A Volume-limited Sample of 63 M7-M9.5 Dwarfs II. Activity, magnetism, and the fade of the rotation-dominated dynamo

TL;DR

This study examines magnetic activity and rotation in ultracool dwarfs M7-M9.5, revealing a shift in dynamo mechanisms and magnetic field characteristics at these spectral types through high-resolution spectroscopic analysis.

Contribution

It provides new measurements of magnetic fields and rotation velocities in ultracool dwarfs, highlighting a transition in dynamo processes compared to earlier stars.

Findings

Magnetic fields range from 0 to 4kG with no temperature dependence.

Halpha luminosity decreases with lower temperature and shows larger scatter.

No correlation between rotation speed and magnetic flux in ultracool dwarfs.

Abstract

In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this second paper, we present projected rotation velocities, average magnetic field strengths, and chromospheric emission from the Halpha line. We confirm earlier results that the mean level of normalized Halpha luminosity decreases with lower temperature, and we find that the scatter among Halpha luminosities is larger at lower temperature. We measure average magnetic fields between 0 and 4kG with no indication for a dependence on temperature between M7 and M9.5. For a given temperature, Halpha luminosity is related to magnetic field strength, consistent with results in earlier stars. A few very slowly rotating stars show very weak magnetic fields and Halpha emission, all stars rotating faster than our detection limit show magnetic fields of at least a few hundred Gauss. In contrast to earlier-type stars, we observe magnetic fields weaker than 1kG in stars rotating faster than ~3km/s, but we find no correlation between rotation and magnetic flux generation among them. We interpret this as a fundamental change in the dynamo mechanism; in ultracool dwarfs, magnetic field generation is predominantly achieved by a turbulent dynamo, while other mechanisms can operate more efficiently at earlier spectral type.