HAT-P-21 is a G-type main-sequence star about 927 light-years away. The star has amount of metals similar to solar abundance. The survey in 2015 has failed to detect any stellar companions.[6] The star is rotating rapidly, being spun up by the tides of giant planet on close orbit.[4]
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 11h 25m 05.9859s[1] |
| Declination | +41° 01′ 40.665″[1] |
| Apparent magnitude (V) | 11.46[2] |
| Characteristics | |
| Spectral type | G3V |
| Astrometry | |
| Radial velocity (Rv) | -51.98 km/s |
| Proper motion (μ) | RA: −1.142(17) mas/yr[1] Dec.: 13.523(24) mas/yr[1] |
| Parallax (π) | 3.5190 ± 0.0228 mas[1] |
| Distance | 927 ± 6 ly (284 ± 2 pc) |
| Details[3][4] | |
| 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±0.02 d[5] |
| Rotational velocity (v sin i) | 3.5±0.5 km/s |
| Age | 10.2±2.5 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Naming
In 2019, the HAT-P-21 star received the proper name Mazalaai while its planet HAT-P-21b received the name Bambaruush at an international NameExoWorlds contest.[7] These names refer to the Mongolian name for the endangered Gobi bear subspecies, and the Mongolian term for 'bear cub', respectively.
Planetary system
In 2010 a transiting hot super-Jovian planet on moderately eccentric orbit was detected.[8] Its equilibrium temperature is 1283±50 K. The transit-timing variation survey in 2011 have failed to rule out or confirm the existence of additional planets in the system, until the orbital parameters of HAT-P-21b are known with better precision.[5]
The planetary orbit is likely aligned with the equatorial plane of the star, misalignment equal to 25±16 degrees.[9]
| 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 |