Low, Low, and Lower: A Binary Ultracool Dwarf System is Detected at 340 MHz
Title: First Detection of an Ultracool Dwarf at 340 MHz: VLITE Observations of EI Cancri AB
Authors: Michele L. Silverstein, Tracy E. Clarke, Wendy M. Peters, Emil Polisensky, Jackie Villadsen, Jordan M. Stone
First Author's Institution: Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, USA
Status: Submitted to AAS Journals [open access]
What makes these dwarfs so cool?
Ultracool dwarfs (UCDs) are the lowest mass stars and brown dwarfs, typically with spectral type M7 or later (lower effective temperature). They have masses of about 0.1 solar masses or less, and are usually limited in size to a few-tenths of the Sun's radius. This makes them appear very red, often peaking in the infrared, and their luminosities are typically only a few tenths of a percent of the Sun's. Some UCDs are just massive enough to fuse hydrogen, while less massive brown dwarfs can sometimes fuse deuterium or don't fuse at all, making them more analogous to planets. Studying these systems is crucial for understanding the differences in their formation processes and evolution.
Magnetism's Role in the Sun
We know that magnetism plays a vital role in the Sun and its activity. The Sun is a differential rotator, which results in a dynamo that generates magnetic fields. Traditional solar dynamo theory invokes the tachocline, the region between a radiatively driven core and the outer convective layer, to produce the large magnetic field we observe from the Sun. However, radio observations and methods like Zeeman-Doppler imaging have identified large-scale magnetic fields in UCDs, challenging the tachocline's role.
The First Radio Star(s) at 340 MHz
The P-band (~340 MHz) is a low frequency where no stars have been detected before. The authors searched for radio emission in this range, focusing on a unique binary system of two nearly identical main-sequence M7 UCDs with 0.12 and 0.10 solar masses, designated EI Cancri A and B. Located at 5.12 parsecs, these stars have a projected separation of approximately 13 AU, making them non-interacting.
Observations and Detection
The Very Large Array (VLA) was used with the VLA Low-band Ionosphere and Transient Experiment (VLITE) commensal system. At an angular separation of 0.874 degrees, they used a VLA observation of OJ 287 to create an image of EI Cancri and identified a source at its position. However, the low frequency limits resolution, making it unclear if the source is attributed to A or B.
Bursts and Emission Origin
After analyzing a 7-hour dataset, three independent bursts were identified. The authors consider incoherent (gyro-radiation) and coherent processes (plasma emission vs. electron cyclotron maser instability) as the emission origin. The brightness temperature is estimated, but the source size is unknown, making a definitive determination challenging.
Further Observations and Interpretations
Additional VLITE observations and VLA Sky Survey (VLASS) images confirmed the detection of both EI Cancri A and B. Further observations with the VLA's dedicated P-band mode and higher frequencies could identify the emission process. Ultra-high-resolution radio observations could map stellar motion and determine orbital properties, while follow-up optical and infrared observations might solidify rotational periods.
This detection offers a unique opportunity to study the system from multiple new perspectives.
