Alight curve for Gliese 229 showing a small flare, adapted from Byrneet al. (1985)[5]
Gliese 229 is known to be a low activityflare star, which means it undergoes random increases in luminosity because ofmagnetic activity at the surface. The spectrum shows emission lines of calcium in theH andK bands. The emission ofX-rays has been detected from thecorona of this star.[16] 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.[5]
Asubstellar companion was discovered in 1994 by Caltech astronomers Kulkarni, Tadashi Nakajima, Keith Matthews, andRebecca Oppenheimer, and Johns Hopkins scientists Sam Durrance and David Golimowski. It was confirmed in 1995 as Gliese 229B,[19][20] It was one of the first brown dwarfs discovered. Although too small to sustainhydrogen-burningnuclear fusion as in amain sequence star, with a mass of around 40 to 60 times that ofJupiter (0.06 solar masses),[10][21] it is still too massive to be aplanet. As a brown dwarf, its core temperature is high enough to initiate the fusion ofdeuterium with a proton to formhelium-3, but it is thought that it used up all its deuterium fuel long ago.[22] This object has a surface temperature of 950 K.[23]
Gliese 229 B was later found to be a binary brown dwarf.[6] Since 2021 it was suggested to be an unresolved binary, given the inconsistency between the object's measured mass and luminosity.[7][30] Further evidence that Gliese 229B is an equal-mass binary comes from high-resolution spectroscopy from theSubaru Telescope.[31] Gliese 229 B was then finally resolved in 2024 withVLT/GRAVITY andVLT/CRIRES+. The components are called Gliese 229 Ba and Gliese 229 Bb. The pair is a tight orbit with an orbital period of 12.1 days and a semi-major axis of 0.042astronomical units (about 16 Earth-Moon distances). The changes inradial velocity extracted from CRIRES+ helped to resolve the orbit of Gliese 229B. The binary has aninclination of31.4°±0.3° and aneccentricity of0.234±0.004. The inclination of the binary is misaligned by37+7 −10° in respect to the orbit of Gliese 229B around Gliese 229A.[6] Additional radial velocity changes between two epochs were detected in Gliese 229B withKeck NIRSPEC. This team independently discovered the binarity of Gliese 229B.[4]
The brown dwarf pair was observed with JWSTMIRI 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]=0.00+0.04 −0.03 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.[12]
In March 2014, asuper-Neptune mass planet candidate was announced in a much closer-in orbit around GJ 229.[32] Given the proximity of the Gliese 229 system to the Sun, the orbit of GJ 229 Ab might be fully characterized by theGaia space-astrometry mission or via direct imaging. In 2020, asuper-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'shabitable zone and in that sense similar toMars in our ownSolar System. While considering GJ 229 Ab unconfirmed, the study estimated a significantly lowerminimum mass for it.[33]
However, a more recent study found that whenstellar 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.[13]
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^Woolley, R.; Epps, E. A.; Penston, M. J.; Pocock, S. B. (1970). "Catalogue of stars within twenty-five parsecs of the Sun".Royal Observatory Annals.5.Bibcode:1970ROAn....5.....W.
^Saumon, D.; Geballe, T. R.; Leggett, S. K.; Marley, M. S.; Freedman, R. S.; Lodders, K.; Fegley, B., Jr.; Sengupta, S. K. (2000-09-01). "Molecular Abundances in the Atmosphere of the T Dwarf GL 229B".The Astrophysical Journal.541 (1):374–389.arXiv:astro-ph/0003353.Bibcode:2000ApJ...541..374S.doi:10.1086/309410.ISSN0004-637X.
^Schultz, A. B.; Allard, F.; Clampin, M.; McGrath, M.; Bruhweiler, F. C.; Valenti, J. A.; Plait, P.; Hulbert, S.; Baum, S.; Woodgate, B. E.; Bowers, C. W.; Kimble, R. A.; Maran, S. P.; Moos, H. W.; Roesler, F. (1998-01-01). "First Results from the Space Telescope Imaging Spectrograph: Optical Spectra of Gliese 229B".The Astrophysical Journal.492 (2):L181 –L184.Bibcode:1998ApJ...492L.181S.doi:10.1086/311103.ISSN0004-637X.
