HD 39118
Triple stellar system in the constellation Orion
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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.
 | |
| 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[3] |
| Characteristics | |
| Cool primary | |
| Evolutionary stage | Horizontal branch[4] |
| Spectral type | K0II[5] |
| Apparent magnitude (B) | 6.83±0.015[6] |
| Apparent magnitude (G) | 5.64[6] |
| Apparent magnitude (J) | 4.193±0.238[6] |
| Apparent magnitude (H) | 3.502±0.194[6] |
| Apparent magnitude (K) | 3.337±0.19[6] |
| B−V color index | 1.12[5] |
| Hot secondary | |
| Evolutionary stage | Main sequence + Main sequence |
| Spectral type | B7/B8V + A0:V[5] |
| B−V color index | −0.09[5] |
| Astrometry | |
| Radial velocity (Rv) | 4.24[5] km/s |
| Proper motion (μ) | RA: +1.097 mas/yr[2] Dec.: −5.161 mas/yr[2] |
| Parallax (π) | 1.4703±0.2299 mas[2] |
| Distance | 2300+350 −270 ly (707.6+107 −83.3 pc)[7] |
| Absolute magnitude (MV) | –2.53 (–2.3 (primary) + –0.75 (secondary))[5] |
| Orbit | |
| Primary | Cool primary |
| Name | Hot secondary |
| Period (P) | 2570±13 d 7.04±0.04 year[8] |
| Semi-major axis (a) | 4.7×108 km 3.14 AU[8] |
| Eccentricity (e) | 0.3±0.007[8] |
| Details | |
| K-type giant | |
| Mass | 3.3+0.3 −0.27[4] M☉ |
| Radius | 23.5+3.7 −1.9[4] R☉ |
| Luminosity | 535[9] L☉ |
| Surface gravity (log g) | 1.52[10] cgs |
| Temperature | 4,550[10] K |
| Metallicity [Fe/H] | −0.34[10] dex |
| Rotational velocity (v sin i) | 4.19[11] km/s |
| Age | 257[4] Myr |
| B-type star | |
| Temperature | 11,300[12] K |
| Other designations | |
| BD+01 1148, 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, Gaia DR2 3316078695157768448, Gaia DR3 3316078695157768448[6] | |
| Database references | |
| SIMBAD | data |
Characteristics
HR 2024 is a spectroscopic binary (more precisely a single-lined spectroscopic binary)[13] made up of a cool primary and a hot secondary, which is also a binary star.[5] The designations “cool” and “hot” refer to the relative 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).[8] The orbital eccentricity is equivalent to 0.3.[8]
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.[6][7][1] It has an apparent magnitude of 5.976. 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.[14] The absolute magnitude, i.e. its brightness if it was seen at a distance or 10 parsecs (32.6 ly), is –2.53.[5] It is moving away from Earth at a velocity of 4.24 km/s.[6]
The Henry Draper Catalogue recognises that the spectrum is composite: the designation HD 39118 is applied to component A with spectral class G0; HD 39119 is applied to component B with spectral class A0, although both entries have the same position and magnitude.[15]
Primary star
The primary has a spectral classification of K0II,[5] meaning that it is a K-type star that has evolved away from the main sequence and is now a bright giant star. Currently, it is on the horizontal branch, fusing helium in its core.[4] It is 3.28 times more massive than the Sun and has expanded to 25 times the Sun's size.[4] It emits a luminosity 535 times the solar luminosity[9] from its photosphere at an effective temperature of 4,550 K, which is around 1,200 K cooler than the Sun.[10][a] It has a subsolar metallicity, having an abundance of iron on its surface equivalent to 46% that of the Sun.[10][b] The age of the primary is estimated at 263 million years,[4] much younger than the Sun (4.6 billion years) despite its advanced evolutionary stage. It rotates on its own axis at a projected velocity of 4.19 km/s.[11] The B-V index is of 1.12,[5] 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).[5] They have a combined brightness about 1.55 magnitudes fainter than the cooler primary star.[5] The B-type star has an effective temperature of 11,300 K.[12] Although stars cannot be resolved, it is calculated that the combined B-V index of the hot companions is –0.09,[5] meaning that it has a typical hue of a B-type star.[c]
Notes
- The Sun's effective temperature is 5772 K.
- 10−0.34
- See the color index article