The lowest mass stars have been well-studied across the mid and high frequency radio bands. However, lower frequencies can reveal larger-scale magnetic structures and may even be the key to the first ...

**Title:**First Detection of an Ultracool Dwarf at 340 MHz: VLITE Observations of EI Cancri AB **Author(s):** 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) consist of the lowest mass stars and brown dwarfs, typically of spectral type M7 or “later” (lower effective temperature). They usually have masses of about 0.1 solar masses or less, about half or less of the Sun’s effective surface temperature, and are typically limited in size to a few-tenths of the Sun’s radius. These facts mean they 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 are just massive enough to fuse Hydrogen, whereas the less massive brown dwarfs can sometimes fuse deuterium or don’t fuse at all, making them more analogous to planets. Studying these kinds of systems–which walk on the boundary of stars and planets–is critical to understanding the differences in their formation processes and evolution. _Figure 1: An annotated diagram of the Sun’s layers. The region between the internal radiative zone and the outer convective zone is called the tachocline. It is theorized to play an essential role in the production of the Sun’s strong magnetic field. Image Credit: NASA_ We’ve long known that magnetism plays an essential role in the Sun and the activity we observe. The Sun is a “differential rotator,” which results in a dynamo that can generate 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; see Figure 1. It is only present in stars with 0.3 solar masses and above since low-mass stars like UCDs don’t have sufficient core conditions to be radiative and are considered “fully” convective. However, radio observations and other methods, such as Zeeman-Doppler imaging, that have identified large-scale magnetic fields in UCDs challenge the tachocline’s role in magnetic field generation. In fact, the coolest known brown dwarf, 2MASS J1047+21, with a temperature of only 900 Kelvin, has a magnetic field of 1.7 kilo-Gauss, or 3000x that of Earth’s magnetic field. # The first radio star(s) at 340 MHz The observed emission frequency, its distribution, and other properties, such as polarization and temporal variability, can be used to infer the origin of a star’s radio emission. In today’s paper, the authors have chosen to search for radio emission in a frequency range in which no stars have been detected before. The P-band (~340 MHz) is a low frequency below the GHz regime, where the vast majority of radio studies have been conducted, including the first detection of radio emission from a UCD. Today’s paper focuses on a unique binary consisting of two nearly identical main-sequence M7 UCDs with 0.12 and 0.10 solar masses, designated EI Cancri A and B. The two stars are located in our solar backyard at 5.12 parsecs (16.7 light-years) and have a projected separation of approximately 13 AU (Earth-Sun distance units, 1 AU = 93 million miles = 150 million kilometers), so they are non-interacting. _Figure 2: (Left): The identification image of the VLITE detection at 340 MHz on 2018 April 26, created from seven hours of data on-source spanning a 28-hour dataset. The peak flux is 2.7+/-0.35 mJy/beam (SNR=7.71). (Right): A zoomed-in image, highlighting the location of the binary components EI Cancri A and B, alongside the marked positions of the three detections from 10-minute time-slices of the same dataset. Figure 1 from today’s paper._ The observations were conducted with the Very Large Array (VLA) using the VLA Low-band Ionosphere and Transient Experiment (VLITE) commensal system, which is present on 18 of the 27 antennas and observes simultaneously with all other VLA observations. Using this method, the authors detected EI Cancri. At an angular separation of 0.874 degrees from the primary target in the observation, they used a VLA observation of the famous blazar OJ 287 to create an image of EI Cancri and identified a source at its position. The low frequency also means lower resolution, so the source cannot unambiguously be attributed to EI Cancri A or B. After time-slicing the 7-hour dataset spanning 28 hours into 10-minute slices, the authors identified three independent bursts at 00:09, 02:48, and 03:41 on 2018 April 27. The image and the best-fitting positions of the three bursts are shown in Figure 2. The authors argue that if both systems are bursting, the image’s apparent central location is naturally explained. The inferred locations from the time-sliced images are consistent with the third burst originating from EI Cancri B. Regardless of the specific association, this represents the first confident radio detection of a UCD at 340 MHz since both stars in the system are UCDs. # The origin of radio emission in EI Cancri AB The authors consider incoherent processes (gyro-radiation) and coherent processes (plasma emission vs. electron cyclotron maser instability) as the origin of the emission. The best-known example is “gyro”/”gyromagnetic” emission, which arises from an electron spiraling along a magnetic field line under the influence of the electromagnetic force. This process can be referred to as cyclotron, gyro-synchrotron, or synchrotron emission, depending on the electron energy. Coherent processes like plasma emission and the electron cyclotron maser instability (ECMI) arise from unstable conditions in the plasma in the star’s atmosphere, and which one gets produced depends on the density and magnetic field strength. These processes are termed coherent because they involve electrons moving in concert, often resulting in highly polarized radio emission. A simple way to estimate which emission process is responsible is to calculate the brightness temperature; if it exceeds 1012 Kelvin, the process is more likely to be coherent than incoherent. The brightness temperature requires an estimate of the source size, which is unknown because there are no other detections at this frequency for comparison. The authors estimate the flaring region in the star’s atmosphere to be 1-5 stellar radii, which causes the resulting brightness temperature to fluctuate around the cutoff value, so a definitive determination isn’t possible yet. Other methods of identifying the emission process rely on frequency-dependent effects, polarization, and periodic signals at the star’s rotation period. Unfortunately, the low signal-to-noise ratio makes it challenging to investigate how the flare appears at different frequencies or polarizations. Since only three flares were detected over several hours and the shortest predicted rotation period of either star is 10 hours, searching for a periodic signal that might favor a coherent process is not yet possible without more data. # More observations and interpretations _Figure 3: Images from the VLA Sky Survey (VLASS) observing at 2-4 GHz (~10x the observing frequency of VLITE) of the EL Cancri system at its three observing epochs in 2019, 2021, and 2024. Both stars are confidently detected in all three epochs. Figure 2 from today’s paper._ In addition to the VLITE observations, the authors examined images available from the VLA Sky Survey (VLASS), an all-sky survey at higher frequencies. In the three observations spanning 2019, 2021, and 2024, both EI Cancri A and B were confidently detected; see Figure 3. The currently available VLASS images are limited to short, single-frequency, single-polarization snapshots and therefore also cannot probe the distinguishing properties of the emission mechanisms. The brightness temperature of the VLASS observations is similarly just on the cusp of the typical dividing value, so both gyro-synchrotron and a coherent process remain equally possible and consistent with the known GHz-emitting population of UCDs. Further observations using the VLA’s more sensitive dedicated P-band mode and higher frequencies over longer times, with accurate polarization measurements, could investigate the radio emission in greater detail and identify the emission process. Ultra-high-resolution radio observations using very-long-baseline interferometry could map stellar motion precisely and determine their orbital properties, and follow-up optical and infrared observations might solidify the true rotational periods. The radio detection of EI Cancri AB at 340 MHz offers a unique opportunity to study the system from multiple new perspectives. _Edited by Margaret Verrico_ ## Author * Will Golay I am a graduate student in the Department of Astronomy at Harvard University and the Center for Astrophysics | Harvard & Smithsonian, advised by Edo Berger. I study radio emission from transient astrophysical objects like tidal disruption events. View all posts

ALT: a large white satellite dish with a blue sky in the background

Magnetically driven phenomena such as flaring events and aurorae lead ultracool dwarfs to emit at radio frequencies. Despite decades of scrutiny, a comprehensive physical understanding of their radio ...

Origin

Astronomers from the University of Hawai'i (UH) at Manoa and elsewhere have observed the Taurus star-forming region, which resulted in the discovery of planetary-mass and stellar companions of two…

See also: The publication in ArXiV.

Astronomers from the University of Hawai'i (UH) at Manoa and elsewhere have observed the Taurus star-forming region, which resulted in the discovery of planetary-mass and stellar companions of two ultracool dwarf stars. The new finding was presented in a paper published December 4 on the pre-print server arXiv.

Astronomers from the University of Hawai'i (UH) at Manoa and elsewhere have observed the Taurus star-forming region, which resulted in the discovery of planetary-mass and stellar companions of two ult...

Astronomers from the University of Hawai'i (UH) at Manoa and elsewhere have observed the Taurus star-forming region, which resulted in the discovery of planetary-mass and stellar companions of two ultracool dwarf stars.