Investigating Chromospheric Activity from Mg II λ2800 in Stars Using Scattered Light Corrected NGSL Spectra from HST

Hubble Space Telescope (HST) observed approximately 556 stellar targets for the Next Generation Spectral Library (NGSL) using Space Telescope Imaging Spectrograph (STIS). NGSL currently contains spectra of 376 targets (corresponding to proposals GO9088, GO9786, and GO10222) with wavelength coverage from 0.2 < λ < 1 μm at λ/?λ ∼ 1000 (Gregg et al., 2006; Heap and Lindler, 2009). The G230LB grating of STIS, used for UV observations, scatters red light. In this work, observations corresponding to proposal GO13776 have been completely reduced which increases the number of stellar spectra in NGSL from 376 to 514. All spectra are also corrected for scattered red light following the prescription by Worthey et al. (2022). The Mg II λ2800 feature is a good indicator for chromospheric activity (Linsky and Ayres, 1978; Fanelli et al., 1990). It has been found in this study that MgII chromospheric emission dominates over photospheric absorption for (B-V)₀ >1.0. This trend is seen for both giants and dwarfs, signifying no dependence of MgII emission on surface gravity. For stars with (B-V)₀ <0, MgII photospheric absorption dominates over emission. Extremely hot stars ((B-V)₀ <0) show neither absorption nor emission for MgII 2800. All cool stars (Teff<5500K) from NGSL are selected to verify the “basal” flux theory by Martinez et al. (2011) and a number of stars with flux lower than the anticipated “basal” flux value are found. No notable dependency is found for MgII 2800 magnitude on metallicity ([Fe/H]) for both dwarfs and giants.