Unlocking the Secrets of Ultracool Dwarfs: A Radio Mystery Unveiled
In the vast cosmic dance, some stars shine with a unique, ultracool glow. But what makes these dwarfs so fascinating? And how can we unravel their mysteries?
Ultracool dwarfs (UCDs) are the smallest of stars and brown dwarfs, with masses around 0.1 solar masses or less, and they're incredibly cool, both in temperature and demeanor. These celestial bodies, often appearing red and peaking in the infrared, are like the shy wallflowers of the stellar world. But don't let their modest appearance fool you; they hold the key to understanding the blurred line between stars and planets.
The magnetic fields of stars have long been a subject of intrigue, and the Sun, our closest star, is no exception. It's a 'differential rotator,' creating a dynamo effect that generates magnetic fields. The traditional solar dynamo theory points to the tachocline, a region between the radiative core and the outer convective layer, as the source of these fields. However, this theory faces a challenge when it comes to UCDs, as their low mass prevents them from having a radiative core. And here's where it gets controversial—radio observations have revealed that UCDs still possess significant magnetic fields, with the coolest brown dwarf, 2MASS J1047+21, boasting a magnetic field 3000 times stronger than Earth's!
The authors of this study decided to explore uncharted territory by searching for radio emissions at 340 MHz, a frequency where no stars had been detected before. They focused on a binary system, EI Cancri AB, consisting of two nearly identical UCDs with masses of 0.12 and 0.10 solar masses, located just 16.7 light-years away. These stars are like cosmic neighbors, yet they don't interact due to their projected separation of approximately 13 AU.
Using the Very Large Array (VLA) and its VLITE commensal system, the researchers detected EI Cancri. By observing the famous blazar OJ 287, they created an image of EI Cancri and identified a source at its position. The low frequency, however, presented a challenge—the source could not be definitively attributed to either EI Cancri A or B due to the lower resolution.
The authors sliced the 7-hour dataset into 10-minute segments and discovered three independent bursts on April 27, 2018. These bursts seemed to originate from the center of the image, suggesting that both stars were active. Further analysis indicated that the third burst likely came from EI Cancri B. This detection is groundbreaking, marking the first confident radio detection of a UCD at 340 MHz in a binary system.
The origin of these radio emissions is a puzzle. The authors consider both incoherent and coherent processes. Incoherent processes, like gyro-radiation, involve electrons spiraling along magnetic field lines. Coherent processes, such as plasma emission and electron cyclotron maser instability (ECMI), arise from unstable plasma conditions. The key difference? Coherent processes produce highly polarized radio emission due to electrons moving in unison.
Determining the emission process is tricky. The authors estimate the brightness temperature, but it hovers around the threshold, making a definitive conclusion difficult. Other methods, like frequency-dependent effects and polarization, are limited by the low signal-to-noise ratio. With only three flares detected and a predicted rotation period of 10 hours or more, identifying a periodic signal that could favor a coherent process is challenging without more data.
The authors also examined images from the VLA Sky Survey (VLASS) at higher frequencies, confidently detecting both EI Cancri A and B. However, these observations also couldn't pinpoint the emission mechanism due to their limited nature. The mystery remains unsolved.
To unlock the secrets of EI Cancri AB's radio emissions, further observations are needed. The VLA's dedicated P-band mode and higher frequencies, combined with accurate polarization measurements, could provide the missing pieces of the puzzle. Ultra-high-resolution radio observations could map the stars' motion and determine their orbital properties, while optical and infrared follow-ups might reveal their true rotational periods. This detection at 340 MHz opens a new window into the world of UCDs, inviting us to explore and understand these intriguing celestial bodies.
