IC 2391 explained

IC 2391
Epoch:	J2000.0
Class:	open cluster
Dist Pc:	176 pc^[1]
Appmag V:	2.5
Size V:	50′
Names:	Caldwell 85, Cr 191, C 0838-528, Lac II 5

IC 2391 (also known as the Omicron Velorum Cluster or Caldwell 85) is an open cluster in the constellation Vela consisting of hot, young, blueish stars, some of which binaries and one of which is a quadruple. Persian astronomer A. a.-R. al-Sufi first described it as "a nebulous star" in .^{[2] [3]} It was re-found by Abbe Lacaille and cataloged as Lac II 5.

It is centred about 490 light-years away from Earth and can be seen with the naked eye. It contains about 30 stars with a total visual magnitude of 2.5, spread out along 50 arcminutes.

Visual and true members

It includes these members:^[4]

Name	Apparent magnitude (V)	Spectral type	Distance (LY)
HD 74195 (ο Velorum)	3.63 varies^[5]	B3/5(V)^[6]	496 ± 30.5
HD 74560 (HY Velorum) sp. binary	4.815^[7]	B3IV	490.4 ± 6.9
HD 74146 (NZ Velorum) sp. binary	5.19^[8]	B5IV	497.3 ± 7.5
HD 74071 (HW Velorum)	5.44	B5V^[9]	519.9 ± 7.5
HD 74196 cepheid variable	5.61	B9/A0	502 ± 4.1
HD 74535 (KT Velorum) alpha2 CVn variable	5.47	B9III(pSi)	421 ± 17.5 & radial velocity +14^[10] so not a true member
	6.27	B8V	479 ± 2.8
	6.43	B8V	483 ± 5.8
HD 74438, youngest known quadruple star.^[11] ^[12] ^[13]	7.58	A2mA5-A8	472 ± 5.2

The stars' era of formation is similar to open cluster IC 2602 in neighbouring Carina,^[14] and has a lithium depletion boundaryage of about 50 million years.^[15] The latter group averages about the same distance, placed at about 485 light years away.

Argus Association

The components formed at about the same time as a nearer group, known as the "Argus Association" which one motion model suggests began in their own nebula cloud. These are in a similar direction, roughly the Vela constellation, within the local galactic arm. The supposed association may chiefly comprise:

Name	Constellation	Distance	Radial velocity
Epsilon Pavonis	Pavo	105	−6.7
HD 88955	Vela	100	+7.4
HD 61005	Puppis	119	+22

See also

• Omicron Velorum

External links

• Web site: Omicron Velorum (IC 2391). SEDS Messier Database. Students for the Exploration and Development of Space. Frommert, Harmut. 29 March 1998.
  • IC 2391.
• Web site: Open Cluster. An Atlas of The Universe.

Notes and References

1. Web site: IC 2391. 2020-06-11. sim-id.
2. Deep-sky Observer's Handbook: Open and globular clusters, Kenneth Glyn Jones, Webb Society (1979), at p3
3. The History of Astronomy, 2017, Anne Rooney, at p162
4. 2007A&A...461..509P. astro-ph/0611584. WIYN open cluster study. XXVI. Improved kinematic membership and spectroscopy of IC 2391. Astronomy and Astrophysics. 461. 2. 509. Platais. I. Melo. C. Mermilliod. J. -C. Kozhurina-Platais. V. Fulbright. J. P. Méndez. R. A. Altmann. M. Sperauskas. J. 17173564. 2007. 10.1051/0004-6361:20065756.
5. B.C. Reed, 'Catalog of galactic OB stars', Astronomy Journal, 125, 2531-2533 (2003/May-0)
6. N. Houk, 'Catalogue of two-dimensional spectral types for the HD stars, Vol. 2', Michigan Spectral Survey##, Ann Arbor, Dep. Astron., University of Michigan, 2, 0 (1978)
7. Hog E., Fabricius, C., Makarov, V., Urban S., Corbin T., Wycoff, G, Bastian, U, Schwekendiek, P., Wicenec, A., 'The Tycho-2 catalogue of the 2.5 million brightest stars' Astronomy and Astrophysics, volume 355, L27-30 (2000/3-2)
8. Ducati, J.R., 'Catalogue of Stellar Photometry in Johnson's 11-color system', CDS/ADC Collection of Electronic Catalogues, 2237, 0 (2002)
9. Houk, N., Cowley A.P., 'Catalogue of two dimentional spectral types for the HD stars, Vol. 1', Michigan Spectral Survey, Ann Arbor, Dep. Astron., University of Michigan, 1, 0 (1975)
10. Randich, S. & others, 'The Gaia-ESO Survey: open clusters in Gaia-DR1. A way forward to stellar age calibration.', Astronomy and Astrophysics, volume 612A, 99-99 (2018/4-1)
11. Merle . T. . Zaggia . S. . The Gaia -ESO Survey: double-, triple-, and quadruple-line spectroscopic binary candidates . Astronomy & Astrophysics . December 2017 . 608 . A95 . 10.1051/0004-6361/201730442. 1707.01720 . 2017A&A...608A..95M . 119502806 . free .
12. Platais . I. . Melo . C. . Mermilliod . J.-C. . Kozhurina-Platais . V. . Fulbright . J. P. . Méndez . R. A. . Altmann . M. . Sperauskas . J. . WIYN open cluster study: XXVI. Improved kinematic membership and spectroscopy of IC 2391 . Astronomy & Astrophysics . January 2007 . 461 . 2 . 509–522 . 10.1051/0004-6361:20065756 . astro-ph/0611584 . 2007A&A...461..509P. 17173564 . free .
13. Web site: Plait . Phil . A weird four-star system may lead the way to catastrophic supernovae . SYFY Official Site . 4 June 2022 . https://web.archive.org/web/20220527005009/https://www.syfy.com/syfy-wire/bad-astronomy-quaternary-star-hd-74438-stars-explode . 27 May 2022 . 26 May 2022.
14. Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters IC 2391 and IC 2602. Stauffer, J.R.. etal. Astrophysical Journal. 479. 2. 776–791. 1997ApJ...479..776S. 10.1086/303930. 1997. free.
15. Navascues. David Barrado y. Stauffer. John R.. Jayawardhana. Ray. 2004-10-10. Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC 2391: Lithium Depletion and Hα Emission. The Astrophysical Journal. en. 614. 1. 386–397. 10.1086/423485. 0004-637X. 2004ApJ...614..386B. astro-ph/0406436. 208867491 .