HD 256

HD 256 is a binary star^[7] system in the equatorial constellation of Cetus. It has a white hue and is dimly visible to the naked eye with an apparent visual magnitude of 6.20.^[2] Based upon parallax measurements, the system is located at a distance of approximately 474 light years from the Sun.^[1] It is drifting closer with a radial velocity of −10 km/s.^[6]

HD 256

Observation data Epoch J2000      Equinox J2000
Constellation	Cetus
Right ascension	00h 07m 18.26172s[1]
Declination	−17° 23′ 13.2424″[1]
Apparent magnitude (V)	6.20[2]
Characteristics
Spectral type	A3Vn sh[3]
Apparent magnitude (B)	6.33[4]
Apparent magnitude (J)	5.858[5]
Apparent magnitude (H)	5.83[5]
Apparent magnitude (K)	5.747[5]
B−V color index	0.133±0.005[2]
Astrometry
Radial velocity (Rv)	−10.2±0.9[6] km/s
Proper motion (μ)	RA: −7.217[1] mas/yr Dec.: 21.155[1] mas/yr
Parallax (π)	6.8882±0.1184 mas[1]
Distance	474 ± 8 ly (145 ± 2 pc)
Absolute magnitude (MV)	0.01[2]
Orbit[7]
Period (P)	747.6 days
Semi-major axis (a)	3.08 AU
Eccentricity (e)	0.23
Inclination (i)	93.34°
Details[7]
A
Mass	1.94±0.15 M☉
Luminosity	57.3±2.0 L☉
Surface gravity (log g)	3.8±0.1 cgs
Temperature	9,000±100 K
Rotational velocity (v sin i)	294±9 km/s
Age	530±50 Myr
B
Mass	1.62±0.13 M☉
Luminosity	13.7±0.5 L☉
Surface gravity (log g)	4.2±0.1 cgs
Temperature	8,250±100 K
Rotational velocity (v sin i)	200±20 km/s
Other designations
BD−18°6428, Gaia DR2 2414558084699665920, GC 103, HD 256, HIP 602, HR 10, SAO 147090, PPM 208364, 2MASS J00071825-1723132[4]
Database references
SIMBAD	data

Originally considered a single star, it was reported to be a shell star in 1982. Circumstellar absorption lines were then found to be variable, showing a similarity to the edge-on debris disk surrounding Beta Pictoris.^[8] The stellar classification of A2 IV/V matched an A-type star near the end of its main sequence lifetime, showing traits of an emerging subgiant star phase.^[9]

A 2019 study using PIONIER (VLTI) and 32 years of radial velocity measurements concluded that HD 256 is instead a binary star. The variable component of the spectral lines do not come from exocomets according to this study, but rather from the binarity. Each individual star holds its own circumstellar shell. The pair have an orbital period of 2.05 years, an eccentricity of around 0.23, and a semimajor axis of 3.08 AU.^[7] The adjusted classification is of a rapidly rotating main sequence shell star of type A3Vn sh.^[3]