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

Federica Chiti, Jennifer L. van Saders, Oleg Kochukhov, Travis S. Metcalfe

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 $B_{\rm dip} = 1.10^{+0.95}_{-0.90}$G ($3σ$). 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.

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 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 signal, assuming a purely axisymmetric dipole, yields a best-fit dipole field strength of G (). 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.
Paper Structure (7 sections, 3 equations, 4 figures, 2 tables)

This paper contains 7 sections, 3 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. SPIRou Observations
  4. ESPaDOnS Observations
  5. Multi-line Polarization Analysis
  6. Forward Modeling of LSD Profiles
  7. Discussion & Conclusions

Figures (4)

  • Figure 1: Coadded LSD profiles of HD 166620 derived from ESPaDOnS data showing Stokes $V$ (top), null profile (middle), and Stokes $I$ (bottom).
  • Figure 2: Coadded LSD profiles of HD 166620 derived from SPIRou data. The Stokes $I$ (left), $V$ (middle) and null profile (right) are offset vertically for display purposes. The different rotational phases are shown in the Stokes $I$ panel. Percentage mean errors on the computed LSD Stokes $V$ profile are indicated with an error bar in the middle panel. The dashed gray vertical line in all panels marks the radial velocity of the star gaia2021.
  • Figure 3: Chi-square difference ($\Delta\chi^2 = \chi^2-\chi^2_{\mathrm{min}}$) as a function of spherical harmonic coefficient $\alpha_{1,0}$ (plotted on a symmetrical logarithmic $x-$scale) for axisymmetric dipole models compared to observed Stokes $V$ and null LSD profiles. Horizontal lines indicate confidence level (CL) thresholds. A power law fit is shown for $\Delta\chi^2$ of Stokes $V$.
  • Figure 4: Grand average with uncertainties of the SPIRou Stokes $V$ polarization profiles for HD 166620. The dashed magenta line is the best-fit axisymmetric model profile of a dipole with $B_{\mathrm{dip}}=1.10\,$G and fixed inclination.