08h40m18s, −52° 54′60″| IC 2391 | |
|---|---|
 | |
| Observation data (J2000.0 epoch) | |
| Right ascension | 08h 40.6m |
| Declination | −53° 02′ |
| Distance | 574 ly (176 pc[1]) |
| Apparent magnitude (V) | 2.5 |
| Apparent dimensions (V) | 50′ |
| Physical characteristics | |
| Other designations | Caldwell 85, Cr 191, C 0838-528, Lac II 5 |
| Associations | |
| Constellation | Vela |
| See also: Open cluster, List of open clusters | |
IC 2391 (also known as the Omicron Velorum ClusterorCaldwell 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 c. 964.[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.
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[7] |
| HD 74560 (HY Velorum) sp. binary | 4.815[8] | B3IV[6] | 490.4 ± 6.9[9] |
| HD 74146 (NZ Velorum) sp. binary | 5.19[10] | B5IV[6] | 497.3 ± 7.5[11] |
| HD 74071 (HW Velorum) | 5.44[8] | B5V[12] | 519.9 ± 7.5[13] |
| HD 74196 cepheid variable | 5.61[10] | B9/A0[6] | 502 ± 4.1[14] |
| HD 74535 (KT Velorum) alpha2 CVn variable | 5.47[8] | B9III(pSi)[6] | 421 ± 17.5[15] & radial velocity +14[16] so not a true member |
| HD 75466 | 6.27[8] | B8V[6] | 479 ± 2.8[17] |
| HD 73952 | 6.43[8] | B8V[6] | 483 ± 5.8[18] |
| HD 74438, youngest known quadruple star.[19][20][21] | 7.58[8] | A2mA5-A8[6] | 472 ± 5.2[22] |
The stars' era of formation is similar to open cluster IC 2602 in neighbouring Carina,[23] and has a lithium depletion boundary age of about 50 million years.[24] The latter group averages about the same distance, placed at about 485 light years away.[25]
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.[26] The supposed association may chiefly comprise:[26]
| Name | Constellation | Distance | Radial velocity |
|---|---|---|---|
| Epsilon Pavonis | Pavo | 105 | −6.7 |
| HD 88955 | Vela | 100 | +7.4 |
| HD 61005 | Puppis | 119 | +22 |
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