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Galactic Evolution Through the Far-Ultraviolet Lens

Presentation #331.09 in the session “Dwarf Galaxies, Spiral Galaxies, and Galaxy Evolution”.

Published onJun 18, 2021
Galactic Evolution Through the Far-Ultraviolet Lens

Stellar ages are a key quantity in astrophysics, as they can be used to track the evolution of stars and galaxies. As stars age, their internal magnetic fields weaken due to a slowing of the star’s rotation. Furthermore, a star’s magnetic field strength is also related to the chromospheric activity of the star. That is, a star with a stronger magnetic field has more intense chromospheric activity that includes flares and spots. We detect the level of stellar chromospheric activity by utilizing far ultraviolet (FUV) photometry obtained from the Galaxy Evolution Explorer (GALEX) and the Global Astrometric Interferometer (Gaia) telescopes. From the FUV emission data, we determined the age of a star using purely empirical methods. By calibrating the GALEX FUV-age relationship with DR2 Gaia data, we were able to establish an FUV-age relationship in conjunction with (GBP - G) colors. We restricted the data points to stars with the color range 0.3 ≤ (GBP - G) ≤ 0.5 in which we see a range of activity levels for a given stellar age. Using Q as an FUV-excess parameter, we determined that the relationship between stellar age, t, and Q is: loge(t) = loge(a) + bQ, where a and b are fit parameters. This method shows that there is a linear relationship between the logarithmic age of a star and its chromospheric activity (FUV emission). As a result, with only FUV observations and readily available Gaia data, one may estimate the age of a single dwarf star. This has important utility such that empirical stellar ages are used to create age-velocity (AVR) plots and age-metallicity relationships. AVRs are generally used with isochrone-determined ages. Such ages often have significant associated errors due to varied assumptions made in the stellar evolution modeling. In contrast, our stellar ages are obtained purely from observational data. This would then test the AVR plot with FUV determined ages, as opposed to previous methods of isochrone determined ages. Our future work consists of constructing AVR plots using Gaia and Galex observations to examine the evolutionary mechanisms of the Milky Way Galaxy.


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