HN_Pegasi_b
HN Pegasi
Star in the constellation Pegasus
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 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 MJ. 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]