The Advanced Spectral Library: The Evolution of Chromospheric Wind Characteristics from Noncoronal to Hybrid Giant Stars*

Abstract

The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph treasury program Advanced Spectral Library enables investigations of the characteristics and dynamics of the wind and chromosphere of cool stars using high-quality spectral data. This paper shows how the wind features change with spectral class when transferring across the Linsky–Haisch dividing line by comparing the noncoronal M3.5 giant γ Cru with the K5 hybrid γ Dra. We compare the presence of high-ionization diagnostics in the two objects in addition to the prominent wind spectrum in predominantly singly ionized species. We study the intrinsic strength and variation of the numerous Fe ii profiles observed in the near-ultraviolet HST spectrum that are sensitive to the wind opacity, turbulence, and flow velocity. The Fe ii relative strengths and wavelength shifts between the absorption and emission components reflect the acceleration of the wind from the base of the chromosphere. Furthermore, we have modeled the wind spectra using the SEI code to derive terminal velocities, wind acceleration, and mass-loss rates. Our results indicate that the chromosphere is denser when moving toward the hybrid objects. The derived average mass-loss rates in the two objects are similar but with a significantly greater terminal wind velocity and acceleration in the hybrid star. We present the analysis of the outflowing wind characteristics when transitioning from the cool noncoronal objects toward the warmer objects with chromospheric emission from a hotter environment.

Resolving Red Giant Winds with the Hubble Space Telescope*

“…However, it is Fe II that is the atomic species with by far the most numerous lines in the E230H spectra, many showing wind absorption (Rau et al 2018;Nielsen et al 2023). In Figure 8(a), we show the 2723-2734 Å spectral region of γ Cru, with four Fe II lines and one Cr II line.…”