CROSSING THE BROWN DWARF DESERT USING ADAPTIVE OPTICS: A VERY CLOSE L DWARF COMPANION TO THE NEARBY SOLAR ANALOG HR 7672

摘要

We have found a very faint companion to the active solar analog HR 7672 (HD 190406; GJ 779; 15 Sge). Three epochs of high-resolution imaging using adaptive optics (AO) at the Gemini-North and Keck II Telescopes demonstrate that HR 7672B is a common proper motion companion, with a separation of 0″.79 (14 AU) and a 2.16μm flux ratio of 8.6 mag. Using follow-up K-band spectroscopy from Keck AO+NIRSPEC, we measure a spectral type of L45 with an uncertainty of +-1.5 subclasses. This is the closest ultracool companion around a main-sequence star found to date by direct imaging. We estimate that the primary has an age of 1-3 Gyr. Assuming coevality, the companion is most likely substellar, with a mass of 55-78 M_J based on theoretical models. The primary star shows a long-term radial velocity trend, and we combine the radial velocity data and AO imaging to set a firm (model-independent) lower limit of 48 M_J. In contrast to the paucity of brown dwarf companions at approx< 4 AU around FGK dwarfs, HR 7672B implies that brown dwarf companions do exist at separations comparable to those of the giant planets in our own solar system. Its presence is at variance with scenarios in which brown dwarfs form as ejected stellar embryos. Moreover, since HR 7672B is likely too massive to have formed in a circumstellar disk as planets are believed to, its discovery suggests that a diversity of physical processes act to populate the outer regions of exoplanetary systems.