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Crossing
the Brown Dwarf Desert: Michael
C. Liu
(IfA/Hawaii), ApJ, 2002, in press. |
[Results] [The Paper] [Press Release + Graphics] [Media Coverage] [Links]
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 2.16 um flux ratio of 8.6 mags. Using follow-up K-band spectroscopy from Keck AO+NIRSPEC, we measure a spectral type of L4.5+/-1.5. This is the closest ultracool companion around a main sequence star found to date by direct imaging. We estimate the primary has an age of 1-3 Gyr. Assuming coevality, the companion is most likely substellar, with a mass of 55-78 Mjup 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 Mjup. In contrast to the paucity of brown dwarf companions at <~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 where 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.
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Gemini North adaptive optics image of 15 Sge and its newly found companion, 15 Sge B. The data were obtained in the K-band (2.2 microns). The image has been computer processed to subtract the light from the much brighter primary star in the vicinity of the companion. Click here for larger versions of the images. |
Keck Telescope adaptive optics K-band image of the 15 Sge system obtained at a later epoch for proper motion confirmation. The orientation and field of view are the same as the Gemini AO images. |
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A spectrum from Keck AO+NIRSPEC shows a very cool temperature for 15 Sge B. From the spectroscopy and photometry, we estimate the companion's spectral type to be L4.5, with an uncertainty of +/-1.5 subclasses. |
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We get a lower mass limit of 48 Mjup for the companion using 13 years of radial velocity data for the primary star and the observed separation in the AO imaging. This mass constraint is model-independent, since it comes strictly from orbital dynamics. Here is one realization of a possible orbit for the companion. |
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The primary star is a solar analog. Given its higher level of activity than the Sun, we estimate an age of 1-3 Gyr. Combining this with theoretical models gives a mass estimate of 55 - 78 Mjup. (Only one set of models shown here. See our paper for more.) Hence, 15 Sge B is most likely substellar. |
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March 4, 2002
