HX Velorum

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^[7]

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

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°.