2022ApJ...932...57H

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 x 10^–11\,{{M}_{\odot }\,{\rm{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 x 10^–11\,{{M}_{\odot }\,{\rm{yr}}}^{-1}. Interpreting the wind acceleration and turbulence in terms of a steady WKB Alfven 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.