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 work provides the first UV spectrum for a magnetic grand minimum star, HD 166620, and compares it to old, inactive solar analogs Tau Ceti and HD 191408 to illuminate UV output during MGM phases. It employs HST/STIS UV spectroscopy, with detailed ISM absorption modeling using Mg II and Fe II to reconstruct intrinsic Lyα and Mg II fluxes, and uses Lyα nondetection toward Tau Ceti to constrain a very weak stellar wind via 2.5-D hydrodynamic astrosphere models, yielding Mdot < $0.1\dot{M}_{\odot}$ and a particularly compact astrosphere. The results show that HD 166620 has Lyα and Mg II fluxes substantially lower than Tau Ceti and HD 191408, suggesting very low chromospheric activity during MGM, while Tau Ceti’s wind and astrosphere imply its debris disk is at least partly exposed to the ISM, with potential dust-sandblasting implications. Overall, the study advances understanding of solar MGM-like phases, their UV signatures, and the environmental impact on surrounding debris disks through wind-ISM interactions, supported by direct UV observations and ISM-informed spectral reconstructions.

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.

Figures (9)

• Figure 2: Slightly smoothed HST/STIS spectra of the H I Ly$\alpha$ line of three stars, plotted in surface flux units on a heliocentric velocity scale. Two separate spectra are shown for $\tau$ Ceti. Very broad H I ISM absorption absorbs the central part of the line profile, with narrow D I absorption also being present blueward of H I. Shaded regions indicate narrow geocoronal emission within the saturated core of the H I absorption.
• Figure 3: HST/STIS spectra of ISM Mg II and Fe II absorption lines observed toward HD 166620, plotted on a heliocentric velocity scale. Blue lines indicate the assumed stellar background above the absorption. The absorption profiles are fitted with two components (dashed lines), with the red line being the combination of the two after instrumental smoothing, which fits the data.
• Figure 4: Fits to the H I Ly$\alpha$ lines of $\tau$ Ceti (for the 2025 spectrum) and HD 166620, with residuals shown underneath. Solid black lines are the inferred intrinsic stellar line profile, and blue lines indicate the ISM absorption line fit to the data. Only a single ISM component is assumed for $\tau$ Ceti, while two are assumed for HD 166620 (red lines), where the Fe II fit from Figure 2 is used to constrain the two-component fit.
• Figure 5: The H I Ly$\alpha$ and Mg II k lines of our three G8-K2 dwarf stars, plotted in surface flux units versus a velocity scale in the stellar rest frame. The bottom panel of the Mg II figure shows the spectra after normalization to a solar Mg abundance.
• Figure 6: Slightly smoothed spectra of the C II 1335.692 Å and C IV 1548.204 Å lines of our three G8-K2 dwarf stars, plotted in surface flux units versus a velocity scale in the stellar rest frame.