HD 156668

HD 156668 is a star in the northern constellation of Hercules constellation. With an apparent visual magnitude of 8.4[2] it is too faint to be viewed with the naked eye, but it can be seen with even a small telescope.[4] The distance to this object has been determined directly using the parallax technique, yielding a value of about 80 light-years (25 parsecs).[1]

HD 156668
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Hercules
Right ascension 17h 17m 40.49053s[1]
Declination +29° 13.6′ 38.0243″[1]
Apparent magnitude (V) +8.424[2]
Characteristics
Spectral type K3 V[2]
U−B color index 0.27
B−V color index 1.015[2]
Astrometry
Radial velocity (Rv)-9.427 km/s
Proper motion (μ) RA: -71.16[1] mas/yr
Dec.: +217.36[1] mas/yr
Parallax (π)40.86 ± 0.86[1] mas
Distance80 ± 2 ly
(24.5 ± 0.5 pc)
Absolute magnitude (MV)6.480[2]
Details
Mass0.772 ± 0.020[2] M☉
Radius0.720 ± 0.013[2] R☉
Luminosity0.230 ± 0.018[2] L☉
Surface gravity (log g)4.60 ± 0.12[2] cgs
Temperature4850 ± 88[2] K
Metallicity [Fe/H]+0.05 ± 0.06[2] dex
Rotation51.5 days[2]
Rotational velocity (v sin i)0.50 ± 1.0[2] km/s
Age8.6 ± 4.8[2] Gyr
Other designations
BD+29° 2979, HD 156668, HIP 84607, SAO 84984, Wolf 646[3]
Database references
SIMBADdata

This star has the stellar classification of a K2 dwarf, with approximately 77% of the mass of the Sun and about 72% of the Sun's diameter. While they are on the main sequence, lower mass stars like this generate energy much more slowly than the Sun. As a result, this star is radiating only 23% of the Sun's bolometric luminosity. HD 156668 is emitting this energy from its outer atmosphere at an effective temperature of around 4850 K,[2] giving it the cool orange glow of a K-type star.[5] It is slightly more enriched in iron compared to the Sun and is rotating at a leisurely rate of once every 51.5 days. Although much older than the Sun, this star is only middle-aged at about 8.6 billion years.[2]

Observations

From 1911 to 1915, Photographic Stellar Spectrum surveys[6] have been carried out by Annie Jump Cannon and Edward Charles Pickering. Anna Mary Palmer who was Henry Draper's wife financed the study, it was also her husband who was responsible for the star's designation in the Henry Draper Catalogue with subsequent extension (HD) although it was first designated as BD+29 2979[7] in the original catalogue that was published in 1863 by Friedrich Wilhelm Argelander due to its position and brightness of 324,198 stars between +90° and −2° declination in the span of an 11-year observation. A number of proper motion surveys was published in 1957 where 9,867 stars in the Southern hemisphere where high proper motion was detected.[8] The name BD was taken from a catalogue of the Bonner Durchmusterung (Bonn Survey)[9] where observations are abbreviated as BD. Latter study expansions were carried out by the Cape Photographic Durchmusterung observatory in Córdoba, Argentina and further extensions by Henry Lee Giclas who designated the star as G 181-34 in the Giclas catalogues. From 1990 to 1993, HD 156668 was observed at least twice and was featured at the 6th annual catalog of the Tokyo Photoelectric Meridian Circle (PMC) where it is one of 6649 stars observed.[10]

Super-Earth
Main article: HD 156668 b

Andrew Howard announced the discovery of a Super-Earth in orbit around HD 156668. This planet is designated HD 156668 b. The announcement was made at the 215th American Astronomical Society meeting on January 4 to 7, 2010 in Washington D.C.. The planet orbits its star in only 4.6 days with a distance approximately 0.05 AU away from the parent star. The researchers used the wobble method where the resulting spectrum showed color shifts which was used to approximate the mass of the astronomical object. The observation revealed data that the exoplanet was at least 4.15 Earth masses.[11] Later observations revealed that the minimum mass is now 3.1 Earth masses.[12]

The HD 156668 planetary system[12]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b ≥3.1 ± 0.4 M⊕ 0.0211 ± 0.0002 1.26984 ± 0.00007 0.000 — —

References
  1. van Leeuwen, F. (November 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. Howard, Andrew W.; et al. (January 2011), "The NASA-UC Eta-Earth Program. II. A Planet Orbiting HD 156668 with a Minimum Mass of Four Earth Masses", The Astrophysical Journal, 726 (2): 73, arXiv:1003.3444, Bibcode:2011ApJ...726...73H, doi:10.1088/0004-637X/726/2/73, S2CID 15559379
  3. "HD 156668". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2010-02-09.
  4. Sherrod, P. Clay; Koed, Thomas L. (2003), A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations, Astronomy Series, Courier Dover Publications, p. 9, ISBN 0-486-42820-6
  5. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on March 10, 2012, retrieved 2012-01-16
  6. The Henry Draper Catalogue, Annie J. Cannon and Edward C. Pickering, Annals of Harvard College Observatory;
    hours 0 to 3, 91 (1918), Bibcode:1918AnHar..91....1C;
    hours 4 to 6, 92 (1918), Bibcode:1918AnHar..92....1C;
    hours 7 to 8, 93 (1919), Bibcode:1919AnHar..93....1C;
    hours 9 to 11, 94 (1919), Bibcode:1919AnHar..94....1C;
    hours 12 to 14, 95 (1920), Bibcode:1920AnHar..95....1C;
    hours 15 to 16, 96 (1921), Bibcode:1921AnHar..96....1C;
    hours 17 to 18, 97 (1922), Bibcode:1922AnHar..97....1C;
    hours 19 to 20, 98 (1923), Bibcode:1923AnHar..98....1C;
    hours 21 to 23, 99 (1924), Bibcode:1924AnHar..99....1C.
  7. "ARICNS ARI Data Base for Nearby Stars". ASTRONOMISCHES RECHEN-INSTITUT HEIDELBERG. Retrieved 2010-02-09.
  8. Luyten, Willem Jacob (1957). A catalogue of 9867 stars in the southern hemisphere with proper motions exceeding 0."2 annually. Harvard.edu. Bibcode:1957cnes.book.....L.
  9. "NASA reference publication -- 1297". Bonner Durchmusterung. Missing or empty |url= (help)
  10. "HD 156668". Sky Map.org. Retrieved 2010-02-09.
  11. "Second smallest exoplanet found to date discovered at Keck". W.M. Keck Observatory. Retrieved 2010-02-09.
  12. Rebekah; et al. (2010). "Radial velocity planets de-aliased. A new, short period for Super-Earth 55 Cnc e". The Astrophysical Journal. 722 (1): 937–953. arXiv:1005.4050. Bibcode:2010ApJ...722..937D. doi:10.1088/0004-637X/722/1/937. S2CID 118592734.
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