Shortest-period Ultracool Dwarf Binary discovered

(10 Jan 2023) Dino Hsu, a PhD graduate of the Cool Star Lab and now postdoctoral scholar at Northwestern University, reported the discovery of the shortest-period ultracool dwarf binary at a press conference at the AAS 241 meeting in Seattle, Washington. The system, LP 413-53, is an M9 dwarf that has long exhibited "odd" features, including varying radial velocities and H-alpha emission measurements in the literature. Using the Keck/NIRSPEC high resolution near-infrared spectrograph, Dino discovered that the system was a double-lined spectroscopic binary, with line centers that could be seen moving apart with every exposure. After obtaining observations over four runs between March and December 2022, Dino was able to piece together an orbit for the system, which is nearly circular with a period of 0.85 days. This beats the previous record-holder for shortest-period ultracool dwarf binary, the young M5 dwarf USco J161630.68-251220.1 with a period of 2.9 days discovered by Lodieu et al. (2015). It also beats out all of the known eclipsing brown dwarf binaries, all of which are young. LP 413-53, in contrast, appears to be fully evolved, and its tiny stars are unlikely to eclipse despite being separated by only 10 stellar diameters. Interestingly, this system is approximately the size of the Galilean moon system, and each star is about the same size as Jupiter; the system also fits within the innermost orbit of the TRAPPIST-1 planetary system. Since the stars would have been close to touching each other in their early evolution, understanding how this system formed (e.g., through orbital decay or three-body dynamics) and whether it is common or a lucky break, are important questions for low-mass star formation theories and the frequency of habitable planetary systems around dim stars.