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

Technical Summary: HST Observations of Chromospheric UV Lines in the AGB Star R Leo

Problem Statement
The physical mechanisms responsible for heating the chromospheres of cool, evolved stars (specifically Asymptotic Giant Branch or AGB stars) and their evolution remain poorly understood. While historical evidence suggests the presence of chromospheres via emission lines (e.g., Fe II, Mg II, C II), the origin of these features is debated. Two primary interpretations exist: (1) heating in quasi-static, semi-empirical atmospheric layers, and (2) heating driven by outward-propagating shocks resulting from stellar pulsation. Although shock-based models have been applied to optical lines in Mira variables, detailed non-LTE (Local Thermodynamic Equilibrium) radiative transfer calculations for ultraviolet (UV) line formation based on time-dependent, dynamical AGB atmospheres have been limited. This study addresses the gap between observed UV diagnostics and dynamical atmospheric models to determine if pulsation-driven shocks alone can explain the UV chromospheric emission in Mira variables.

Methodology
The authors analyzed high-resolution UV spectra of the nearby Mira-type AGB star R Leo obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The observations cover the wavelength range 2121.2–2941.5 Å with a resolving power of $R \approx 30,000$.

The study focuses on two primary chromospheric diagnostics:

1. The C II] 2325 Å multiplet (semi-forbidden intercombination lines), used as an electron density diagnostic.
2. The Mg II h & k resonance lines (2796 and 2803 Å), which probe extended atmospheric regions and dynamic phenomena.

To model the formation of these lines, the authors employed the RH non-LTE radiative-transfer code. The input atmospheric structures were derived from the phase-dependent, one-dimensional, spherically symmetric hydrodynamic models of Bowen (1988). These models simulate pulsation-driven shocks without imposing an additional semi-empirical chromospheric heating term, allowing the authors to test if the dynamical atmosphere itself produces the observed UV emission.

Key modeling steps included:

• Atomic Setup: Full non-LTE treatment for H, Mg, and C, with other species (He, Fe, Na, O, Si, Al, Ca, S) treated as passive background atoms for opacity and charge balance.
• Scattering Treatment: Partial Frequency Redistribution (PRD) was used for the Mg II lines to accurately model the resonance scattering essential for strong resonance lines.
• Phase Matching: The analysis focused on a model snapshot corresponding to pulsation phase $\phi \approx 0.377$, closely matching the observation epoch ($\phi \approx 0.37$).
• Corrections: Flux losses in the C II] 2326.133 Å component due to circumstellar Fe I absorption were corrected using fluorescent Fe I line measurements.

Key Contributions and Results

• Electron Density from C II]: The observed ratios of the C II] multiplet components imply an electron density of order $n_e \sim 10^9 \text{ cm}^{-3}$. After correcting for flux contamination, the diagnostic ratios ($R_1, R_2, R_3$) are consistent with this density range.
• C II] Formation Region: Contribution function analysis reveals that the C II] emission is strongly localized in a compact, shock-heated shell at $R \approx 1.8 R_{\text{phot}}$. In this region, the temperature peaks at $T \sim 10^4 \text{ K}$ and the local electron density is a few $\times 10^8 \text{ cm}^{-3}$. The emission is optically thin ($\tau_\nu \approx 0.08\text{--}0.13$) and arises in a geometrically narrow layer rather than throughout the extended atmosphere.
• Mg II h & k Formation: Unlike the compact C II] region, the Mg II lines probe a much more extended region ($R < 16 R_{\text{phot}}$). The contribution functions show that line cores form at greater radial distances than the wings, consistent with the behavior of strong resonance lines in the Sun. The Mg II h line is reproduced more reliably than the k line; the k line is significantly affected by circumstellar and interstellar absorption (specifically from Fe I and Mn I), leading to an overestimation of flux in the model compared to observations.
• Kinematics: The semi-forbidden Al II] $\lambda$2669 line, being less affected by opacity, shows a projected stellar-rest-frame blueshift of $\sim 6 \text{ km s}^{-1}$. This is consistent with shock-related motions expected in Mira atmospheres and comparable to the outward expansion velocity of the millimeter-wavelength surface measured by ALMA.
• Phase Dependence: The synthetic Mg II profiles exhibit strong phase dependence, matching the observed variability where emission peaks around $\phi \approx 0.3\text{--}0.45$. The model successfully reproduces the general shape and phase behavior of the Mg II lines, supporting the dynamical origin of the emission.

Significance and Claims
The paper claims that the combination of a compact C II] formation region, extended Mg II contribution functions, and the strong phase dependence of the synthetic profiles supports a picture in which pulsation-driven shocks shape the ultraviolet chromospheric emission lines in Mira variable stars.

The study demonstrates that detailed non-LTE modeling based on hydrodynamic atmospheric structures (without ad hoc chromospheric heating terms) can reproduce the observed UV diagnostics. Specifically, the results suggest that the high temperatures required for UV emission are produced self-consistently in the post-shock regions of the dynamical atmosphere. The work provides a physical link between the dynamical structure of the extended atmosphere (shocks) and the observed UV emission, reinforcing the interpretation that the "chromosphere" in these evolved stars is a dynamic, shock-heated region rather than a static layer.

The authors conclude that while the current model provides a reasonable match, future progress requires time-monitoring observations combining UV, optical, and mm/sub-mm diagnostics over the full pulsation cycle, alongside detailed non-LTE modeling for a larger sample of AGB stars to fully connect chromospheric emission, shock propagation, and mass loss.