Massive brown dwarf HD 203771 b confirmed in eccentric orbit around orange dwarf

Researchers have measured the mass of HD 203771 b, a rare brown dwarf orbiting an orange dwarf star. By combining precise astrometry and radial velocity data, the study confirms the companion has a mass roughly 70 times that of Jupiter. The analysis also explores how brown dwarfs migrate inward over time, suggesting that disk friction plays a key role in shaping their final orbital paths.

Star system: HD 203771 A | Distance from the Sun: 184 light years

The massive brown dwarf known as HD 203771 b has been characterized as orbiting its host orange dwarf star once every 1140 days. Brown dwarfs are often called failed stars because they are more massive than planets but not heavy enough to sustain the nuclear fusion that powers stars. This particular companion follows an eccentric, or elongated, path that is inclined at an angle of approximately 130 degrees relative to its host. The host star itself has a mass estimated at 0.79 times that of the Sun.

To determine the properties of this system, researchers utilized a joint analysis approach combining radial velocity measurements and astrometric data. Radial velocity tracks the slight wobbles of a star caused by the gravitational pull of an orbiting body. Astrometry measures the precise position of a star in the sky over time. By combining these two techniques using data from the Gaia and Hipparcos missions, the team calculated the true mass of the brown dwarf to be between 69 and 74 times the mass of Jupiter.

The findings extend beyond this single system by comparing it to a broader group of 159 brown dwarfs. By examining their orbital characteristics, the analysis shows that these objects tend to have more circular orbits when they are closer to their host stars. This shift toward lower eccentricity as distance decreases suggests that these bodies migrate inward through a process involving disk friction. This process implies that brown dwarfs likely move toward their stars while remaining aligned within the disk of material surrounding the young star.

Key findings
HD 203771 b is a massive brown dwarf with a mass approximately 69 to 74 times that of Jupiter.
The object completes one orbit around its orange dwarf host every 1140 days on an eccentric path.
The orbital tilt of the companion is approximately 130 degrees.
Data across 159 brown dwarfs suggests that orbits become more circular as objects migrate inward.
The migration of brown dwarfs is likely driven by friction within the protoplanetary disk.

Scientific study

An Intermediate-orbit Brown Dwarf Confirmed with Astrometry+Radial Velocity Joint Analysis, and the Eccentricity Distribution of Brown Dwarfs Across Different Locations (external link)