English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

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 LargeTelescope and HIRES at Keck Observatory. In this second paper, we presentprojected rotation velocities, average magnetic field strengths, andchromospheric emission from the Halpha line. We confirm earlier results thatthe mean level of normalized Halpha luminosity decreases with lowertemperature, and we find that the scatter among Halpha luminosities is largerat lower temperature. We measure average magnetic fields between 0 and 4kG withno indication for a dependence on temperature between M7 and M9.5. For a giventemperature, Halpha luminosity is related to magnetic field strength,consistent with results in earlier stars. A few very slowly rotating stars showvery weak magnetic fields and Halpha emission, all stars rotating faster thanour detection limit show magnetic fields of at least a few hundred Gauss. Incontrast to earlier-type stars, we observe magnetic fields weaker than 1kG instars rotating faster than ~3km/s, but we find no correlation between rotationand magnetic flux generation among them. We interpret this as a fundamentalchange in the dynamo mechanism; in ultracool dwarfs, magnetic field generationis predominantly achieved by a turbulent dynamo, while other mechanisms canoperate more efficiently at earlier spectral type.

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