Story
Spectropolarimetric Constraints on the Maunder Minimum Analog HD 166620: Evidence for Weakened Magnetic Braking
Key takeaway
Astronomers find evidence that a star similar to our Sun experienced a long period of unusually low magnetic activity, which could provide insights into past changes in the Earth's climate.
Quick Explainer
The researchers studied the magnetic properties of HD 166620, a Sun-like star that has entered the "weakened magnetic braking" (WMB) phase of its life cycle. Using spectropolarimetric observations, they were able to detect the star's faint, large-scale magnetic field, which their models indicate is comparable to the Sun's field during its historical Maunder Minimum period of reduced activity. This provides direct observational evidence that the WMB transition involves a weakening of the star's overall magnetic field. The consistency between the single-snapshot and time-series analyses suggests the magnetic fields of these inactive stars are remarkably stable, offering a unique window into the magnetic characteristics of Sun-like stars during grand minimum phases.
Deep Dive
Spectropolarimetric Constraints on the Maunder Minimum Analog HD 166620: Evidence for Weakened Magnetic Braking
Overview
This work presents the first spectropolarimetric time-series analysis of the Maunder Minimum analog star HD 166620. The authors used observations from the SPIRou and ESPaDOnS instruments at the Canada-France-Hawaii Telescope to investigate the star's magnetic field properties and confirm it as a close analog to the Sun during its Maunder Minimum grand activity minimum phase.
Problem & Context
- Sun-like stars undergo a "rotational crisis" in the middle of their main-sequence lives, where the efficiency of magnetic braking drastically decreases, causing them to rotate faster than predicted. This phenomenon is known as "weakened magnetic braking" (WMB).
- HD 166620 is the oldest known K dwarf residing in the WMB regime, with a Rossby number just past the critical threshold for this magnetic transition.
- HD 166620 is also the first identified true analog of the Sun during its historical Maunder Minimum period (1645-1715), characterized by dramatically reduced sunspot activity.
Methodology
- The authors obtained 12 nights of SPIRou near-infrared spectropolarimetry and a single epoch from ESPaDOnS optical spectropolarimetry of HD 166620.
- They used Least-Squares Deconvolution (LSD) to extract high signal-to-noise Stokes I and V profiles from the observations.
- To search for the expected weak polarization signatures, they performed forward modeling of the LSD Stokes V profiles, assuming a purely axisymmetric dipole magnetic field geometry.
Results
- While individual Stokes V profiles showed no significant polarization, the authors were able to recover a faint signal by computing a grand average LSD profile from the full SPIRou time series.
- Forward modeling of the cumulative Stokes V signal yielded a best-fit dipole field strength of $B{dip} = 1.10^{+0.95}{-0.90}$ G (3σ confidence).
- This field strength matches simulations of the solar dipole during the Maunder Minimum phase.
Interpretation
- The authors' results provide direct empirical evidence that the transition to weakened magnetic braking involves a weakening of the large-scale magnetic field.
- The inferred dipole field strength for HD 166620 falls within the upper range of the simulated Maunder Minimum field strength for the Sun, suggesting HD 166620 represents a state comparable to the Sun near the peak activity of its own grand minimum.
- The consistency between the dipole field strength measured from a single snapshot and the time series analysis indicates the large-scale field of these old, inactive stars is remarkably stable.
Limitations & Uncertainties
- Observing the ultra-weak fields of inactive K dwarfs like HD 166620 challenges the limits of current instrumentation.
- The authors found the optical ESPaDOnS instrument to be more sensitive than the near-infrared SPIRou for this target, despite the theoretical advantages of infrared spectropolarimetry.
- Practical challenges also exist, as monitoring stars with long rotation periods requires extended instrument availability, which has been limited for ESPaDOnS in recent years.
What Comes Next
- The planned VISION instrument at CFHT, which will combine the strengths of SPIRou and ESPaDOnS, may provide the sensitivity and scheduling flexibility required to better map the subtle, evolving magnetic fields of stars like HD 166620 entering the WMB regime.
