HAT-P-21

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]

HAT-P-21 / Mazalaai

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
Mazalaai, Gaia DR3 770622651659107712, TYC 3013-1229-1, GSC 03013-01229, 2MASS J11250598+4101406[2]
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]

Size comparison of HAT-P-21 b and Jupiter

The HAT-P-21 planetary system^[8][10]

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

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. HAT-P-21 -- Star
3. Stassun, Keivan G.; Collins, Karen A.; Gaudi, B. Scott (2016), "Accurate Empirical Radii and Masses of Planets and Their Host Stars with Gaia Parallaxes", The Astronomical Journal, 153 (3): 136, arXiv:1609.04389, Bibcode:2017AJ....153..136S, doi:10.3847/1538-3881/aa5df3, S2CID 119219062
4. Maxted, P. F. L.; Serenelli, A. M.; Southworth, J. (2015), "A comparison of gyrochronological and isochronal age estimates for transiting exoplanet host stars", Astronomy & Astrophysics, 577: A90, arXiv:1503.09111, Bibcode:2015A&A...577A..90M, doi:10.1051/0004-6361/201525774, S2CID 53324330
5. Damiani, C.; Lanza, A. F. (2011), "Prospecting transit duration variations in extrasolar planetary systems", Astronomy & Astrophysics, 535: A116, arXiv:1109.0936, Bibcode:2011A&A...535A.116D, doi:10.1051/0004-6361/201117207, S2CID 118460941
6. Wöllert, Maria; Brandner, Wolfgang; Bergfors, Carolina; Henning, Thomas (2015), "A Lucky Imaging search for stellar companions to transiting planet host stars", Astronomy & Astrophysics, 575: A23, arXiv:1507.01938, Bibcode:2015A&A...575A..23W, doi:10.1051/0004-6361/201424091, S2CID 119250579
7. "IAU 100 NameExoWorlds Approved Names". Archived from the original on 2019-12-19. Retrieved 2020-07-08.
8. Bakos, G. Á.; Hartman, J.; Torres, G.; Latham, D. W.; Kovács, Géza; Noyes, R. W.; Fischer, D. A.; Johnson, J. A.; Marcy, G. W.; Howard, A. W.; Kipping, D.; Esquerdo, G. A.; Shporer, A.; Béky, B.; Buchhave, L. A.; Perumpilly, G.; Everett, M.; Sasselov, D. D.; Stefanik, R. P.; Lázár, J.; Papp, I.; Sári, P. (2010), "HAT-P-20b–HAT-P-23b: FOUR MASSIVE TRANSITING EXTRASOLAR PLANETS", The Astrophysical Journal, 742 (2): 116, arXiv:1008.3388, Bibcode:2011ApJ...742..116B, doi:10.1088/0004-637X/742/2/116, S2CID 119182075
9. Mancini, L.; et al. (2022), "The GAPS Programme at TNG", Astronomy & Astrophysics, 664: A162, arXiv:2205.10549, doi:10.1051/0004-6361/202243742, S2CID 248986121
10. Davoudi, F.; Jafarzadeh, S.J.; Poro, A.; Basturk, O.; Mesforoush, S.; Fasihi Harandi, A.; Gozarandi, M.J.; Zare Mehrjardi, Z.; Maley, P.D.; Khakpash, S.; Rokni, K.; Sarostad, A. (2020), "Light Curve Analysis of Ground-Based Data from Exoplanets Transit Database", New Astronomy, 76: 101305, arXiv:1910.11438, Bibcode:2020NewA...7601305D, doi:10.1016/j.newast.2019.101305, S2CID 202931761