The First Millimeter Detection of a Non-Accreting Ultracool Dwarf

TL;DR

This paper reports the first millimeter detection of a non-accreting ultracool dwarf, revealing synchrotron emission linked to magnetic activity and particle acceleration, expanding observational capabilities in this stellar class.

Contribution

It presents the first ALMA millimeter detection of an ultracool dwarf, demonstrating synchrotron emission and its connection to magnetic activity in such objects.

Findings

Detection of 95 GHz emission from the ultracool dwarf TVLM 513-46546.

The emission spectrum extends from centimeter to millimeter wavelengths.

Flux density correlates with optical variability, indicating magnetic activity.

Abstract

The well-studied M9 dwarf TVLM 513-46546 is a rapid rotator (P_rot ~ 2 hr) hosting a stable, dipolar magnetic field of ~3 kG surface strength. Here we report its detection with ALMA at 95 GHz at a mean flux density of $56 \pm 12$ uJy, making it the first ultracool dwarf detected in the millimeter band, excluding young, disk-bearing objects. We also report flux density measurements from unpublished archival VLA data and new optical monitoring data from the Liverpool Telescope. The ALMA data are consistent with a power-law radio spectrum that extends continuously between centimeter and millimeter wavelengths. We argue that the emission is due to the synchrotron process, excluding thermal, free-free, and electron cyclotron maser emission as possible sources. During the interval of the ALMA observation that phases with the maximum of the object's optical variability, the flux density is higher at a ~1.8 sigma significance level. These early results show how ALMA opens a new window for studying the magnetic activity of ultracool dwarfs, particularly shedding light on the particle acceleration mechanism operating in their immediate surroundings.