HST observations of chromospheric UV lines in the AGB star R Leo

The role of stellar chromospheres in the chemistry, mass loss, and evolution of cool evolved stars remains poorly understood. We present high-resolution ultraviolet spectra of the nearby Mira-type AGB star R~Leo obtained with STIS on board the $\mathit{\text{Hubble}}\mathit{\text{Space}}\mathit{\text{Telescope}}$. We focus on two strong chromospheric diagnostics, the Mg,$\text{textsc}ii$ h & k resonance lines and the C,$\text{textsc}ii$] 2325,Å multiplet, and model their formation with the NLTE radiative-transfer code RH using phase-dependent hydrodynamic atmospheric structures that include pulsation-driven shocks. The observed C,$\text{textsc}ii$] multiplet ratios imply an electron density of order ${10}^9\mathrm{c}{\mathrm{m}}^{-3}$. The RH contribution functions show that the C,$\text{textsc}ii$] emission is strongly localized in a compact shock-heated shell at the first temperature peak of the model atmosphere, near $R\simeq 1.8R_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}$, where $T\sim {10}^4$,K and the local electron density is a few ${10}^8\mathrm{c}{\mathrm{m}}^{-3}$. In contrast, the Mg,$\text{textsc}ii$ h & k lines probe a more extended region ($R<16R_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}$), with the line cores forming at greater radial distances than the wings. The Mg,$\text{textsc}ii$ 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,$\text{textsc}ii$] $\lambda 2669$ line shows a projected stellar-rest-frame blueshift of $\sim 6\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$, consistent with shock-related motions expected in Mira atmospheres. Overall, the compact C,$\text{textsc}ii$] formation region, the extended Mg,$\text{textsc}ii$ contribution functions, and the strong phase dependence of the synthetic Mg,$\text{textsc}ii$ profiles support a picture in which pulsation-driven shocks shape the ultraviolet chromospheric emission lines in Mira variable stars.