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]
The Coolest Discovery Yet: Unveiling the Mysteries of Ultracool Dwarfs
Imagine stars and brown dwarfs that are so cold and faint they appear almost invisible. These are the ultracool dwarfs (UCDs), the smallest and coldest celestial bodies that are still considered stars. They are so cool that their effective temperatures are just a fraction of the Sun's, and their masses are typically less than 0.1 solar masses. But what makes them truly fascinating is their magnetic properties, which challenge our understanding of stellar dynamos.
Magnetic Mysteries and Radio Waves
The Sun's magnetic field is a powerful force, generated by its differential rotation and the tachocline, a region between its radiative core and outer convective layer. However, UCDs, being low-mass stars, don't have the core conditions to produce large magnetic fields like the Sun. Yet, radio observations have detected significant magnetic fields in UCDs, including the incredibly cool 2MASS J1047+21, with a magnetic field 3000 times stronger than Earth's.
A New Frequency Frontier
Today's paper takes us on a journey to a frequency range where no stars have been detected before: 340 MHz. This low-frequency band is a frontier for radio astronomy, and the authors searched for radio emission in a unique binary system, EI Cancri AB, consisting of two nearly identical main-sequence M7 UCDs. Located just 16.7 light-years away, these stars are non-interacting and have a projected separation of approximately 13 AU.
The First Radio Detection
Using the Very Large Array (VLA) and the VLA Low-band Ionosphere and Transient Experiment (VLITE) commensal system, the authors detected EI Cancri AB. By creating an image of the system using a VLA observation of the blazar OJ 287, they identified a source at its position. However, the low frequency limits the resolution, making it challenging to attribute the source to either star.
Bursting with Activity
The authors identified three independent bursts at 00:09, 02:48, and 03:41 on April 27, 2018. These bursts suggest that both stars in the system are active, and the inferred locations from time-sliced images support the idea that the third burst originates from EI Cancri B. This discovery represents the first confident radio detection of a UCD at 340 MHz.
Unraveling the Emission Origins
The authors consider various emission processes, including incoherent (gyro-radiation) and coherent (plasma emission vs. electron cyclotron maser instability) processes. The brightness temperature, a key indicator, suggests that both processes could be at play. However, the lack of other detections at this frequency makes it challenging to determine the exact emission mechanism.
Further Observations and Interpretations
Additional observations using the VLA's dedicated P-band mode and higher frequencies are needed to investigate the radio emission in greater detail. Ultra-high-resolution radio observations could provide valuable insights into the stars' orbital properties and rotational periods. The radio detection of EI Cancri AB at 340 MHz opens up exciting possibilities for studying these fascinating celestial bodies from multiple perspectives.
