HN Pegasi

HN Pegasi is the variable star designation for a young, Sun-like star in the northern constellation of Pegasus. It has an apparent visual magnitude of 5.9,[4] which, according to the Bortle scale, indicates that it is visible to the naked eye from suburban skies. Parallax measurements put the star at a distance of around 59 light years from the Sun,[1] but it is drifting closer with a radial velocity of −16.7 km/s.[6]

HN Pegasi

HN Peg B (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Pegasus
Right ascension	21h 44m 31.330s[1]
Declination	+14° 46′ 18.98″[1]
Apparent magnitude (V)	5.92 - 5.95[2]
Characteristics
Spectral type	G0 V CH-0.5[3]
U−B color index	+0.031[4]
B−V color index	+0.588[4]
Variable type	BY Dra[5]
Astrometry
Radial velocity (Rv)	−16.68±0.09[6] km/s
Proper motion (μ)	RA: +231.108 mas/yr[1] Dec.: −113.200 mas/yr[1]
Parallax (π)	55.148 ± 0.0348 mas[1]
Distance	59.14 ± 0.04 ly (18.13 ± 0.01 pc)
Absolute magnitude (MV)	4.70[7]
Details
HN Peg A
Mass	1.085±0.091[5] M☉
Radius	1.002±0.018[5] R☉
Luminosity (bolometric)	1.090[7] L☉
Surface gravity (log g)	4.45[8] cgs
Temperature	5,961[8] K
Metallicity [Fe/H]	−0.06[8] dex
Rotation	4.84 d[9]
Rotational velocity (v sin i)	12.81[10] km/s
Age	237±33[5] Myr
HN Peg B
Radius	0.101[11] R☉
Surface gravity (log g)	4.81[11] cgs
Temperature	1,115[11] K
Other designations
HN Peg, BD+14°4668, FK5 3737, GJ 836.7, HD 206860, HIP 107350, HR 8314, SAO 107364[12]
Database references
SIMBAD	HN Peg A
	HN Peg B

This is a G-type main sequence star with a stellar classification of G0 V CH-0.5[3] and an estimated age of just 237 million years. It has slightly more mass and a slightly larger radius than the Sun,[5] but a somewhat lower metallicity.[8] It is spinning relatively quickly, with an estimated rotation period of 4.84[9] days.

The surface magnetic field of the star has a complex and variable geometry. It is a BY Draconis variable star with an active chromosphere, which means there is a rotational modulation of its luminosity due to star spots. Much like the Sun, the star spot activity undergoes a periodic cycle of maxima and minima lasting roughly 5.5±0.3 yr.[5] Its apparent magnitude varies between a maximum of 5.92 and a minimum of 5.95 over a period of 24.9 days.[2] However, the rotation period is on average 4.84 days. The star shows an anti-solar pattern of rotation, with the rotation rate steadily increasing during each cycle before dropping back to the initial value upon the start of a new cycle.[9]

In 2007, the discovery of a brown dwarf companion was announced. HN Peg B was spotted using the Spitzer Space Telescope at an angular separation of 43.2 arc sec, showing a methane emission characteristic of T-type dwarfs. The separation corresponds to a projected physical distance of 795 AU,[13] which is uncommonly wide for such brown dwarf companions. The estimated mass of the object is 28 M_J. Based upon its spectrum, HN Peg B has relatively thin cloud decks.[11]

This star displays an emission of infrared excess that suggests there is a circumstellar disk of debris in orbit.[7] HN Pegasi is most likely a thin disk population star.[8] It is a member of the nearby Hercules-Lyra association of stars that share a common motion through space.[5]