HD 4747 is a star that lies approximately 61light-years away in the constellation of Cetus. The star is a low-amplitudespectroscopic binary, with the secondary being a directly detectedbrown dwarf.
The binarity of HD 4747 was announced in 2002, based on observations with the HIRES spectrograph at theW. M. Keck Observatory.[10] The radial velocity variation caused by the companion was found to have a semi-amplitude of approximately 0.65 km/s, which evaded detection by earlier spectrographs due to their precision being poorer than about 1 km/s, but was easily detectable by the 3 m/s precision of HIRES. With a period of 6832 ± 653 days (the large error due to the orbit being unclosed over 1731 day span of observations) and an eccentricity of 0.64 ± 0.06, assuming a primary mass of 0.83 M☉ led to a minimum secondary mass of42.3MJ - well within the brown dwarf regime. HD 4747 B became one of the few brown dwarf candidates orbiting within a few AU of any type of star.
An updated orbital solution was provided in 2010, based on observations with theCORALIE spectrograph.[5] With a 3068-day extension to the radial velocity time series, the orbital period was found to be about twice as long as previously thought due to an increase in the fitted eccentricity. The minimum mass of the companion was increased slightly to46.1 ± 2.3MJ, remaining in the brown dwarf regime.
In 2013 an attempt to directly image the secondary usingadaptive optics imaging at the W. M. Keck Observatory resulted in a non-detection, thus effectively eliminating the possibility of the companion being a low inclination stellar binary. A follow-up attempt in 2014 resulted in a candidate source that together with additional images taken in 2015 confirmed the companion as havingcommon proper motion and also showed orbital motion in a counter-clockwise direction.[8]
Photometry indicates that HD 4747B is most likely aL-typebrown dwarf and may possibly be close to the transition between L andT-types. A preliminary dynamical mass was found to be67.2±1.8 times the mass ofJupiter.[7]
^abcdefAn, Qier; Brandt, Timothy D.; Brandt, G. Mirek; Venner, Alexander (2025-08-11). "Orbits and Masses for 156 Companions from Combined Astrometry and Radial Velocities, and A Validation of Gaia Non-Single Star Solutions".The Astrophysical Journal Supplement Series.arXiv:2508.08374.
