Chi Ophiuchi

Chi Ophiuchi, Latinized from χ Ophiuchi, is a variable star in the equatorial constellation of Ophiuchus.[13] It has a blue-white hue and is faintly visible to the naked eye with an apparent visual magnitude that fluctuates around 4.22.[3] The distance to this object, as determined from parallax measurements, is approximately 500 light years,[2] but it is moving closer to the Sun with a radial velocity of −19 km/s.[8] This star is a proper motion member of the Upper Scorpius sub-group in the Scorpius–Centaurus OB association; the nearest such co-moving association of massive stars to the Sun.[14]

Chi Ophiuchi

A blue-light light curve for Chi Ophiuchi, adapted from Cuypers et al. (1989)[1]
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Ophiuchus
Right ascension 16h 27m 01.43557s[2]
Declination −18° 27′ 22.4500″[2]
Apparent magnitude (V) 4.22[3] (4.18 to 5.0)[4]
Characteristics
Evolutionary stage main sequence
Spectral type B2Vne[5]
U−B color index −0.75[6]
B−V color index +0.28[6]
R−I color index +0.22[6]
Variable type GCAS[4] + LERI[7]
Astrometry
Radial velocity (Rv)−19.0±2.1[8] km/s
Proper motion (μ) RA: −5.378[2] mas/yr
Dec.: −22.308[2] mas/yr
Parallax (π)6.5402 ± 0.1961 mas[2]
Distance500 ± 10 ly
(153 ± 5 pc)
Absolute magnitude (MV)−2.27[9]
Details
Mass10.1±0.7[10] M☉
Radius4.44±0.09[11] R☉
Luminosity55,847[12] L☉
Surface gravity (log g)3.53±0.04[11] cgs
Temperature30,000±300[11] K
Rotational velocity (v sin i)150±3[11] km/s
Age22.5±2.6[10] Myr
Other designations
χ Oph, 7 Ophiuchi, BD−18°4282, FK5 3298, GC 22117, HD 148184, HIP 80569, HR 6118, SAO 159918, PPM 231703[13]
Database references
SIMBADdata

This is a massive Be star with a stellar classification of B2Vne,[5] where the 'n' indicates "nebulous" lines due to rapid rotation. It is spinning with a projected rotational velocity of 144 km/s. As the critical velocity for the star is 477 km/s, the inclination angle of its poles must be small; estimated as ~20°.[15] It is a Gamma Cassiopeiae variable with an amplitude of 0.15 magnitude.[16] The brightness has been measured varying from magnitude 4.18 down to 5.0.[4]

Chi Ophiuchi is 22.5 million years old with 10.1 times the mass of the Sun.[10] It has four times the Sun's radius and is radiating 56,000 times the luminosity of the Sun from its photosphere at an effective temperature of about 30,000 K. A weak magnetic field has been detected in the chromosphere of this star.[17] It is being orbited by a symmetrical disk of ejected gas extending out to 0.52 AU (112 R☉),[15] and excess radio and infrared emission has been detected from this structure.[18]

Abt and Levy (1978) cataloged Chi Ophiuchi as a single-lined spectroscopic binary with a period of 138.8 days and an eccentricity of 0.44, although the orbital elements are considered marginal.[19] Eggleton and Tokovinin (2008) list it as a single star.[20]

References
  1. Cuypers, J.; Balona, L. A.; Marang, F. (December 1989). "Intensive photometry of southern Be variables. I. Winter objects". Astronomy and Astrophysics Suppl. Ser. 81: 151–186. Bibcode:1989A&AS...81..151C.
  2. 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.
  3. Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644.
  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. Note: catalogued as 'ksi Oph'.
  5. Houk, Nancy; Smith-Moore, M. (1978). Michigan catalogue of two-dimensional spectral types for the HD stars. Vol. 4. Ann Arbor: Dept. of Astronomy, University of Michigan. Bibcode:1988mcts.book.....H.
  6. Hoffleit, D.; Warren, Jr., W. H. "The Bright Star Catalogue" (5th Revised ed.). CDS. Retrieved 2019-10-24. HR 6118, database entry.
  7. Watson, C. L. (2006). "The International Variable Star Index (VSX)". The Society for Astronomical Sciences 25th Annual Symposium on Telescope Science. Held May 23–25. 25: 47. Bibcode:2006SASS...25...47W.
  8. Gontcharov, G. A. (November 2006). "Pulkovo Compilation of Radial Velocities for 35495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
  9. Melnik, A. M.; Dambis, A. K. (2020). "Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions". Astrophysics and Space Science. 365 (7): 112. arXiv:2006.14649. Bibcode:2020Ap&SS.365..112M. doi:10.1007/s10509-020-03827-0. S2CID 220128144.
  10. Tetzlaff, N.; et al. (January 2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190–200. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. S2CID 118629873.
  11. Arcos, C.; et al. (March 2018), "Stellar parameters and H α line profile variability of Be stars in the BeSOS survey", Monthly Notices of the Royal Astronomical Society, 474 (4): 5287–5299, arXiv:1711.08675, Bibcode:2018MNRAS.474.5287A, doi:10.1093/mnras/stx3075, S2CID 74872624.
  12. Zorec, J.; et al. (November 2016). "Critical study of the distribution of rotational velocities of Be stars. I. Deconvolution methods, effects due to gravity darkening, macroturbulence, and binarity". Astronomy & Astrophysics. 595: 26. Bibcode:2016A&A...595A.132Z. doi:10.1051/0004-6361/201628760.
  13. "chi Oph". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved January 12, 2011.
  14. de Geus, E. J.; et al. (June 1989). "Physical parameters of stars in the Scorpio-Centaurus OB association". Astronomy and Astrophysics. 216 (1–2): 44–61. Bibcode:1989A&A...216...44D.
  15. Tycner, C.; et al. (December 2008). "Constraining the Physical Parameters of the Circumstellar Disk of χ Ophiuchi". The Astrophysical Journal. 689 (1): 461–470. arXiv:0807.3971. Bibcode:2008ApJ...689..461T. doi:10.1086/592097. S2CID 16779318.
  16. Adelman, S. J.; et al. (December 2000). "On the Variability of O4-B5 Luminosity Class III-V Stars". Information Bulletin on Variable Stars. 5008: 1. Bibcode:2000IBVS.5008....1A.
  17. Hubrig, S.; et al. (December 2007). "Evidence for weak magnetic fields in early-type emission stars". Astronomische Nachrichten. 328 (10): 1133. arXiv:0711.2085. Bibcode:2007AN....328.1133H. doi:10.1002/asna.200710877. S2CID 15213781.
  18. Taylor, A. R.; et al. (October 1987). "Radio detection of the Be star psi Persei". Monthly Notices of the Royal Astronomical Society. 228 (4): 811–817. Bibcode:1987MNRAS.228..811T. doi:10.1093/mnras/228.4.811.
  19. Abt, H. A.; Levy, S. G. (February 1978). "Binaries among B2 - B5 IV, V absorption and emission stars". Astrophysical Journal Supplement Series. 36: 241–258. Bibcode:1978ApJS...36..241A. doi:10.1086/190498.
  20. Eggleton, P. P.; Tokovinin, A. A. (September 2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869–879. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. S2CID 14878976.
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