High-Resolution Spectroscopy of Raman-scattered He II Lines in the Symbiotic Nova RR Telescopii

Raman-scattered emission features in symbiotic stars provide a powerful diagnostic of mass-loss and transfer processes, as they uniquely probe both ionized and neutral regions within interacting binaries. When resolved with high-resolution spectroscopy, these features encode detailed information on the physical properties of the neutral hydrogen medium. In this work, we present high-resolution spectroscopic observations of the symbiotic nova RR Telescopii obtained with FEROS in 2004 and GHOST in 2024, providing a about 20 yr baseline. We report the clear detection of all three Raman-scattered He II lines at 6545, 4851, and 4332, and constrain the distribution and kinematics of H I through line profile analysis. The three Raman lines exhibit distinct relative velocities, indicating that they trace different depths within the H I region. The Raman conversion efficiencies of the three Raman He II lines in 2024 are significantly lower than those in 2004, indicating substantial changes in the physical properties of the neutral hydrogen region. In addition, radiative transfer modeling implies a larger covering factor (opening angle) of the neutral region in 2004 than in 2024. These results indicate that the neutral hydrogen region cannot be characterized by a single H I column density, emphasizing the need for advanced radiative transfer modeling that accounts for the complex kinematics and geometry of the H I region. Overall, these results establish Raman-scattered He II lines as a powerful tool for spectroscopic tomography, allowing for direct constraints on the structure and kinematics of neutral hydrogen in symbiotic binaries.