2025ApJ...992..126N

Investigating the atmospheres of giants and supergiants is critical to expanding our knowledge of stellar life cycles, galaxy formation, and the origin of material that forms exoplanets. This paper presents the analysis of the evolved K-type giant β Cet using high-quality spectral data collected by the Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS), including an ultraviolet atlas with line identifications that will support future studies of cool, evolved stars. Beta Cet exhibits chromosphere emission, transition region diagnostics, and coronal X-ray flux, placing it near the Linsky-Haisch dividing line that separates objects with strong coronal signatures from the cooler giants and supergiants with massive stellar winds. The transition region in β Cet produces clear features in the STIS spectra that are formed at ${\mathrm{log}}\,T\lt $ 5.40 K, in addition to the highly ionized Fe XXIλ1354. It is stronger and more dominant than observed in the K5III hybrid star γ Dra, with a line surface flux as much as 2-3 orders of magnitude greater. The majority of the transition region lines are significantly broader than expected from thermal motion alone, and this nonthermal broadening appears to increase with formation temperature. The nonthermal width of the C IV and Si IV doublets is used to investigate the dominant heating mechanisms in β Cet's transition region. We find that the radiative losses observed in the ultraviolet are balanced by an energy influx from sound waves and turbulence, while the Alfvén waves supported by a magnetic field are required to account for the reported X-ray flux.