SY Equulei

SY Equulei, also known as HD 203664, is a single variable star located in the equatorial constellation Equuleus. It has an average apparent magnitude of about 8.5, varying by a few hundredths of a magnitude, making it readily visible in binoculars and small telescopes, but not to the naked eye. The star is relatively far away at a distance of 8,000 light years[2] and is receding with a heliocentric radial velocity of 48 km/s.[7] At that distance, SY Equulei is dimmed by 0.19 magnitudes due to interstellar dust.[16]

SY Equulei

A light curve for SY Equulei, plotted from Hipparcos data[1]
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Equuleus
Right ascension 21h 23m 28.8086s[2]
Declination +09° 55′ 54.9204″[2]
Apparent magnitude (V) 8.52 - 8.58[3]
Characteristics
Spectral type B0.5 IIIn[4]
U−B color index −1.00[5]
B−V color index −0.20[5]
Variable type β Cephei[6]
Astrometry
Radial velocity (Rv)48±5.5[7] km/s
Proper motion (μ) RA: −0.185 mas/yr[2]
Dec.: −9.049 mas/yr[2]
Parallax (π)0.3869 ± 0.0558 mas[2]
Distanceapprox. 8,000 ly
(approx. 2,600 pc)
Absolute bolometric
magnitude (Mbol)		−6.04[8]
Details
Mass14[9] M☉
Radius10.44[10] R☉
Luminosity2,490[11] L☉
Surface gravity (log g)3.8±0.2[12] cgs
Temperature28,184+1,328
−1,268[12] K
Metallicity [Fe/H]0.00[13] dex
Rotational velocity (v sin i)180[14] km/s
Age~7.5[12] Myr
Other designations
SY Equueli, AG+09°2978, BD+09°4793, HD 203664, HIP 105614, SAO 126757[15]
Database references
SIMBADdata

Throughout the late 20th century, HD 203664 was known to have a dust cloud surrounding it. Subsequent observations from Kenneth R. Sembach (1995) reveal it to contain high abundances of calcium as well as traces of magnesium, aluminum, and silicon.[17] The cloud probably came from outside the galactic plane and is moving towards the star at a rate of 70 km/s.[17] The star has a high galactic latitude, indicating its location in the galactic halo. HD 203664 was most likely ejected from its birthplace to its current distance.[9] However, its status as a Beta Cephei variable wasn't discovered until a survey of 2000 using Hipparcos data.[8] It was then given the designation SY Equulei.[18]

SY Equulei has a stellar classification of B0.5 IIIn,[4] indicating an evolved B-type star with nebulous (broad) absorption lines due to rapid rotation. Unlike most stars of its type, it spins rapidly with a projected rotational velocity of 180 km/s,[14] which is 40% of its break-up velocity.[12] It has 14 times the mass of the Sun[9] and a radius of 10.4 radius.[10] It radiates at 2,490 times the luminosity of the Sun[11] from its photosphere at an effective temperature of 28,184 K,[12] giving a whitish blue hue.

SY Equueli is a variable star with an amplitude of 0.07 magnitudes[6] and an average period of 3.98 hours.[19] In later observations, SY Equulei was found to have multiple periods.[12]

References
  1. "Light Curve". Hipparcos ESA. ESA. Retrieved 17 February 2022.
  2. Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
  3. "SY Equ". International Variable Star Index. AAVSO. Retrieved 2022-07-31.
  4. Walborn, Nolan R. (August 1971). "Some Spectroscopic Characteristics of the OB Stars: an Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification". The Astrophysical Journal Supplement Series. 23: 257. Bibcode:1971ApJS...23..257W. doi:10.1086/190239. eISSN 1538-4365. ISSN 0067-0049.
  5. Guetter, H. H. (October 1974). "UBV photometry of 180 early-type stars". Publications of the Astronomical Society of the Pacific. 86: 795. Bibcode:1974PASP...86..795G. doi:10.1086/129675. eISSN 1538-3873. ISSN 0004-6280.
  6. Samus’, N. N.; Kazarovets, E. V.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. (January 2017). "General catalogue of variable stars: Version GCVS 5.1". Astronomy Reports. 61 (1): 80–88. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. eISSN 1562-6881. ISSN 1063-7729. S2CID 125853869.
  7. Kharchenko, N.V.; Scholz, R.-D.; Piskunov, A.E.; Röser, S.; Schilbach, E. (November 2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ~55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten. 328 (9): 889–896. arXiv:0705.0878. Bibcode:2007AN....328..889K. doi:10.1002/asna.200710776. ISSN 0004-6337.
  8. Aerts, C. (September 2000). "Follow-up photometry of six new beta Cephei stars discovered from the HIPPARCOS mission". Astronomy and Astrophysics. 361: 245–257. Bibcode:2000A&A...361..245A. ISSN 0004-6361.
  9. Keenan, F. P.; Dufton, P. L.; McKeith, C. D. (1 October 1982). "Atmospheric parameters and chemical compositions of eighteen halo OB stars". Monthly Notices of the Royal Astronomical Society. 200 (3): 673–685. Bibcode:1982MNRAS.200..673K. doi:10.1093/mnras/200.3.673. eISSN 1365-2966. ISSN 0035-8711.
  10. Kervella, Pierre; Arenou, Frédéric; Thévenin, Frédéric (2022). "Stellar and substellar companions from Gaia EDR3". Astronomy & Astrophysics. 657: A7. arXiv:2109.10912. Bibcode:2022A&A...657A...7K. doi:10.1051/0004-6361/202142146. eISSN 1432-0746. ISSN 0004-6361.
  11. McDonald, I.; Zijlstra, A. A.; Boyer, M. L. (21 November 2012). "Fundamental parameters and infrared excesses of Hipparcos stars: Parameters and IR excesses from Hipparcos". Monthly Notices of the Royal Astronomical Society. 427 (1): 343–357. arXiv:1208.2037. Bibcode:2012MNRAS.427..343M. doi:10.1111/j.1365-2966.2012.21873.x. ISSN 0035-8711.
  12. Aerts, C.; De Cat, P.; De Ridder, J.; Van Winckel, H.; Raskin, G.; Davignon, G.; Uytterhoeven, K. (16 March 2006). "Multiperiodicity in the large-amplitude rapidly-rotating β Cephei star HD 203664". Astronomy & Astrophysics. 449 (1): 305–311. arXiv:astro-ph/0511306. Bibcode:2006A&A...449..305A. doi:10.1051/0004-6361:20054142. eISSN 1432-0746. ISSN 0004-6361.
  13. Anders, F.; et al. (February 2022). "Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia EDR3 stars brighter than G = 18.5". Astronomy & Astrophysics. 658: A91. arXiv:2111.01860. Bibcode:2022A&A...658A..91A. doi:10.1051/0004-6361/202142369. eISSN 1432-0746. ISSN 0004-6361.
  14. Walker, G. A. H.; Hodge, S. M. (1965). "Equivalent widths and halfwidths of the λ4388 and λ4471 He I lines, rotational velocities and λ4430 central depths for 450O to B5 stars". Publications of the Dominion Astrophysical Observatory Victoria. 12: 401. Bibcode:1965PDAO...12..401W. ISSN 0078-6950.
  15. "SY Equulei". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved July 29, 2022.
  16. Neckel, T.; Klare, G.; Sarcander, M. (July 1980). "Catalogue of Extinction Data of 12547 O-Stars to F-Stars Galactic Clusters and Delta-Cephei Stars". Bulletin d'Information du Centre de Donnees Stellaires. 19: 61. Bibcode:1980BICDS..19...61N. ISSN 1169-8837.
  17. Sembach, Kenneth R. (May 1995). "Properties of the +70 kilometers per second cloud toward HD 203664". The Astrophysical Journal. 445: 314. Bibcode:1995ApJ...445..314S. doi:10.1086/175695. eISSN 1538-4357. ISSN 0004-637X.
  18. Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V.; Frolov, M. S.; Antipin, S. V.; Kireeva, N. N.; Pastukhova, E. N. (January 1999). "The 74th Special Name-list of Variable Stars". Information Bulletin on Variable Stars. 4659: 1. Bibcode:1999IBVS.4659....1K. ISSN 0374-0676.
  19. Watson, C. L.; Henden, A. A.; Price, A. (May 2006). "The International Variable Star Index (VSX)". Society for Astronomical Sciences Annual Symposium. 25: 47. Bibcode:2006SASS...25...47W.
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