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  • 21 Aug, 2019

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HD 176693
HD 176693, also known as Kepler-408, is a star with a close orbiting exoplanet in the northern constellation of Draco. It is located at a distance of 291 light years from the Sun based on parallax measurements, but it is drifting closer with a radial velocity of −55 km/s. The star is predicted to come as close as 23.1 light-years in 1.6 million years. It has an apparent visual magnitude of 8.83, which is too faint to be viewed with the naked eye.

The spectrum of HD 176693 matches an F-type main-sequence star with a stellar classification of F8V. The star is older than the Sun, at 7.15 billion years. It is slightly and uniformly depleted in heavy elements compared to the Sun, having about 75% of the solar abundance of iron and other heavy elements. HD 176693 is a chromospherically inactive star, although there is weak evidence for tidal spin-up due to star-planet interaction.

HD 176693 is 5% more massive than the Sun and has a 25% larger radius. It is radiating 1.9 times the luminosity of the Sun from its photosphere at an effective temperature of 6,080 K. The star is spinning with a rotation period of 12.89 days. As of 2016, multiplicity surveys have not detect any stellar companions to HD 176693.

Planetary system

In 2014, a transiting Sub-Earth planet b was detected on a tight 2.5 day orbit. Initially reported with a relatively low confidence of 97.9%, it was confirmed in 2016.

The planetary orbit is inclined to the equatorial plane of the star by 41.7
−3.5°. Such strong spin-orbit misalignment is unique for a sub-Earth transiting planet, and needs either additional giant planets in the system or a history of close stellar encounters to explain it. The planet may also be a captured body originating from elsewhere.

The Kepler-408 planetary system
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b ≥ 0.02 MJ — 2.465024±0.000005 — 81.85±0.10° 0.86±0.04 R🜨

References

  1. ^ Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ Høg, E.; et al. (March 2000), "The Tycho-2 catalogue of the 2.5 million brightest stars", Astronomy and Astrophysics, 355: L27–L30, Bibcode:2000A&A...355L..27H, doi:10.1888/0333750888/2862.
  3. ^ Molenda-Żakowicz, J.; et al. (2013), "Atmospheric parameters of 169 F-, G-, K- and M-type stars in the Kepler field", Monthly Notices of the Royal Astronomical Society, 434 (2): 1422, arXiv:1306.6011, Bibcode:2013MNRAS.434.1422M, doi:10.1093/mnras/stt1095, S2CID 59269553.
  4. ^ Booth, R. S.; et al. (2020), "Chromospheric emission of solar-type stars with asteroseismic ages", Monthly Notices of the Royal Astronomical Society, 491 (1): 455–467, arXiv:1910.12557, Bibcode:2020MNRAS.491..455B, doi:10.1093/mnras/stz3039.
  5. ^ Gaia Collaboration (May 2022), "Gaia DR3 Part 1. Main source", VizieR Online Data Catalog, Bibcode:2022yCat.1355....0G, doi:10.26093/cds/vizier.1355.
  6. ^ Kamiaka, Shoya; et al. (2019), "The Misaligned Orbit of the Earth-sized Planet Kepler-408b", The Astronomical Journal, 157 (4): 137, arXiv:1902.02057, Bibcode:2019AJ....157..137K, doi:10.3847/1538-3881/ab04a9, S2CID 118909208.
  7. ^ Brito, Ana; Lopes, Ilídio (2019), "The partial ionization zone of heavy elements in F-stars: A study on how it correlates with rotation", Monthly Notices of the Royal Astronomical Society, 488 (2): 1558–1571, arXiv:1906.12308, Bibcode:2019MNRAS.488.1558B, doi:10.1093/mnras/stz1804.
  8. ^ "Kepler-408", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 1 July 2021
  9. ^ Bailer-Jones, C.A.L.; et al. (2018), "New stellar encounters discovered in the second Gaia data release", Astronomy & Astrophysics, 616: A37, arXiv:1805.07581, Bibcode:2018A&A...616A..37B, doi:10.1051/0004-6361/201833456, S2CID 56269929.
  10. ^ Ramírez, I.; et al. (2020), "Detailed chemical compositions of planet-hosting stars – I. Exploration of possible planet signatures", Monthly Notices of the Royal Astronomical Society, 495 (4): 3961–3973, arXiv:2005.09846, Bibcode:2020MNRAS.495.3961L, doi:10.1093/mnras/staa1420.
  11. ^ Metcalfe, Travis S.; Egeland, Ricky (2019), "Understanding the Limitations of Gyrochronology for Old Field Stars", The Astrophysical Journal, 871 (1): 39, arXiv:1811.11905, Bibcode:2019ApJ...871...39M, doi:10.3847/1538-4357/aaf575, S2CID 119405127.
  12. ^ Kraus, Adam L.; et al. (2016), "The Impact of Stellar Multiplicity on Planetary Systems. I. The Ruinous Influence of Close Binary Companions", The Astronomical Journal, 152 (1): 8, arXiv:1604.05744, Bibcode:2016AJ....152....8K, doi:10.3847/0004-6256/152/1/8, S2CID 119110229.
  13. ^ Marcy, Geoffrey W.; et al. (2014), "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets", The Astrophysical Journal Supplement Series, 210 (2): 20, arXiv:1401.4195, Bibcode:2014ApJS..210...20M, doi:10.1088/0067-0049/210/2/20, S2CID 10760418.
  14. ^ Campante, T. L.; et al. (2016), "Spin–Orbit Alignment of Exoplanet Systems: Ensemble Analysis Using Asteroseismology", The Astrophysical Journal, 819 (1): 85, arXiv:1601.06052, Bibcode:2016ApJ...819...85C, doi:10.3847/0004-637X/819/1/85, S2CID 56307453.
  15. ^ Petrovich, Cristobal; et al. (2020), "A Disk-driven Resonance as the Origin of High Inclinations of Close-in Planets", The Astrophysical Journal Letters, 902 (1): L5, arXiv:2008.08587, Bibcode:2020ApJ...902L...5P, doi:10.3847/2041-8213/abb952, S2CID 221186597.


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