S Monocerotis, also known as 15 Monocerotis, is a massive multiple and variable star system located in the constellation Monoceros. It is the brightest star in the Christmas Tree open cluster in the area catalogued as NGC 2264.
Multiple, variable star in the constellation Monoceros
S Monocerotis
S Monocereotis lies in NGC 2264 at the very northern edge of Monoceros.
NGC 2264 including the Cone Nebula and the Christmas Tree Cluster (upside down in this image) with S Monocerotis at the very top of the image (and the base of the Christmas tree)
S Monocerotis is found within an open cluster and the Washington Double Star Catalog lists many companion stars.[18] The closest and brightest is S Mon B, magnitude 7.8 and 3 arcseconds away. It is classified as B2 main sequence star with a mass of 7.31M☉. Designated component C is an 11th-magnitude B8V star.[6] The cluster contains another dozen or so 9th and 10th magnitude stars and many fainter stars.
S Monocerotis A is a spectroscopic binary system with an eccentric orbit of about 112 years.[13] Since 1943, the spectrum of this star has served as the MK standard for O7 by which other stars are classified.[19] It is also an irregular variable star with a range of less than a tenth of a magnitude. The orbital parameters can be used to derive the masses of the two stars, giving 31M☉ and 11M☉.[6]
The distance to S Monocerotis and NGC 2264 has been derived in various ways, including dynamical parallax and isochrone fitting. These consistently give estimates of 700 - 900 parsecs, although this is double the likely distance derived from the Hipparcos parallax measurements.[9] Gaia Early Data Release 3 contains parallaxes for the companions components B and C of 1.4mas and 1.5mas respectively, consistent with the expected distance to the cluster.
References
Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862. ISBN0333750888.
Samus, N. N.; Durlevich, O. V.; etal. (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. Bibcode:2009yCat....102025S.
Sota, A.; Maíz Apellániz, J.; Walborn, N. R.; Alfaro, E. J.; Barbá, R. H.; Morrell, N. I.; Gamen, R. C.; Arias, J. I. (2011). "The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-violet at R ~ 2500". The Astrophysical Journal Supplement. 193 (2): 24. arXiv:1101.4002. Bibcode:2011ApJS..193...24S. doi:10.1088/0067-0049/193/2/24. S2CID119248206.
Oja, T. (August 1985). "Photoelectric photometry of stars near the north Galactic pole. II". Astronomy and Astrophysics Supplement Series. 61: 331–339. Bibcode:1985A&AS...61..331O.
Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; Levato, H.; Morrell, N. I.; Torres, G.; Udry, S. (2004). "SB9: The ninth catalogue of spectroscopic binary orbits". Astronomy and Astrophysics. 424 (2): 727–732. arXiv:astro-ph/0406573. Bibcode:2004A&A...424..727P. doi:10.1051/0004-6361:20041213. S2CID119387088.
Feinstein, A.; Vazquez, R. A.; Benvenuto, O. G. (1986). "Of-stars in young open clusters". Astronomy and Astrophysics. 159: 223. Bibcode:1986A&A...159..223F.
Maíz Apellániz, J. (2019). "Gaia DR2 distances to Collinder 419 and NGC 2264 and new astrometric orbits for HD 193 322 Aa,Ab and 15 Mon Aa,Ab". Astronomy and Astrophysics. 630: A119. arXiv:1908.02040. Bibcode:2019A&A...630A.119M. doi:10.1051/0004-6361/201935885. S2CID247759810.
Herrero, A.; Kudritzki, R. P.; Vilchez, J. M.; Kunze, D.; Butler, K.; Haser, S. (1992). "Intrinsic parameters of galactic luminous OB stars". Astronomy and Astrophysics. 261: 209. Bibcode:1992A&A...261..209H.
Sung, Hwankyung; Stauffer, John R.; Bessell, Michael S. (2009). "A Spitzer View of the Young Open Cluster NGC 2264". The Astronomical Journal. 138 (4): 1116–1136. arXiv:0906.3072. Bibcode:2009AJ....138.1116S. doi:10.1088/0004-6256/138/4/1116. S2CID5272305.
Bernacca, P. L.; Perinotto, M. (1970). "A catalogue of stellar rotational velocities". Contributi Osservatorio Astronomico di Padova in Asiago. 239 (1): 1. Bibcode:1970CoAsi.239....1B.
Hohle, M. M.; Neuhäuser, R.; Schutz, B. F. (2010). "Masses and luminosities of O- and B-type stars and red supergiants". Astronomische Nachrichten. 331 (4): 349–360. arXiv:1003.2335. Bibcode:2010AN....331..349H. doi:10.1002/asna.200911355. S2CID111387483.
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