HD 233731

HD 233731, or HAT-P-22, is a suspected multiple star system in the northern circumpolar constellation of Ursa Major. It is invisible to the naked eye, having an apparent visual magnitude of 9.732.^[3] This system is located at a distance of 267 light years from the Sun based on parallax, and is drifting further away with a radial velocity of +13 km/s.^[2]

HD 233731

Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major[1]
Right ascension	10h 22m 43.5923s[2]
Declination	+50° 07′ 42.062″[2]
Apparent magnitude (V)	9.732[3]
Characteristics
Spectral type	G5V[4]
B−V color index	0.86[4]
Astrometry
Radial velocity (Rv)	12.86(15)[2] km/s
Proper motion (μ)	RA: −26.110(14) mas/yr[2] Dec.: 83.806(16) mas/yr[2]
Parallax (π)	12.2731±0.0155 mas[2]
Distance	265.7 ± 0.3 ly (81.5 ± 0.1 pc)
Absolute magnitude (MV)	5.22±0.14[3]
Details[4]
Mass	0.936+0.028 −0.033 M☉
Radius	1.062+0.046 −0.013 R☉
Luminosity	0.77±0.09[3] L☉
Surface gravity (log g)	4.357+0.039 −0.005 cgs
Temperature	5,314±50 K
Metallicity [Fe/H]	0.30±0.09 dex
Rotation	28.7±0.4 d
Rotational velocity (v sin i)	1.65±0.26 km/s
Age	9.0+1.4 −2.2 Gyr[4] 12.4±2.6[3] Gyr
Other designations
HAT-P-22, Gaia DR2 846946629987527168, HD 233731, TYC 3441-925-1, GSC 03441-00925, 2MASS J10224361+5007420[5]
Database references
SIMBAD	data

The stellar classification of the primary is G5V,^[4] matching an ordinary G-type main-sequence star. The star has a low level of stellar activity with an estimated age of 9 to 12 billion years old. Its metallicity is twice that of the Sun, unusual for its advanced age.^[4] HD 233731 has a similar mass and radius as the Sun, and is spinning with a rotation period of 28.7 days.^[4] It is radiating 77%^[3] of the luminosity of the Sun from its photosphere at an effective temperature of 5314 K.^[4]

A faint stellar companion (2MASS J10224397+5007504) with a red hue is located at an angular separation of 9 arcseconds from the primary.^[3] In 2015, a spectroscopic stellar companion was reported with a semimajor axis of less than 33 AU. This star has an effective temperature of 4,000+250
−400 K with a mass of 0.63+0.07
−0.17 M_☉.^[6]

Planetary system

In 2010 a transiting hot Jupiter like planet was detected, designated HAT-P-22b.^[3] It has an equilibrium temperature of 1,463±19 K, and planetary atmosphere is cloudy.^[7] The measurement of Rossiter-McLaughlin effect in 2018 has allowed to detect what the planetary orbit is well aligned with the equatorial plane of the star, with a misalignment angle equal to 25°±18°.^[4]

In 2017, analysis of additional HARPS data showed a long-term trend that suggested the presence of an additional orbiting companion, HAT-P-22c.^[8]

Size comparison of HAT-P-22 b and Jupiter

The HAT-P-22 planetary system^[3][4][8]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	2.192+0.073 −0.013 MJ	0.04171+0.00042 −0.00050	3.21223328	0.016±0.009	86.46±0.41°	1.060±0.048 RJ
c (unconfirmed)	≥3.0 MJ	—	≥20.8 years	—	—	—