NGTS-7Ab: an ultrashort-period brown dwarf transiting a tidally locked and active M dwarf
Jackman, James A. G.; Wheatley, Peter J.; Moyano, Maximiliano; Günther, Maximilian N.; Cabrera, Juan; Csizmadia, Szilard; Eigmüller, Philipp; Erikson, Anders; Rauer, Heike; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Anderson, David R.; Burleigh, Matthew R.; Gillen, Edward; Jenkins, James S.; McCormac, James; Udry, Stéphane; Vines, Jose I.; West, Richard G.; Smith, Alexis M. S.; Bryant, Edward M.; Gänsicke, Boris T.; Queloz, Didier; Watson, Christopher A.; Bouchy, François; Chaushev, Alexander; Hodgkin, Simon T.; Louden, Tom; Nielsen, Louise D.; Poppenhaeger, Katja; Raynard, Liam; Pollacco, Don; Briegal, Joshua T.; Turner, Oliver; Costes, Jean C.; Belardi, Claudia; Bayliss, Dan; Braker, Ian P.
United Kingdom, United States, Switzerland, Germany, Chile
Abstract
We present the discovery of NGTS-7Ab, a high-mass brown dwarf transiting an M dwarf with a period of 16.2 h, discovered as part of the Next Generation Transit Survey (NGTS). This is the shortest period transiting brown dwarf around a main or pre-main sequence star to date. The M star host (NGTS-7A) has an age of roughly 55 Myr and is in a state of spin-orbit synchronization, which we attribute to tidal interaction with the brown dwarf acting to spin-up the star. The host star is magnetically active and shows multiple flares across the NGTS and follow-up light curves, which we use to probe the flare-star-spot phase relation. The host star also has an M star companion at a separation of 1.13 arcsec with very similar proper motion and systemic velocity, suggesting that the NGTS-7 system is a hierarchical triple. The combination of tidal synchronisation and magnetic braking is expected to drive on-going decay of the brown dwarf orbit, with a remaining lifetime of only 5-10 Myr.