The Coolest Discovery Yet: Unveiling the Binary Ultracool Dwarf System at 340 MHz
The universe is full of mysteries, and one of the most intriguing is the nature of ultracool dwarfs (UCDs). These celestial bodies, with their low masses and even lower temperatures, are the focus of a groundbreaking study that has just been submitted to AAS Journals. The paper, titled 'First Detection of an Ultracool Dwarf at 340 MHz: VLITE Observations of EI Cancri AB', is a collaboration between researchers from the Naval Research Laboratory and other esteemed institutions.
But what makes these dwarfs so fascinating? Well, they are the lowest mass stars and brown dwarfs, with masses of about 0.1 solar masses or less. Their effective surface temperatures are half or less of the Sun's, and their sizes are limited to a few-tenths of the Sun's radius. This results in a very red appearance, with peak radiation in the infrared spectrum. Their luminosities are typically only a few tenths of a percent of the Sun's, and some are just massive enough to fuse hydrogen, while others can fuse deuterium or don't fuse at all, making them more like planets.
The study focuses on a unique binary system, EI Cancri AB, consisting of two nearly identical main-sequence M7 UCDs with masses of 0.12 and 0.10 solar masses. Located in our solar backyard at 5.12 parsecs (16.7 light-years) with a projected separation of approximately 13 AU, these stars are non-interacting.
The authors used the Very Large Array (VLA) and the VLA Low-band Ionosphere and Transient Experiment (VLITE) commensal system to detect EI Cancri AB. At an angular separation of 0.874 degrees from the primary target, they created an image of EI Cancri using a VLA observation of the blazar OJ 287. The low frequency of 340 MHz resulted in lower resolution, making it challenging to attribute the source to either EI Cancri A or B.
After analyzing a 7-hour dataset, the researchers identified three independent bursts at 00:09, 02:48, and 03:41 on April 27, 2018. The image and best-fitting positions of these bursts are presented in Figure 2. The authors suggest that if both systems are bursting, the apparent central location of the image is explained. The inferred locations from the time-sliced images support the idea that the third burst originates from EI Cancri B.
The origin of radio emission in EI Cancri AB is a subject of debate. The authors consider incoherent processes (gyro-radiation) and coherent processes (plasma emission vs. electron cyclotron maser instability) as potential sources. The best-known example is 'gyro' or 'gyromagnetic' emission, where electrons spiral along a magnetic field line under electromagnetic force influence. Coherent processes involve electrons moving in concert, resulting in highly polarized radio emission.
To determine the emission process, the researchers calculate the brightness temperature. If it exceeds 10^12 Kelvin, the process is more likely to be coherent. However, estimating the source size is challenging due to the lack of other detections at this frequency for comparison. The authors estimate the flaring region in the star's atmosphere to be 1-5 stellar radii, causing the brightness temperature to fluctuate around the cutoff value, making a definitive determination impossible.
Further observations using the VLA's dedicated P-band mode and higher frequencies could provide more insights. Ultra-high-resolution radio observations using very-long-baseline interferometry could map stellar motion precisely and determine their orbital properties. Follow-up optical and infrared observations might solidify the true rotational periods.
This discovery opens up exciting possibilities for studying the EI Cancri AB system from multiple perspectives, shedding light on the mysteries of ultracool dwarfs and their radio emissions.
