Using Mg II k Lines to Probe Stellar Chromopheres

Stellar chromospheres constitute a complex and crucial part of stellar atmospheres. Understanding stellar chromospheres could improve our understanding of solar weather, plasma, and stellar magnetic fields. Past research has used the Mg II k spectral lines (at ∼ 2796.34 [A]), to gauge upper solar chromosphere activity associated with spatially resolved solar features (e.g. sunspots, plage, etc.). This study aimed to use Mg II k spectral analysis to gauge the likely chromospheric activity of other stars. We examined the spectra of 141 stars observed by NASA's Hubble Space Telescope. The most common Mg II k spectral profile was a broad line with a central reversal, forming two peaks with a central depression. The remaining 27 Mg II k spectra consisted of symmetrical or asymmetrical single peak profiles. We used a double gaussian curve fit model to, where relevant, extract each star's Mg II k spectra characteristics: Mg II k line spectral shape, peak asymmetry, relative depression depth, peak to peak width, full width at half maximum, and full width at half maximum for each individual peak. We identified how the properties of the Mg II k spectra varied with stellar characteristics, such as temperature, metallicity, age, mass, radius, and rotational period.