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 made by the Hipparcos spacecraft put the star at a distance of around 58 light years.[1]

HN Pegasi
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Pegasus
Right ascension 21h 44m 31.32919s[1]
Declination +14° 46′ 18.9806″[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: +229.93[1] mas/yr
Dec.: −113.46[1] mas/yr
Parallax (π)55.91 ± 0.45[1] mas
Distance58.3 ± 0.5 ly
(17.9 ± 0.1 pc)
Absolute magnitude (MV)4.70[7]
Details
HN Peg A
Mass1.085±0.091[5] M☉
Radius1.002±0.018[5] R☉
Luminosity (bolometric)1.090[7] L☉
Surface gravity (log g)4.45[8] cgs
Temperature5,961[8] K
Metallicity [Fe/H]−0.06[8] dex
Rotation4.84 d[9]
Rotational velocity (v sin i)12.81[10] km/s
Age237±33[5] Myr
HN Peg B
Radius0.101[11] R☉
Surface gravity (log g)4.81[11] cgs
Temperature1,115[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
SIMBADHN 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 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]

References
  1. van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600.
  2. Samus, N. N.; Durlevich, O. V.; et al. (2009), "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)", VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S, 1: B/gcvs, Bibcode:2009yCat....102025S.
  3. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992.
  4. Blanco, C.; et al. (1979), "Photoelectric observations of stars with variable H and K emission components. III", Astronomy and Astrophysics Supplement Series, 36: 297–306, Bibcode:1979A&AS...36..297B.
  5. Boro Saikia, S.; et al. (January 2015), "Variable magnetic field geometry of the young sun HN Pegasi (HD 206860)", Astronomy & Astrophysics, 573: 16, arXiv:1410.8307, Bibcode:2015A&A...573A..17B, doi:10.1051/0004-6361/201424096, S2CID 53316047, A17.
  6. Nidever, David L.; et al. (August 2002), "Radial Velocities for 889 Late-Type Stars", The Astrophysical Journal Supplement Series, 141 (2): 503–522, arXiv:astro-ph/0112477, Bibcode:2002ApJS..141..503N, doi:10.1086/340570, S2CID 51814894.
  7. Ertel, S.; et al. (May 2012), "A peculiar class of debris disks from Herschel/DUNES. A steep fall off in the far infrared", Astronomy & Astrophysics, 541: 14, arXiv:1203.6784, Bibcode:2012A&A...541A.148E, doi:10.1051/0004-6361/201118077, S2CID 5457432, A148.
  8. Ramírez, I.; et al. (February 2013), "Oxygen abundances in nearby FGK stars and the galactic chemical evolution of the local disk and halo", The Astrophysical Journal, 764 (1): 78, arXiv:1301.1582, Bibcode:2013ApJ...764...78R, doi:10.1088/0004-637X/764/1/78, S2CID 118751608.
  9. Messina, S.; Guinan, E. F. (October 2003), "Magnetic activity of six young solar analogues II. Surface Differential Rotation from long-term photometry", Astronomy and Astrophysics, 409 (3): 1017–1030, Bibcode:2003A&A...409.1017M, doi:10.1051/0004-6361:20031161.
  10. Martínez-Arnáiz, R.; et al. (September 2010), "Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter", Astronomy and Astrophysics, 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725, S2CID 43455849.
  11. Leggett, S. K.; et al. (August 2008), "HN Peg B: A Test of Models of the L to T Dwarf Transition", The Astrophysical Journal, 682 (2): 1256–1263, arXiv:0804.1386, Bibcode:2008ApJ...682.1256L, doi:10.1086/589146, S2CID 15701830.
  12. "V* HN Peg -- Variable of BY Dra type", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2015-12-07.
  13. Luhman, K. L.; et al. (January 2007), "Discovery of Two T Dwarf Companions with the Spitzer Space Telescope", The Astrophysical Journal, 654 (1): 570–579, arXiv:astro-ph/0609464, Bibcode:2007ApJ...654..570L, doi:10.1086/509073, S2CID 11576708.
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