Primary HD 4747 A | Companion HD 4747 B | |
HD 4747 is a star that lies approximately 61 light-years away in the constellation of Cetus. The star is a low-amplitude spectroscopic binary, with the secondary being a brown dwarf candidate.
Binarity
The binarity of HD 4747 was announced in 2002, based on observations with the HIRES spectrograph at the W. M. Keck Observatory. The radial velocity variation caused by the companion was found to have an 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 of 42.3 MJ - 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 the CORALIE spectrograph. 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 to 46.1 ± 2.3 MJ, remaining in the brown dwarf regime.
While the minimum mass of the object is that of a brown dwarf, this depends on the inclination of the orbit relative to observations from the Solar System. Because of this, if the orbit of HD 4747 B is inclined by less than 40°, the mass of the object will be above the hydrogen burning limit at approximately 80 MJ that marks the transition between stars and brown dwarfs. Long-term astrometry or direct detection could solve this problem, but no attempt to do so has yet appeared in literature.