HD 187474

HD 187474, also known as HR 7552 and V3961 Sagittarii, is a star about 315 light years from the Earth, in the constellation Sagittarius.[3] It is a 5th magnitude star, so it will be faintly visible to the naked eye of an observer far from city lights. It is a variable star, whose brightness varies slightly from magnitude 5.28 to 5.34.[4] HD 187474 is classified as an Alpha2 Canum Venaticorum variable star, but it has a rotation period of 2345 days - more than an order of magnitude longer than is typical for that class.[6][9] HD 187474 is an Ap star.[10]

HD 187474

Visual band light curves for V3961 Sagittarii. The red points show the Hipparcos data,[1] and the blue points show data from Mikulášek et al.[2]
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittatius
Right ascension 19h 51m 50.61067s[3]
Declination −39° 52′ 27.7383″[3]
Apparent magnitude (V) 5.28 – 5.34[4]
Characteristics
Spectral type A0 EuCrSr[5]
Variable type α2 CVn[4]
Astrometry
Proper motion (μ) RA: 17.944±0.460[3] mas/yr
Dec.: −11.878±0.278[3] mas/yr
Parallax (π)10.3412 ± 0.3119 mas[3]
Distance315 ± 10 ly
(97 ± 3 pc)
Absolute bolometric
magnitude (Mbol)		+0.1[6]
Orbit[7]
Period (P)690 d
Eccentricity (e)0.45
Details
Mass2.7[3] M☉
Radius3.7[3] R☉
Luminosity75[3] L☉
Surface gravity (log g)3.80[3] cgs
Temperature10,350[6] K
Rotational velocity (v sin i)5[7] km/s
Age318[3] Myr
Other designations
V3961 Sgr, HR 7552, HIP 97749, SAO 229903[8]
Database references
SIMBADdata

In 1958, Horace Babcock announced that HD 187474 has a magnetic field, the strength of which he estimated to be 1867 gauss. He found the star to be remarkable, because it was the only A-type star he had found that seemed to have a magnetic field which did not vary in time.[11] However this perceived consistency turned out to be the result of Babcock's observations covering only a small portion of the star's unexpectedly long variability period.[12]

HD 187474 is a single-lined spectroscopic binary. This was discovered by Sylvia Burd at Palomar Observatory, and the result was communicated to S. Leeman by Babcock. Leeman published the finding in 1964, along with orbital elements derived by Burd. The initial estimate of the orbital period was 700 days.[12]

The variability of HD 187474 was apparently discovered by Babcock sometime prior to 1976, the year when it was given the variable star designation V3961 Sagittarii,[13] but the result was never published by him.[14][15]

Several groups have tried to determine the strength and geometry of HD 187474's magnetic field. In 1987 Pierre Didelon derived a surface field strength of 5.0±0.4 kilogauss, from observations of Zeeman splitting of spectral lines.[16] In the year 2000, John Landstreet and Gautier Mathys found that the variation of the measured magnetic field as the star rotated was far from sinusoidal. They obtained an acceptable fit to the data with a model of the field which contained colinear dipole, quadrupole and octupole terms.[17] Two years later, Stefano Bagnulo et al. modeled the field as a dipole inclined 80° with respect to the rotation axis plus a nonlinear quadrupole term.[18] The next year, V. R. Khalack et al. modeled the field as a set of virtual magnetic charges, with the constraint that the total magnetic charge must be zero.[5] In 2005, Yu. V. Glagolevskij modeled the field as a dipole displaced from the star's center, and inclined relative to the rotation axis by 24°.[6]

References
  1. EAS (1997). "The HIPPARCOS and TYCHO catalogues". Astrometric and Photometric Star Catalogues Derived from the ESA Hipparcos Space Astrometry Mission. ESA SP Series. Noordwijk, Netherlands: ESA Publications Division. 1200. Bibcode:1997ESASP1200.....E. ISBN 9290923997. Retrieved 15 October 2022.
  2. Mikulášek, Z.; Janík, J.; Zverko, J.; Žižňovský, J.; Zejda, M.; Netolicky, M.; Vaňko, M. (January 2007). "On-line database of photometric observations of magnetic chemically peculiar stars". Astronomische Nachrichten. 328 (1): 10–15. Bibcode:2007AN....328...10M. doi:10.1002/asna.200610705. Retrieved 18 March 2023.
  3. 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.
  4. Samus', N. N.; Kazarovets, E. V.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. (2017). "General catalogue of variable stars: Version GCVS 5.1". Astronomy Reports. 61 (1): 80. Bibcode:2017ARep...61...80S. doi:10.1134/S1063772917010085. S2CID 125853869.
  5. Khalack, V. R.; Zverko, J.; Žižňovský, J. (May 2003). "Structure of the magnetic field in the Ap star HD 187474" (PDF). Astronomy and Astrophysics. 403: 179–185. Bibcode:2003A&A...403..179K. doi:10.1051/0004-6361:20030322. S2CID 122211894. Retrieved 18 March 2023.
  6. Glagolevskij, Yu. V. (October 2005). "Model of the Magnetic Field of HD 187474". Astrophysics. 48 (4): 483–490. Bibcode:2005Ap.....48..483G. doi:10.1007/s10511-005-0047-y. S2CID 120770653. Retrieved 18 March 2023.
  7. Budsj, Ján (December 1999). "Do the physical properties of Ap binaries depend on their orbital elements?". Monthly Notices of the Royal Astronomical Society. 310 (2): 419–427. Bibcode:1999MNRAS.310..419B. doi:10.1046/j.1365-8711.1999.02985.x. S2CID 53701368. Retrieved 18 March 2023.
  8. "V* V3961 Sgr -- alpha2 CVn Variable". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2023-03-16.
  9. Variability types, General Catalogue of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line March 16, 2023.
  10. Bidelman, William P.; Böhm, Karl-Heinz (June 1955). "Spectral Classification of Some Peculiar A Stars". Publications of the Astronomical Society of the Pacific. 67 (396): 179–180. Bibcode:1955PASP...67..179B. doi:10.1086/126797. S2CID 120459197. Retrieved 18 March 2023.
  11. Babcock, Horace W. (September 1958). "Magnetic Fields of the A-Type Stars". Astrophysical Journal. 128: 228–260. Bibcode:1958ApJ...128..228B. doi:10.1086/146539. Retrieved 18 March 2023.
  12. Leeman, S. (1964). "An Analysis of the Magnetic Star HD 187474". Monthly Notes of the Astronomical Society of South Africa. 23: 6. Bibcode:1964MNSSA..23....6L. Retrieved 18 March 2023.
  13. Kukarkin, B. V.; Kholopov, P. N.; Fedorovich, V. P.; Kireyeva, N. N.; Kukarkina, N. P.; Medvedeva, G. I.; Perova, N. B. (March 1977). "62nd Name-List of Variable Stars" (PDF). Information Bulletin on Variable Stars. 1248: 1. Bibcode:1977IBVS.1248....1K. Retrieved 18 March 2023.
  14. Hensberge, H.; Manfroid, J.; Schneider, H.; Maitzen, H. M.; Catalano, F. A.; Renson, P.; Weiss, W. W.; Floquet, M. (March 1984). "The frequency of Ap-stars with long rotation periods". Astronomy and Astrophysics. 132: 291–295. Bibcode:1984A&A...132..291H. Retrieved 18 March 2023.
  15. Wolff, S. C. (November 1975). "The distribution of periods of the magnetic A-type stars". Astrophysical Journal. 202: 127–136. Bibcode:1975ApJ...202..127W. doi:10.1086/153958. Retrieved 18 March 2023.
  16. Didelon, P. (September 1987). "HD 187474 - The first results of surface magnetic field measurements". The Messenger. 49: 5–10. Bibcode:1987Msngr..49....5D. Retrieved 18 March 2023.
  17. Landstreet, J. D.; Mathys, G. (July 2000). "Magnetic models of slowly rotating magnetic Ap stars: aligned magnetic and rotation axes". Astronomy and Astrophysics. 359: 213–226. Bibcode:2000A&A...359..213L. Retrieved 18 March 2023.
  18. Bagnulo, S.; Landi Degl'Innocenti, M.; Landolfi, M.; Mathys, G. (November 2002). "A statistical analysis of the magnetic structure of CP stars" (PDF). Astronomy and Astrophysics. 394 (3): 1023–1037. Bibcode:2002A&A...394.1023B. doi:10.1051/0004-6361:20021221. Retrieved 18 March 2023.
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