How much do underestimated field strengths from Zeeman-Doppler imaging affect spin-down torque estimates? (2002.11774v1)

Abstract: Numerous attempts to estimate the rate at which low-mass stars lose angular momentum over their lifetimes exist in the literature. One approach is to use magnetic maps derived from Zeeman-Doppler imaging (ZDI) in conjunction with so-called "braking laws". The use of ZDI maps has advantages over other methods because it allows information about the magnetic field geometry to be incorporated into the estimate. However, ZDI is known to underestimate photospheric field strengths due to flux cancellation effects. Recently, Lehmann et al. (2018) conducted synthetic ZDI reconstructions on a set of flux transport simulations to help quantify the amount by which ZDI underestimates the field strengths of relatively slowly rotating and weak activity solar-like stars. In this paper, we evaluate how underestimated angular momentum-loss rate estimates based on ZDI maps may be. We find that they are relatively accurate for stars with strong magnetic fields but may be underestimated by a factor of up to $\sim$10 for stars with weak magnetic fields. Additionally, we re-evaluate our previous work that used ZDI maps to study the relative contributions of different magnetic field modes to angular momentum-loss. We previously found that the dipole component dominates spin-down for most low-mass stars. This conclusion still holds true even in light of the work of Lehmann et al. (2018).

• The paper finds that Zeeman-Doppler imaging (ZDI) can underestimate angular momentum-loss rates for weak-field stars by up to a factor of 10 due to underestimated magnetic field strengths.
• For stars with strong magnetic fields, ZDI-derived spin-down estimates are relatively accurate despite limitations in measuring total field flux.
• The study reconfirms that the stellar dipole magnetic field component is the primary driver of angular momentum loss for most low-mass stars, even with ZDI's underestimation.

Impact of Underestimated Magnetic Field Strengths on Stellar Spin-Down Torque

The paper authored by See et al. explores the implications of inaccuracies in estimating stellar magnetic field strengths using Zeeman-Doppler Imaging (ZDI) on the calculation of spin-down torques. The primary focus is on quantifying how these underestimated field strengths affect the angular momentum-loss rate estimates for low-mass stars.

Context and Motivation

Stellar spin-down is a critical process in the evolution of low-mass stars, driven by the loss of angular momentum through stellar winds. Theoretical models, often relying on magnetohydrodynamic (MHD) simulations, employ braking laws that incorporate stellar magnetic field parameters to estimate angular momentum-loss rates. ZDI, a widely used technique to map these magnetic fields, suffers from limitations such as flux cancellation, which leads to underestimates of the large-scale field strengths.

Methodology

See et al. utilize synthetic ZDI reconstructions to analyze how these limitations impact angular momentum-loss estimates. The paper builds on the findings of Lehmann et al., who previously quantified ZDI's underestimation of field strengths using flux transport simulations. Specifically, the paper assesses how large-scale field components (dipole, quadrupole, and octupole) reconstructed by ZDI compare to more comprehensive simulation maps and their subsequent influence on calculated spin-down torques.

Key Findings

1. Accuracy of ZDI for Strong Fields: It is found that for stars with strong magnetic fields, ZDI-derived angular momentum-loss rate estimates are relatively accurate.
2. Underestimation for Weak Fields: For stars with weak magnetic fields, ZDI may underestimate the angular momentum-loss by up to a factor of approximately 10. This highlights a significant limitation while applying available braking laws to ZDI data for such stars.
3. Magnetic Field Component Contribution: The paper reconfirms previous assertions that the dipolar component predominantly dictates the angular momentum-loss for most low-mass stars, even with ZDI's underestimation of field strengths.
4. Critical Mass-Loss Rate: A crucial finding is the underestimation of the critical mass-loss rate below which only the dipole component contributes to angular momentum-loss. This suggests that even fewer stars than previously thought surpass this critical mass-loss rate, reinforcing the dominance of the dipole component.

Implications and Future Directions

The implications of this research underscore the need for cautious interpretation of angular momentum-loss estimates derived from ZDI data. It suggests a possible reevaluation of existing models, especially for stars with weak fields, to accommodate the limitations of ZDI. Furthermore, the paper calls for improvements in observational techniques or the development of alternative methods to better capture the full magnetic energy, especially in stars with weaker fields.

Future research could focus on extending the analysis to a broader range of stellar masses and rotations. Exploring the influence of non-axisymmetric fields and incorporating more realistic stellar magnetic geometries could also refine the applicability of existing models.

In conclusion, this paper contributes significantly to our understanding of stellar wind-braking mechanisms and the limitations of current imaging techniques, setting a foundation for more robust interpretations of stellar evolution metrics in the presence of ZDI-induced underestimations.