HST observations of chromospheric UV lines in the asymptotic giant branch star R Leo I. C II] multiplet and Mg II h and k lines

Journal article, 2026

The role of stellar chromospheres in the chemistry, mass loss, and evolution of cool evolved stars remains poorly understood. We present high-resolution ultraviolet (UV) spectra of the nearby Mira-type asymptotic giant branch (AGB) star R Leo obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We focus on two strong chromospheric diagnostics, the Mg II h and k resonance lines and the C II] 2325 & Aring; multiplet, and model their formation with the non-local-thermal-equilibrium (NLTE) radiative-transfer code RH using phase-dependent hydrodynamic atmospheric structures that include pulsation-driven shocks. The observed C II] multiplet ratios imply an electron density of order of 10(9) cm(-3). The calculated contribution functions show that the C II] emission is strongly localised in a compact shock-heated shell at the first temperature peak of the model atmosphere, near R similar or equal to 1.8 R-phot, where T similar to 10(4) K and the local electron density is a few 10(8) cm(-3). In contrast, the Mg II h and k lines probe a more extended region (R < 16 R-phot), with the line cores forming at greater radial distances than the wings. The Mg II h line is reproduced more reliably than the k line, which is more strongly affected by circumstellar and interstellar absorption. As an additional low-opacity kinematic check, the semi-forbidden Al II] lambda 2669 line shows a projected stellar-rest-frame blueshift of similar to 6 kms(-1), consistent with shock-related motions expected in Mira atmospheres. Overall, the compact C II] formation region, the extended Mg II contribution functions, and the strong phase dependence of the synthetic Mg II profiles support a picture in which pulsation-driven shocks shape the UV chromospheric emission lines in Mira variable stars.