HX Velorum

HX Velorum

HX Velorum is the brightest star in this image of IC 2395.
Observation data Epoch J2000       Equinox J2000
Constellation	Vela
Right ascension	08h 42m 16.19252s [1]
Declination	−48° 05′ 56.7481″ [1]
Apparent magnitude  (V)	5.48 - 5.53 [2]
Characteristics
Spectral type	B1.5V [3]
U−B color index	−0.9 [3]
B−V color index	−0.17 [3]
Variable type	ELL [2]
Astrometry
Radial velocity (Rv)	27.1±0.7 [4] km/s
Proper motion (μ)	RA: −3.714±0.128 mas/yr [1] Dec.: 4.758±0.138 mas/yr [1]
Parallax (π)	0.9479±0.1121  mas [1]
Distance	2,990±150  ly (917±45  pc) [4]
Absolute magnitude  (MV)	−2.32 [5]
Orbit [4]
Period (P)	1.124480(1) days
Semi-major axis (a)	13.6 R☉ [a]
Eccentricity (e)	0.0
Inclination (i)	20.6±0.8°
Semi-amplitude (K1) (primary)	83.3±0.6 km/s
Semi-amplitude (K2) (secondary)	131.6±0.6 km/s
Details [4]
Aa
Mass	16.30±0.03  M☉
Radius	5.66±0.02  R☉
Luminosity	29,700±610  L☉
Surface gravity (log g)	4.08±0.03  cgs
Temperature	25,000±1,300  K
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {M}}/{\ce {H}}\right]\end{smallmatrix}}}$ = +0.083±0.025
Rotational velocity (v sin i)	130±3 km/s
Ab
Mass	10.32±0.04  M☉
Radius	4.84±0.02  R☉
Luminosity	6,170+590 −540  L☉
Surface gravity (log g)	4.14±0.04  cgs
Temperature	22,000±640  K
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {M}}/{\ce {H}}\right]\end{smallmatrix}}}$ = +0.083±0.025
Rotational velocity (v sin i)	93±6 km/s
Other designations
HD  74455, HR  3462, HIP  42712, SAO  220313 [6]
Database references
SIMBAD	data

HX Velorum, also known as HR 3462 and HD 74455, is a star in the constellation Vela. 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, with its brightness varying slightly from magnitude 5.48 to 5.53 over a period of 1.12 days. ^[2]

A light curve for HX Velorum, plotted from TESS data

In 1981, Robert Shobbrook announced that HR 3462 is a variable star based on observations made in 1976. He correctly classified it as an ellipsoidal variable, but the period he derived, 0.56205±0.00005 days, was half the actual orbital period because his data did not allow him to distinguish between primary and secondary minima in the light curve. ^[8] It was given the variable star designation HX Velorum in 1980. ^[9] In 1983, Christoffel Waelkens and Frédy Rufener published the correct period of variability, 1.124 days. ^[10]

HX Velorum is a triple star, consisting of a pair (components A, magnitude 5.5, and B, magnitude 8.28) separated by 0.5 arc seconds. Component A is itself a close binary pair (components Aa and Ab). ^{[11] [3]} The system's brightness variation is caused by the ellipsoidal Aa and Ab components orbiting each other. ^[3]

HX Velorum is only about 2 arc minutes from the center of IC 2395, so it appears to be within that cluster. However the Gaia DR3 dataset lists the parallax of HX Velorum as 0.9479±0.1121  mas , yielding a distance of 3,400+500
−300 light years, while the distance to IC 2395 has been estimated to be 4,560±200 ^[12] light years, so HX Velorum might be a foreground object rather than a true cluster member. Mark Blackford et al. concluded HX Velorum is a member of the cluster, but that conclusion was based in part on earlier, significantly different distance estimates for both the star and the cluster. ^[3]

Notes

1. from a sin i / sin(i), with a sin i = 4.8 R_☉ and i = 20.6°.

References

1. 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.
2. 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.
3. Blackford, M. G.; Erdem, A.; Sürgit, D.; Özkardeş, B.; Budding, E.; Butland, R.; Demircan, O. (July 2019). "Absolute parameters of young stars: HX Velorum". Monthly Notices of the Royal Astronomical Society. 487 (1): 161–167. Bibcode:2019MNRAS.487..161B. doi: 10.1093/mnras/stz1136 .
4. Cakirli, O.; Hoyman, B.; Ozdarcan, O.; Bilir, S. (2025-09-22). "Selected β Cephei pulsators in eclipsing binaries". arXiv: 2509.18277 [astro-ph.EP].
5. 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.
6. "V* HX Vel -- Double or Multiple Star". SIMBAD . Centre de données astronomiques de Strasbourg . Retrieved 2023-03-18.
7. "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 19 March 2023.
8. Shobbrook, R. R. (July 1981). "Short period variability of some early B stars". Monthly Notices of the Royal Astronomical Society. 196 (2): 129–134. Bibcode:1981MNRAS.196..129S. doi: 10.1093/mnras/196.2.129 .
9. Kholopov, P. N.; Samus, N. N.; Kukarkina, N. P.; Medvedeva, G. I.; Perova, N. B. (February 1981). "65th Name-List of Variable Stars" (PDF). Information Bulletin on Variable Stars. 1921: 1. Bibcode:1981IBVS.1921....1K . Retrieved 21 March 2023.
10. Waelkens, C.; Rufener, F. (May 1983). "An observational study of the influence of close companions on the pulsations of beta Cephei stars". Astronomy and Astrophysics. 121: 45–50. Bibcode:1983A&A...121...45W.
11. "The Washington Double Star Catalog". The Washington Double Star Catalog. Georgia State University. Retrieved 21 March 2023.
12. Jaehnig, Karl; Bird, Jonathan; Holley-Bockelmann, Kelly (December 2021). "Membership Lists for 431 Open Clusters in Gaia DR2 Using Extreme Deconvolution Gaussian Mixture Models". The Astrophysical Journal. 923 (1): 129. arXiv: 2108.02783 . Bibcode:2021ApJ...923..129J. doi: 10.3847/1538-4357/ac1d51 .