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The details of the processes responsible for heating the chromospheres of evolved cool stars remain uncertain. While most spectroscopic diagnostics measure spatially-integrated emission, here we examine diagnostics sensitive to localized atmospheric regions that are specific to cool evolved stars in binary systems with hot main-sequence companions: emission from Si i and C i resulting from the ultraviolet irradiation of the evolved star’s atmosphere. HST Space Telescope Imaging Spectrograph (STIS) high-spectral-resolution near-ultraviolet observations of ζ Aurigae A+B (K4 Ib + B5 V) were obtained at three orbital phases, including total eclipse, to search for Si i and the corresponding C i line emission. Si i 2987.645 Å emission was detected at phases ϕ = .101 and .448 (from periastron) in-line with predictions from a previous study of optical Si i 3905 Å and Si i 4102 Å emission lines. No other Si i line emission is apparent, and the analogous C i lines at 2478.561 Å and 2582.901 Å also are not detected. High-spectral-resolution HST STIS and Goddard High Resolution Spectrograph spectra confirm the results of a previous study that showed that the intrinsic chromospheric fluxes on the visible hemisphere of ζ Aur A, observed during total eclipse, are representative of the single K supergiant λ Vel (K4 Ib). Furthermore, the HST spectra show that the chromospheric turbulent velocities are very close to those in this spectral-type proxy. These combined results highlight the importance of detailed spatially-resolved chromospheric models of ζ Aur systems, based on sequences of atmospheric eclipse spectra, to help constrain the poorly understood mechanisms that heat the atmospheres and drive stellar winds in cool evolved stars.

In this paper we aim to constrain the wind temperature, outflow and turbulent velocities, ionization state, and mass-loss rate of the single red giant Arcturus (α Boo K1.5 III) using high spectral resolution Hubble Space Telescope Space Telescope Imaging Spectrograph profiles of Si iii 1206.5 Å , O i 1304 Å and 1306 Å, C ii 1334 Å and 1335 Å, and Mg ii h 2802 Å. The use of the E140-H setting for α Boo allows the Si iii 1206.5 Å line to be cleanly extracted from the echelle format for the first time. The ratios of the wind optical depths of lines from different species constrain the temperature at the base of the wind to T wind ∼ 15,400 K. The mass-loss rate derived is 2.5 × 10−11 M ⊙ yr − 1 for Epoch 2018–2019, smaller than previous semiempirical estimates. These results can be reconciled with multiwavelength Very Large Array radio continuum fluxes for Epoch 2011–2012 by increasing the temperature to T wind ∼ 18,000 K, or increasing the mass-loss rate to 4.0 × 10−11 M ⊙ yr − 1 . Interpreting the wind acceleration and turbulence in terms of a steady WKB Alfvén wave–driven wind reveals that the wave energy damping length increases with increasing radius, opposite to the trend expected for ion-neutral damping of monochromatic waves, confirming a previous result by Kuin and Ahmad derived for ζ Aur binaries. This implies that a spectrum of waves is required in this framework with wave periods in the range of hours to days, consistent with the photospheric granulation timescale. Constraints on a radial magnetic field (B) at 1.2 R * are an upper limit of B ≤ 2 G from the implied wave heating, and B ≥ 0.3 G to avoid excessive wave amplitudes.