Strong dipole magnetic fields in fast rotating fully convective stars (1801.08571v1)

Abstract: M dwarfs are the most numerous stars in our Galaxy with masses between approximately 0.5 and 0.1 solar mass. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes, and large-scale magnetic fields. Such activity is driven by a dynamo powered by the convective motions in their interiors. Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including, e.g., theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation-activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days. The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that due to a tight empirical correlation between X-ray and magnetic flux, the stellar magnetic fields will also saturate, to values around ~4kG. Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M-dwarfs. By combining results from spectroscopic and polarimetric studies we explain our findings in terms of bistable dynamo models: stars with the strongest magnetic fields are those in a dipole dynamo state, while stars in a multipole state cannot generate fields stronger than about four kilogauss. Our study provides observational evidence that dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large scale component, but also in the total magnetic energy.

Overview of "Strong dipole magnetic fields in fast rotating fully convective stars"

The paper "Strong dipole magnetic fields in fast rotating fully convective stars" provides a detailed investigation into the magnetic field characteristics of M dwarf stars, which are the most common type of star in the Galaxy. M dwarfs, particularly the fully convective ones, exhibit various magnetic activities akin to the Sun, such as flares, high X-ray fluxes, and significant magnetic fields. The magnetic activity in these stars is attributed to the dynamo processes driven by convective motions within their interiors.

Key Findings

The research highlights two main empirical observations that inform the models of rotationally driven convective dynamos in M dwarfs:

1. Magnetic Field Geometry: The paper identifies a tendency towards simple, axisymmetric, poloidal-dominated magnetic fields in fully convective M dwarfs via analysis of circular polarization in spectral lines. Conversely, stars that are only partially convective display more complex field geometries with strong toroidal components.
2. Rotation-Activity Relationship: A well-established rotation-activity relationship is observed where activity markers saturate in stars with rapid rotation (periods shorter than a few days), and activity diminishes with slower rotation rates. This relationship is typically correlated with X-ray flux and magnetic field strength.

Remarkably, the paper presents evidence of magnetic fields surpassing the previously accepted saturation limit of approximately 4 kilogauss (kG) in four fully convective M-dwarfs. These M-dwarfs exhibit stronger magnetic fields when in a dipole dynamo state compared to a multipole state.

Methodology

The research utilizes both spectroscopic and polarimetric data collected from ESPaDOnS and NARVAL spectropolarimeters. These instruments facilitate precise measurements of magnetic fields on the surface of stars. By employing a combination of spectroscopic diagnostics and radiative transfer modeling, the authors measured the average magnetic fields in several M dwarfs, discerning substantial deviations from previously determined maximum field strengths.

Implications

The detection of exceptionally strong magnetic fields in these stars challenges the prevailing assumption about magnetic saturation in fully convective stars. It provides observational backing for bistable dynamo models, suggesting that these stars can exist in either dipole or multipole magnetic states with differing magnetic field strengths. Consequently, M dwarfs in a dipole state can generate stronger fields than those in a multipole state, reaffirming the model's hypothesis.

Furthermore, this research has broader implications for the understanding of stellar dynamos and their contributions to stellar magnetic activity, potentially affecting models of stellar evolution and planetary habitability around such stars.

Future Directions

One critical avenue for future research highlighted by this paper is the long-term monitoring of these stars to provide more data on their magnetic cycles. Such observations could uncover whether M dwarfs shift dynamically between their magnetic states or maintain these configurations indefinitely. The insights from this monitoring are also crucial to distinguishing between bistable and cyclic dynamo theories in fully convective stars.

In conclusion, the paper robustly advances our understanding of magnetic dynamics in M dwarfs and underscores the diversity of magnetic behaviors in fully convective stars. It encourages further theoretical and observational work to untangle the complexities of stellar magnetism and its broader astrophysical implications.