Quiet, but not silent. The X-ray activity of the Maunder minimum star HD 166620

TL;DR

This study investigates whether Sun-like stars enter Maunder minimum–like states by characterizing the X-ray and chromospheric activity of HD 166620, the only unambiguous MM-like star. Using a deep XMM-Newton observation, the authors measure its coronal X-ray emission during a grand minimum–like phase and contextualize the result with a 10 pc FGK dwarf comparison sample and long-term $S_{\rm HK}$ monitoring. They find $\log L_X = 26.56^{+0.10}_{-0.12}$ (erg s$^{-1}$), $\log (L_X/L_{bol}) = -6.58^{+0.10}_{-0.12}$, and $\log F_X = 3.97^{+0.10}_{-0.12}$, with the corona resembling the solar background corona and a brightness decrease by a factor of $\sim 2.5$ since the pre-MM maximum. These results imply that the Sun’s Maunder minimum may not have involved a dramatic suppression of coronal activity relative to present minima and support a link between weakened magnetic braking near the critical Rossby number and the occurrence of grand minima in Sun-like stars.

Abstract

As the only known unambiguous star in a Maunder minimum-like chromospheric activity state, the properties of HD 166620 can provide valuable insight into the behaviour of the Sun during the historic extended low-states of its activity cycle. The coronal X-ray activity of HD 166620 has so far only been probed with a ROSAT/HRI observation in 1996, near the chromospheric activity maximum before the star entered its grand minimum around 2004. We conducted a deep {\it XMM-Newton} observation of HD 166620 during its chromospheric Ca II H&K activity grand minimum to achieve a better understanding of its magnetic activity. We detected HD 166620 with an X-ray luminosity of ${{\rm log}\,L_{\rm X}\,\rm{(erg\,s^{-1})}=26.56^{+0.10}_{-0.12}}$, corresponding to ${{\rm log}\,(L_{\rm X}/L_{\rm bol}) = -6.58^{+0.10}_{-0.12}}$ and an X-ray surface flux of log Fx (erg/cm^2/s) = 3.97+0.10-0.12. With respect to the earlier ROSAT observation, the X-ray brightness of HD 166620 has decreased by a factor of 2.5 during its Maunder minimum-like state. To place its X-ray properties into context, we constructed an X-ray sample of late-type stars within 10 pc of the Sun. The activity of HD 166620 is below the levels of all other K dwarfs in the 10 pc sample. The corona of HD 166620 during its grand minimum emits at the level of the solar background corona, which implies that it has no large active magnetic structures. Along with long-term Ca II H&K monitoring of HD 166620, this result provides evidence that the solar activity during the Maunder minimum was not reduced significantly below the levels seen during its present-day cycle minima. The similar X-ray surface flux of HD 166620 and the modern quiet Sun, and also their Rossby number near the critical value of spin-down models, suggest a connection between the regime of weakened magnetic braking and the occurrence of Maunder minimum states.

Figures (7)

• Figure 1: $S_{\!{\rm HK}}$ and $R_{\rm X}$ time series for HD 166620 with data compiled by Luhn_2022 and more recent data from Keck/HIRES Isaacson_2024 and the APF Vogt_2014. The small transparent symbols mark individual $S_{\!{\rm HK}}$ measurements, and the larger opaque symbols are the same data, binned in 120 d bins. A black outline denotes a bin containing only a single measurement. The dashed blue line shows the extrapolated trend of the two maxima (see Sect. \ref{['subsect:data-SHK']}). The two X-ray measurements are overplotted with black squares, and their epochs are marked on the axis.
• Figure 2: Top: XMM-Newton EPIC/pn spectrum of HD 166620 and best-fit 2T-APEC thermal model. Bottom: Residuals.
• Figure 3: X-ray over bolometric luminosity for HD 166620 in its two detections compared with the range of values observed throughout the solar cycle Peres_2000 and for FGK (Bennedik et al., in prep.) and M0--M4 dwarfs Caramazza_2023 within 10 pc of the Sun.
• Figure 4: Same as Fig. \ref{['fig:Rx-plot']}, but for X-ray surface fluxes $F_{\!\rm X}$. The typical ranges of $F_{\!\rm X}$ exhibited by different types of solar coronal structures Caramazza_2023 are depicted as coloured stripes.
• Figure 5: $S_{\!{\rm HK}}$ time series for HD 166620 showing the Keck/HIRES data in comparison with the newly obtained APF data. The dotted vertical line marks the time of the XMM-Newton observation.