2024ApJ...964...80T

The Mg II h&k emission lines (2803, 2796 Å) are a useful tool for understanding stellar chromospheres and transition regions due to their intrinsic brightness, relatively low interstellar medium (ISM) absorption interference, and abundance of archival spectra available. Similar to other optically thick chromospheric emission lines such as H I Lyα, Mg II emissions commonly present with a self-reversed line core, the depth and shape of which vary from star to star. We explore the relationship between self-reversal and the stellar atmosphere by investigating the extent to which fundamental stellar parameters affect self-reversal. We present a search for correlations between photospheric parameters such as effective temperature, surface gravity, and metallicity with the Mg II k self-reversal depth for a group of 135 FGKM main-sequence stars with high-resolution near-ultraviolet spectra from the Hubble Space Telescope. We modeled the observed Mg II k line profiles to correct for ISM attenuation and recover the depth of the emission line's self-reversal in relation to the intensity of the line. We used the PHOENIX atmosphere code to homogeneously determine the stellar parameters by computing a suite of stellar atmosphere models that include a chromosphere and transition region, and using archival photometry to guide the models of each star. We quantify the sensitivity of the visible and near-infrared photometry to chromospheric and photospheric parameters. We find weak trends between Mg II k self-reversal depth and age, rotation period, Mg II luminosity, temperature, and mass. All stars in our sample older than ∼2 Gyr or rotating slower than ∼10 days exhibit self-reversal.