Gliese 229

Template:Short description Template:Sky Template:Starbox begin Template:Starbox image Template:Starbox observe Template:Starbox character Template:Starbox astrometry Template:Starbox orbit Template:Starbox orbit Template:Starbox detail Template:Starbox detail Template:Starbox catalog Template:Starbox reference Template:Starbox image Template:Starbox end Gliese 229 (also written as Gl 229 or GJ 229) is a multiple system composed of a red dwarf and two brown dwarfs, located 18.8 light years away in the constellation Lepus. The primary component has 58% of the mass of the Sun, 55% of the Sun's radius, and a very low projected rotation velocity of 1 km/s at the stellar equator.

The star is known to be a low activity flare star, which means it undergoes random increases in luminosity because of magnetic activity at the surface. The spectrum shows emission lines of calcium in the H and K bands. The emission of X-rays has been detected from the corona of this star. These may be caused by magnetic loops interacting with the gas of the star's outer atmosphere. No large-scale star spot activity has been detected.

The space velocity components of this star are U = +12, V = –11 and W = –12 km/s. The orbit of this star through the Milky Way galaxy has an eccentricity of 0.07 and an orbital inclination of 0.005.

Brown dwarfs

A substellar companion was discovered in 1994 by Caltech astronomers Kulkarni, Tadashi Nakajima, Keith Matthews, and Rebecca Oppenheimer, and Johns Hopkins scientists Sam Durrance and David Golimowski. It was confirmed in 1995 as Gliese 229B, It was one of the first brown dwarfs discovered. Although too small to sustain hydrogen-burning nuclear fusion as in a main sequence star, with a mass of around 40 to 60 times that of Jupiter (0.06 solar masses), it is still too massive to be a planet. As a brown dwarf, its core temperature is high enough to initiate the fusion of deuterium with a proton to form helium-3, but it is thought that it used up all its deuterium fuel long ago. This object has a surface temperature of 950 K.

A light curve for Gliese 229 showing a small flare, adapted from Byrne et al. (1985)

Gliese 229B is the prototype of the T-dwarfs, due to the detection of methane in its spectrum. It also shows other molecules in its atmosphere, namely water vapor, carbon monoxide and ammonia. Atomic absorption lines of caesium, sodium and potassium are also detected.

Gliese 229 B was later found to be a binary brown dwarf. Since 2021 it was suggested to be an unresolved binary, given the inconsistency between the object's measured mass and luminosity. Further evidence that Gliese 229B is an equal-mass binary comes from high-resolution spectroscopy from the Subaru Telescope. Gliese 229 B was resolved in 2024 with VLT/GRAVITY and VLT/CRIRES+. The components are called Gliese 229 Ba (Template:Val) and Gliese 229 Bb (Template:Val). The pair is a tight binary with an orbital period of 12.1 days and a semi-major axis of 0.042 astronomical units (about 16 Earth-Moon distances). The changes in radial velocity extracted from CRIRES+ helped to resolve the orbit of Gliese 229B. The binary has an inclination of 31.4 ±0.3° and an eccentricity of 0.234 ±0.004. The inclination of the binary is misaligned by Template:Val° in respect to the orbit of Gliese 229B around Gliese 229A. Additional radial velocity changes between two epochs were detected in Gliese 229B with Keck NIRSPEC. This team independently discovered the binarity of Gliese 229B.

The brown dwarf pair was observed with JWST MIRI low resolution spectroscopy. Previous works showed a difference in abundances between host star and companion in Gliese 229 from near-infrared spectra. This new study using mid-infrared data showed that the pair has abundances consistent with the host star. The metallicities were measured to be C/O = 0.65 ±0.05 and [M/H]=Template:Val and are equal for each brown dwarf in the pair. The host star has C/O = 0.68 ± 0.12 and [M/H] = −0.02 ± 0.06.

Search for planets

In March 2014, a super-Neptune mass planet candidate was announced in a much closer-in orbit around GJ 229. Given the proximity of the Gliese 229 system to the Sun, the orbit of GJ 229 Ab might be fully characterized by the Gaia space-astrometry mission or via direct imaging. In 2020, a super-Earth mass planet was discovered around GJ 229. GJ 229 Ac orbits the star closer in than GJ 229 Ab, located towards the outer edge but still well inside the star's habitable zone and in that sense similar to Mars in our own Solar System. While considering GJ 229 Ab unconfirmed, the study estimated a significantly lower minimum mass for it.

However, a more recent study found that when stellar activity was taken in account, the radial velocity signals corresponding to the planets' orbital periods disappeared. Therefore, intrinsic activity of the host star or errors in the previous observations are the cause of the radial velocity variations, instead of planets, which mean Gliese 229 Ab and Gliese 229 Ac likely do not exist.

See also

• Epsilon Indi

External links

• Brown dwarfs (NASA)
• It's Twins! Mystery of Famed Brown Dwarf Solved press release by Caltech