astrobites | Daily Paper Summaries | 2020 Oct 21
Ali Crisp wrote:
What do you get when you have too much mass to form a planet, but not enough mass to form a star? A brown dwarf! First theorized in the 1960s and observed in the 1990s, brown dwarfs (BDs) – a subclass of ultra-cool dwarfs – are substellar objects around 13-80 times the mass of Jupiter (or 10-90 times, depending on who you ask). They are special because, though they are thought to form in a similar manner to stars, they aren’t massive enough to trigger sustained hydrogen fusion in their cores. Instead, they are thought to fuse deuterium or lithium. This means that, unlike our Sun or other stars, they will gradually cool and fade rather than becoming white dwarfs, neutron stars, or black holes.
Despite not being stars, BDs are still self-luminous – meaning they emit energy in the form of light rather than just reflecting it back from a host star like planets do – and therefore can have spectral classifications like stars. Depending on how much light they emit and their temperatures, BDs are classified as either L, T, or Y type. Each class shows different dominant absorption lines, with L dwarfs being water and carbon monoxide-dominated, T dwarfs being methane-dominated, and Y dwarfs potentially being ammonia-dominated.
Like stars, some BDs are known to have strong magnetic fields, and even instances of potential aurorae. In addition to being observable by some optical instruments, this magnetic activity allows some BDs to be detectable in the radio and – if the magnetic field activity is strong enough – X-ray bands. However, radio observations of these objects have previously been performed primarily to follow-up known BDs. The authors of today’s paper use the Low Frequency Array (LOFAR) to make the first direct radio observation of a brown dwarf, BDR 1750+3809. They specifically looked at circularly polarized (CP) radio sources in the LOFAR Two-meter Sky Survey (LoTSS), because known brown dwarfs have highly circularly polarized radio emission. They followed up the LoTSS data with near-infrared observations using the Wide-field Infrared Camera (WIRC) at Palomar, and the NIRI imager at Gemini-North. They also obtained a spectrum using NASA’s Infrared Telescope Facility (IRTF). ...
Direct Radio Discovery of a Cold Brown Dwarf ~ H. K. Vedantham et al
- arXiv.org > astro-ph > arXiv:2010.01915 > 05 Oct 2020