Binary star system in the constellation Orion
! style="text-align: center; background-color: #FFFFC0;" colspan="2" | Observation dataEpoch J2000.0      Equinox J2000.0
! style="text-align:left" | Constellation
! style="text-align:left" | Right ascension
! style="text-align:left" | Declination
! style="text-align:left" | Apparent magnitude (V)
! style="background-color: #FFFFC0; text-align: center;" colspan="2"| Characteristics
! style="text-align:left" | Spectral type
! style="text-align:left" | B−V 
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HD 33636 is a G-type main-sequence star located approximately 96.5 light-years away in the Orion constellation. It is a 7th magnitude star with a metallicity of −0.05 ± 0.07. A likely substellar companion was discovered in 2002.
Companion.
HD 33636 b was discovered in 2002 by the Keck telescope in Hawaii using the radial velocity method. It was independently detected at the Haute-Provence Observatory in France. With this method it showed a minimum mass of 9.28 Jupiter masses, and was initially assumed to be a planet and labelled "HD 33636 b" (lower-case).
In 2007, Bean et al. used the Hubble Space Telescope (HST) astrometry to find that this body has an inclination as little as 4.1 ± 0.1°, which yielded a true mass of 142 MJ. This is too high to be a planet. It was classified by this study as an M-dwarf star of likely spectral type M6V, "HD 33636 B" (upper-case).
This picture was further revised in the 2020s. A 2023 study using astrometry from Hipparcos and Gaia found that the mass had likely been overestimated, and found a lower true mass of about 77.8 MJ. This would place HD 33636 b near the borderline between stars and brown dwarfs. A 2024 study using Gaia astrometry even excluded the possibility of a companion mass greater than 40 MJ, instead finding a mass range more compatible with the initial minimum mass estimate. This study estimated a mass of about 15.4 MJ, near the borderline between brown dwarfs and planets.
This object takes 2121 days or 5.807 years to orbit at a semimajor axis of 3.33 astronomical units (AU).
References.
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