HST Observations of HD 166620 and τ Ceti: First UV Spectra of a Magnetic Grand Minimum Star and the Extent of τ Ceti’s Astrosphere

Abstract We present new Hubble Space Telescope (HST) UV spectra of the K2 V star HD 166620, the first star clearly recognized to be in a “magnetic grand minimum” state analogous to the Sun’s “Maunder minimum” in the late 1600s. The stellar H I Ly α surface fluxes are extremely low, about a factor of 2 below fluxes observed during solar minimum, and also significantly lower than those of τ Ceti (G8 V) and HD 191408 (K2.5 V), two stars more similar to HD 166620 in spectral type and age (∼10 Gyr) than the Sun. The τ Ceti data that are compared with HD 166620 include both old archival data and a new HST observation as well. The Ly α data are used to confirm a nondetection of astrospheric Ly α absorption for this star, suggesting a very weak wind with M ̇ < 0.1 M ̇ ⊙ . The very compact astrosphere inferred for τ Ceti indicates that the star’s debris disk is at least partly exposed to the interstellar medium (ISM), and we discuss possible consequences.