Spectropolarimetric Constraints on the Maunder Minimum Analog HD 166620: Evidence for Weakened Magnetic Braking

Abstract

We present the first spectropolarimetric time-series analysis of the Maunder Minimum analog HD 166620, using 12 nights of data from CFHT/SPIRou and a single epoch from CFHT/ESPaDOnS. While individual Stokes $V$ profiles exhibit no significant polarization signatures, we leverage the rotational coverage of the SPIRou dataset to compute a grand average LSD profile. Forward modeling of the cumulative Stokes $V$ signal, assuming a purely axisymmetric dipole, yields a best-fit dipole field strength of $B_{\mathrm{dip}}=1.10_{-0.90}^{+0.95}$G ($3\sigma$). This field strength matches simulations of the solar dipole during the Maunder Minimum phase. Our results are consistent with independent constraints on the dipole field strength from an LBT/PEPSI snapshot and exclude the presence of strong non-axisymmetric fields potentially missed by this single-epoch observation. These findings provide direct empirical evidence that the transition to weakened magnetic braking involves a weakening of the large-scale magnetic field and suggest that HD 166620 represents a state comparable to the Sun near the peak activity of a grand minimum.

9 pages, 4 figures, 2 tables, accepted for publication in ApJ