| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major[1] |
| Right ascension | 11h 25m 05.9859s[2] |
| Declination | +41° 01′ 40.665″[2] |
| Apparent magnitude (V) | 11.46[3] |
| Characteristics | |
| Spectral type | G3V[4] |
| Astrometry | |
| Radial velocity (Rv) | −51.87±0.48[2] km/s |
| Proper motion (μ) | RA: −1.142(17)mas/yr[2] Dec.: 13.523(24)mas/yr[2] |
| Parallax (π) | 3.5190±0.0228 mas[2] |
| Distance | 927 ± 6 ly (284 ± 2 pc) |
| Details[5][6] | |
| Mass | 0.947±0.042 M☉ |
| Radius | 1.105±0.083 R☉ |
| Luminosity | 1.06+0.20 −0.16 L☉ |
| Surface gravity (log g) | 4.33±0.06 cgs |
| Temperature | 5634±67 K |
| Metallicity | 0.04±0.08 |
| Rotation | 15.88±0.02d[7] |
| Rotational velocity (v sin i) | 3.5±0.5 km/s |
| Age | 10.2±2.5 Gyr |
| Other designations | |
| Mazaalai,HAT-P-21,TOI-1800,TIC 17993892,TYC 3013-1229-1,GSC 03013-01229,2MASS J11250598+4101406[3] | |
| Database references | |
| SIMBAD | data |
| Exoplanet Archive | data |
HAT-P-21, also namedMazaalai, is aG-type main-sequence star about 927 light-years away. The star has amount of metals similar tosolar abundance. The survey in 2015 has failed to detect any stellar companions.[8] The star is rotating rapidly, being spun up by the tides of giant planet on close orbit.[6]
In 2019, the star HAT-P-21 received the proper nameMazaalai while its planetHAT-P-21b received the nameBambaruush in the internationalNameExoWorlds contest. These names refer to the Mongolian name for the endangeredGobi bear subspecies, and the Mongolian term for 'bear cub', respectively.Mazaalai was also the name of Mongolia's first satellite.[9] Some IAU publications have misspelled the star's name asMazalaai, but the latest IAU Catalog of Star Names listsMazaalai.[10]
In 2010 atransiting hotsuper-Jovian planet on moderately eccentric orbit was detected.[4] Its equilibrium temperature is 1283±50K. Thetransit-timing variation survey in 2011 have failed to rule out or confirm the existence of additional planets in the system, until the orbital parameters ofHAT-P-21b are known with better precision.[7]
The planetary orbit is likely aligned with the equatorial plane of the star, misalignment equal to 25±16 degrees.[11]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b (Bambaruush) | 4.063±0.161 MJ | 0.0494±0.0007 | 4.124481±0.000007 | 0.228±0.016 | 88.6° | 1.08±0.18 RJ |
