HST STIS Observations of zeta Aurigae A's Irradiated Atmosphere

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 zeta 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 angstrom emission was detected at phases phi = .101 and .448 (from periastron) in-line with predictions from a previous study of optical Si i 3905 angstrom and Si i 4102 angstrom emission lines. No other Si i line emission is apparent, and the analogous C i lines at 2478.561 angstrom and 2582.901 angstrom 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 zeta Aur A, observed during total eclipse, are representative of the single K supergiant lambda 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 zeta 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.