Trends in Ultracool Dwarf Magnetism. Ii. The Inverse Correlation Between X-Ray Activity and Rotation as Evidence for a Bimodal Dynamo

Abstract

Observations of magnetic activity indicators in solar-type stars exhibit a relationship with rotation with an increase until a "saturation" level and a moderate decrease in activity in the very fastest rotators ("supersaturation"). While X-ray data have suggested that this relationship is strongly violated in ultracool dwarfs (UCDs; spectral type M7), the limited number of X-ray detections has prevented firm conclusions.In this paper, we analyze the X-ray activity-rotation relation in 38 UCDs. Our sample represents the largest catalog of X-ray active UCDs to date, including seven new and four previously unpublished Chandra observations presented in a companion paper. We identify a substantial number of rapidly rotating UCDs with X-ray activity extending two orders of magnitude below the expected saturation level and measure a "supersaturation"-type anticorrelation between rotation and X-ray activity. The scatter in UCD X-ray activity at a fixed rotation is ∼3 times larger than that in earlier-type stars. We discuss several mechanisms that have been proposed to explain the data, including centrifugal stripping of the corona, and find them to be inconsistent with the observed trends. Instead, we suggest that an additional parameter correlated with both X-ray activity and rotation is responsible for the observed effects. Building on the results of Zeeman-Doppler imaging of UCD magnetic fields and our companion study of radio/X-ray flux ratios, we argue that this parameter is the magnetic field topology, and that the large scatter in UCD X-ray fluxes reflects the presence of two dynamo modes that produce distinct topologies.

A Panchromatic View of Brown Dwarf Aurorae

“…The presence of coronal structures, like those in the Sun, in M-dwarf atmospheres, is further corroborated by the detections of X-ray emission in observations of early M-dwarfs (e.g., James et al 2000;Pizzolato et al 2003). Like the optical emission features, UV and X-ray emission are also strongly correlated with rotation/age, with observations showing constant emission levels for young objects rotating more quickly than ∼5 days, and the emission declining for more slowly rotating objects as they age (Pizzolato et al 2003;Cook et al 2014;Shkolnik & Barman 2014).…”

Dynamical masses of a nova-like variable on the edge of the period gap

“…For example, LMC 335 and 2XMM J043527.2-144301 have temperatures of kT = 1.22 keV and kT = 1.22 keV, and X-ray luminosities of 1.7 × 10 29 erg s −1 (0.1-7 keV) and 5 × 10 27 erg s −1 (0.2-2 keV), respectively (Tsang et al 2012;Gupta, Galeazzi & Williams 2011). When placing HS 0220+0603 in the LX vs rotation period plot by Pizzolato et al (2003) (also see Cook, Williams & Berger 2014), we see that the luminosity is also higher than rapidly rotating active stars. We therefore cannot discard the presence of X-ray emission due to very low-level accretion.…”

Extreme Water Loss and Abiotic O2Buildup on Planets Throughout the Habitable Zones of M Dwarfs

“…Flares could also remove some of the oxygen, lowering the amount that builds up in the atmosphere or at the surface, particularly around the lowest-mass M dwarfs. On the other hand, stars less massive than about 0.1 M 1 may be in a supersaturation regime early on, saturating at XUV fluxes 1 or even 2 orders of magnitude below those of higher-mass M dwarfs (Cook et al, 2014). This could reduce water loss rates from planets around these lowest-mass stars.…”