 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 | |
| Database references | |
| SIMBAD | HN Peg A |
| HN Peg B | |
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]
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 absorption characteristic of T-type dwarfs. The separation corresponds to a projected physical distance of 795 AU, [14] which is uncommonly wide for such brown dwarf companions. The estimated mass of the object is 28 MJ. Based upon its spectrum, HN Peg B has relatively thin cloud decks. [11] HN Peg B was observed with Hubble WFC3 and found to be variable in the 1.1–1.7 μm range. It varied with an amplitude of 1.206 ±0.025% and a rotation period of 15.4 ±0.5 hours. The water absorption band at 1.4 μm had a reduced amplitude. The condensation particles causing the variation have a particle size larger than 1 μm. [15] It also showed variability in a Spitzer light curve, with an amplitude of 1.1 ±0.5% at 4.5 μm and 0.77 ±0.15% at 3.6 μm. [16]
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]
