallicity determined through purely empirical methods, one gains an understanding of the stellar evolution and the greater context of the Milky Way's formation and evolution.

This thesis first calibrates a relationship between stellar age and far-ultraviolet observations. I utilized far-ultraviolet photometry acquired by the Galaxy Evolution Explorer (GALEX) space telescope as an indicator of chromospheric activity to infer ages of late-F, G, and K type dwarf stars. I derived a purely empirical correlation between FUV magnitudes and stellar age in conjunction with (B-V) color. The correlation is defined in terms of a FUV-excess parameter Q(FUV-B, B-V). I related stellar age to Q. This correlation is functional up to 6 Gyr for FGK dwarfs. With such a correlation, one only need Johnson (B-V) and FUV measurements to estimate stellar age for Population I dwarf stars of solar-like temperature and metallicity.

The nature of the activity-age relationship is more elusive for giants. The difficulties in analyzing the age-activity relationship of giants stem from the challenges in determining low activity levels from observed activity indicators. In this thesis I have examined correlations between four chromospheric and coronal activity indicators. I found an evident correlation between X-ray luminosity from ROSAT observations (a coronal activity indicator), and flux from Mg II h and k emission lines (a chromospheric activity indicator). Using GALEX far-ultraviolet (FUV) magnitudes I also constrained a relationship between FUV magnitudes as a chromospheric activity indicator and X-ray luminosities. I found a varied range of FUV emission for core-helium burning stars (CHeB) indicating that giants in this phase may be receiving a magnetic field strength ``boost." Lastly I endeavored to constrain a relationship between ultraviolet emission and rotational velocities of giants, but did not find a clear relationship. Obscurities in this relationship may be resulting from a lack of sin(i) measurements and/or too few stars with vsin(i)>7 km/s.

Such a calibration between GALEX FUV magnitudes and stellar age has utility in population studies of FGK dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application I have investigated a population of solar neighborhood stars for their metallicities and velocity dispersions. I have cross-matched a sample of FGK type dwarf stars from Casagrande (2011) with the Gaia and GALEX catalogs. Using calibrated relationships between FUV magnitudes and age, I determined a chromospheric activity indicator, Q, and stellar age for each dwarf. I further investigate the activity-velocity and activity-metallicity relations with [Fe/H] and velocities from Casagrande (2011) and empirically-determined FUV ages. I show that perigalactic and eccentricity versus FUV-age plots are consistent to an ``inside out" formation history model.

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