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

TL;DR

This study presents the first UV spectra of a star in a magnetic grand minimum state, revealing extremely low stellar activity and a very weak stellar wind, with implications for the star's astrosphere and debris disk exposure.

Contribution

It provides the first UV spectral data of a magnetic grand minimum star and analyzes its stellar wind and astrosphere, offering new insights into stellar magnetic activity and its effects.

Findings

HD 166620 exhibits extremely low Lyman-alpha flux, indicating minimal stellar activity.

The star has a very weak stellar wind, with mass loss rate less than 0.1 times the solar rate.

Tau Ceti's compact astrosphere suggests its debris disk is partially exposed to the interstellar medium.

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 1600's. The stellar H I Lyman-alpha surface fluxes are extremely low, about a factor of two below fluxes observed during solar minimum, and also significantly lower than those of Tau 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 Tau Ceti data that are compared with HD 166620 include both old archival data and a new HST observation as well. The Lyman alpha data are used to confirm a nondetection of astrospheric Lyman-alpha absorption for this star, suggesting a very weak wind with Mdot<0.1 Mdot_sun. The very compact astrosphere inferred for Tau Ceti indicates that the star's debris disk is at least partly exposed to the ISM, and we discuss possible consequences.