HST-STIS Spectra of Antares ( <i>α</i> Scorpii: M1.5 Iab)—Type II Supernova Progenitor and Betelgeuse’s Chromospheric Twin

Abstract We present the first Hubble Space Telescope - Space Telescope Imaging Spectrograph (HST-STIS) far-ultraviolet (FUV) and near-ultraviolet (NUV) high-resolution spectra of the red supergiant (RSG) Antares (M1.5 Iab), isolated from its long-period B-star binary companion, and compare them to spectra of Betelgeuse (M2 Iab) to assess the similarities and differences in chromospheric heating and atmospheric dynamics. While Betelgeuse has two well-known circumstellar CO outflows, previous radio and infrared observations of Antares did not detect the presence of circumstellar CO. The Antares FUV STIS spectra reveal CO electronic Fourth-Positive absorption bands formed exterior to the chromosphere. The derived column density is 4 orders of magnitude smaller than the CO MOLsphere located interior to 1.7 R * . The numerous NUV Fe ii emission lines show classical wind-scattering profiles, with velocities close to the star of v wind ≃ 19 km s −1 , consistent with that previously inferred from absorption spectra observed against the B star at a projected radius of ∼150 R * . Surprisingly, the NUV semi-forbidden emission lines show evidence of wind scattering, indicative of the massive column densities present in the winds of low-surface-gravity RSGs. Differential comparison with Betelgeuse of chromospheric fluxes from emission lines excited by electron collisions, electron collisions combined with photospheric photoexcitation, and coincidental line pumping reveals similar levels of chromospheric heating. C ii ] indicates comparable chromospheric electron densities and column densities. With its well-constrained distance and stellar parameters, Antares provides a good source for understanding the processes driving RSG outflows. Chromospherically, Antares is effectively a twin of Betelgeuse.