HD 39118

HD 39118 (HD 39119, HR 2024) is a spectroscopic binary star in the constellation Orion, close to the celestial equator. It is made up from three stars: a cool primary (a K-type giant star) and a hot secondary, which is a binary star formed from a B-type main-sequence star and an A-type main-sequence star. A 2021 estimate derive a distance of 708 parsecs (2,310 ly) to HD 39118, and it is moving away from Earth at a speed of 4.24 km/s. The apparent magnitude is 5.976, making it visible to the naked eye only from dark skies.

HD 39118

Basic layout of the HR 2024 system.
Observation data Epoch J2000      Equinox J2000
Constellation	Orion[1]
Right ascension	05h 50m 30.03s[2]
Declination	+02° 01′ 28.9″[2]
Apparent magnitude (V)	5.976[2]
Characteristics
Cool primary
Evolutionary stage	Horizontal branch[3]
Spectral type	K0II[4]
Apparent magnitude (B)	6.83±0.015[2]
Apparent magnitude (G)	5.64[2]
Apparent magnitude (J)	4.193±0.238[2]
Apparent magnitude (H)	3.502±0.194[2]
Apparent magnitude (K)	3.337±0.19[2]
B−V color index	1.12[4]
Hot secondary
Evolutionary stage	B-type main-sequence star + A-type main-sequence star
Spectral type	B7V/B8V + A0V[4]
B−V color index	-0.09[4]
Astrometry
Radial velocity (Rv)	4.24[4] km/s
Proper motion (μ)	RA: 1.097 mas/yr[2] Dec.: -5.161 mas/yr[2]
Parallax (π)	1.47±0.23 mas[5]
Distance	2300+350 −270 ly (707.6+107 −83.3 pc)[5]
Absolute magnitude (MV)	–2.53 (–2.3 (primary) + –0.75 (secondary))[4]
Orbit
Primary	Cool primary
Companion	Hot secondary
Period (P)	2570±13 d 7.04±0.04 year[6]
Semi-major axis (a)	4.7×108 km 3.14 AU[6]
Eccentricity (e)	0.3±0.007[6]
Details
K-type giant
Mass	3.3+0.3 −0.27[3] M☉
Radius	23.5+3.7 −1.9[3] R☉
Luminosity	535[7] L☉
Surface gravity (log g)	1.52[8] cgs
Temperature	4,550[8] K
Metallicity [Fe/H]	-0.34[8] dex
Rotational velocity (v sin i)	4.19[9] km/s
Age	257[3] Myr
B-type star
Temperature	11,300[10] K
Other designations
BD+01 1148, Gaia DR2 3316078695157768448, Gaia DR3 3316078695157768448, HD 39118/39119, HIP 27588, HR 2024, SAO 113198, PPM 149543, TIC 158867386, TYC 120-877-1, GSC 00120-00877, IRAS 05478+0200, 2MASS J05503003+0201290
Database references
SIMBAD	data

Characteristics

HR 2024 is a spectroscopic binary (more precisely a single-lined spectroscopic binary)^[11] made up of a cool primary and a hot secondary, which is also a binary star.^[4] The designations “cold” and “hot” refer to the effective temperature of the components. They are separated by 4.7×10⁸ kilometres (3.1 AU), and complete an orbit around each other every 2,570 days (7 years).^[6] The orbital eccentricity is equivalent to 0.3.^[6]

HD 39118 can be seen in the northern celestial hemisphere, close to the celestial equator, at a distance of 708 parsecs (2,310 light-years) in the constellation Orion.^[2][5][1] It has an apparent magnitude of 5.976.^[2] At this magnitude, it is visible to the naked eye only in dark skies, being close to the limiting magnitude to naked-eye vision of 6.5.^[12] The absolute magnitude, i.e. its brightness if it was seen at a distance or 10 parsecs (32.6 ly), is –2.53.^[4] It is moving away from Earth at a velocity of 4.24 km/s.^[2]

It was double cataloged in the Henry Draper Catalogue, receiving the designations HD 39118 and HD 39119.^[2]

Primary star

The primary has an spectral classification of K0II,^[4] meaning that it is a K-type star that has left the main sequence, being now a bright giant star. Currently, it is in the horizontal branch of evolution.^[3] It is 3.28 times more massive has expanded to 25 times the Sun's size.^[3] It emitts a luminosity 535^[7] times the solar luminosity from its photosphere at an effective temperature of 4,550 K, which is around 1,200 K cooler than the Sun.^[8][a] It has a subsolar metallicity, having an abundance of iron on its surface equivalent to 46% that of the Sun.^[8][b] The age of the primary is estimated at 263 million years,^[3] much younger than the Sun (4.6 billion years) despite its advanced evolutionary stage. It rotates under its own axis at a velocity of 4.19 km/s.^[9] The B-V index is of 1.12,^[4] giving it the typical orange hue of a K-type star.^[c]

Hot companion

The hot companion is made up of two other stars, one is a late B-type main-sequence star (spectral type B7V/B8V) and the other is an early A-type main-sequence star (spectral type A0V).^[4] It has an absolute magnitude about 1.55 magnitudes fainter than the primary companion.^[4] The B-type star has an effective temperature of 11,300 K.^[10] The B-V index of the hot companion is of –0.09,^[4] meaning that it has a typical hue of a B-type star.^[c]

Notes

1. The Sun's effective temperature is 5772 K.
2. From a logarithm of -0.34
3. See the color index article