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

Abstract

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 \textit{Hubble Space Telescope}. We focus on two strong chromospheric diagnostics, the Mg\,\textsc{ii} h \& k resonance lines and the C\,\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\,\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\,\textsc{ii}] emission is strongly localized in a compact shock-heated shell at the first temperature peak of the model atmosphere, near $R\simeq 1.8R_{\mathrm{phot}}$, 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\,\textsc{ii} h \& k lines probe a more extended region ($R<16R_{\mathrm{phot}}$), with the line cores forming at greater radial distances than the wings. The Mg\,\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\,\textsc{ii}] $\lambda 2669$ line shows a projected stellar-rest-frame blueshift of $\sim 6\mathrm{km}{\mathrm{s}}^{-1}$, consistent with shock-related motions expected in Mira atmospheres. Overall, the compact C\,\textsc{ii}] formation region, the extended Mg\,\textsc{ii} contribution functions, and the strong phase dependence of the synthetic Mg\,\textsc{ii} profiles support a picture in which pulsation-driven shocks shape the ultraviolet chromospheric emission lines in Mira variable stars.

Submitted to Astronomy & Astrophysics