Can FAST detect auroras on brown dwarfs?
Brown dwarfs are known as "failed stars", owing to the lack of central hydrogen burning. They bridge the gap between planets and stars. Some brown dwarfs are found to maintain kilogauss magnetic fields and produce flaring radio emissions, similar to aurora on magnetized planets in solar system, arousing astronomers’ curiosities about their field properties and dynamos.
Radio emissions from brown dwarfs reflect their magnetic activities. For solar-type stars, radio, optical and X-ray emissions are all used as magnetic indicators, while for brown dwarfs, optical and X-ray decrease dramatically, and radio becomes the most efficient probe.
Dr. Jing Tang and the colleagues from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) carried out a statistical analysis of radio-flaring brown dwarf population, which helped quantify the potential of finding such objects in FAST surveys.
This study was published this June in Research in Astronomy and Astrophysics, which is a peer-reviewed international journal in astronomy and astrophysics supported by NAOC and the Chinese Astronomical Society, and has a distinguishededitorial board with extensive academic qualifications, ensuring high academic standards and a braod international coverage.
The traditional way is to select a number of brown dwarfs and track them for several hours to catch the possible flares. It is very expensive. Till now, the detected flaring brown dwarfs are less than 20. The so-called Commensal Radio Astronomy FasT Survey (CRAFTS) promises to increase the number by almost one order of magnitude, according to the published study.
Led by Dr. Di Li, Chief Scientist of FAST, CRAFTS utilizes a novel and unprecedented mode to realize simultaneous data taking for pulsar and FRB search, Galactic HI mapping, and HI galaxy study. It is designed to cover 60% of the sky in drfit-scan mode.
For FAST, the most significant problem for a point source is the severe confusion due to the large beam size. However, the flaring radio emission is highly circularly polarized, suffering little confusion. Circular polarization can be calculated from the orthogonally polarized outputs, independent of system fluctuation, and is a good method to search for flares.
If we find some highly circularly polarized signal in the survey, we can cross match the archival optical/infrared counterpart for identification. FAST is expected to detect flaring brown dwarfs as far as 180 pc.
Most flaring brown dwarfs are detected at high frequencies. Though some efforts have been made on low frequencies, the flaring emission at L band has not been detected yet. FAST may fill in this gap. If successful, it also bodes well for FAST’s potential to discovery exoplanets with strong magnetic fields.
This paper is available at https://doi.org/10.1088/1674-4527/ac66bd
Image: The conceptual plot of the CRAFTS survey. The scanning pattern of FAST's beams across the sky, revealing galaxies, pulsars, and brown dwarfs. (Credit: Di Li)