HD 18438 is a red giant star in the deep northern constellation of Cepheus, located about 730 light-years (220 parsecs) from Earth. With an apparent magnitude of 5.49, it is visible by the naked eye under dark skies as a red-hued dot of light about 10 degrees away from the celestial north pole. It is part of a wide binary system with an F-type subgiant star. In 2023, HD 18438 was discovered to be orbited by a 21 MJ substellar object, potentially making it the largest host star to an exoplanet ever found as of September 2024.[2][6]
  | |
| Observation data Epoch J2000.0[1] Equinox J2000.0[1] | |
|---|---|
| Constellation | Cepheus |
| HD 18438 | |
| Right ascension | 03h 06m 07.84053s |
| Declination | +79° 25′ 06.7270″ |
| Apparent magnitude (V) | +5.49[1] |
| TYC 4516-2148-1 | |
| Right ascension | 03h 06m 06.49835s |
| Declination | +79° 25′ 03.7722″ |
| Apparent magnitude (V) | +9.08[1] |
| Characteristics | |
| HD 18438 | |
| Spectral type | M2.5 III |
| B−V color index | +1.569[2] |
| J−H color index | +0.864[1] |
| J−K color index | +1.145[1] |
| TYC 4516-2148-1 | |
| Spectral type | F7 IV[3] |
| B−V color index | +0.61 |
| Astrometry | |
| HD 18438 | |
| Radial velocity (Rv) | -37.6[1] km/s |
| Proper motion (μ) | RA: -35.523[1] mas/yr Dec.: 10.383[1] mas/yr |
| Parallax (π) | 4.443 ± 0.171 mas[2] |
| Distance | 730 ± 30 ly (225 ± 9 pc) |
| Absolute magnitude (MV) | -1.0[4] |
| TYC 4516-2148-1 | |
| Proper motion (μ) | RA: -35.639[1] mas/yr Dec.: 10.069[1] mas/yr |
| Parallax (π) | 4.3427 ± 0.0176 mas[1] |
| Distance | 751 ± 3 ly (230.3 ± 0.9 pc) |
| Absolute magnitude (MV) | 2.5[4] |
| Orbit[2] | |
| Primary | HD 18438 |
| Companion | TYC 4516-2148-1 |
| Semi-major axis (a) | 4.7[3]" (1100 AU) |
| Details[2] | |
| HD 18438 | |
| Mass | 1.84±0.09 M☉ |
| Radius | 88.475±4.424 R☉ |
| Luminosity | 929±41 L☉ |
| Surface gravity (log g) | 0.9±0.1 cgs |
| Temperature | 3860±100 K |
| Metallicity [Fe/H] | −0.4±0.1 dex |
| Rotation | 637 d |
| Rotational velocity (v sin i) | 5.5±0.2 km/s |
| Age | 5.5±2.4 Gyr |
| TYC 4516-2148-1 | |
| Mass | 1.174±0.184 M☉ |
| Radius | 2.554±0.182 R☉ |
| Luminosity | 7.558±0.095 L☉ |
| Surface gravity (log g) | 3.7±0.1 cgs |
| Temperature | 6164±211 K |
| Other designations | |
| HD 18438: AG+79°95, BD+78°103, FK5 105, Gaia DR3 567664766657362304, GC 3638, HD 18438, HIP 14417, HR 881, SAO 4810, PPM 5183, TIC 297820335, TYC 4516-2147-1, 2MASS J03060788+7925066[1] | |
| TYC 4516-2148-1: Gaia DR3 567664762361981312, TYC 4516-2148-1[5] | |
| Database references | |
| SIMBAD | HD 18438 |
| TYC 4516-2148-1 | |
Stellar characteristics
editHD 18438 is a red giant on the asymptotic giant branch (AGB)[7] that has already gone through the helium shell flash, with a spectral type of M2.5 III. Thus, it has evolved past the main sequence after using up its core hydrogen. It has 1.84 times the mass of the Sun but 88.48 times its girth and, at an effective temperature of 3,860 K (3,590 °C; 6,490 °F) it radiates 929 times Sun's luminosity from its photosphere.[2] The star is aged 5.5±2.4 billion years, making it likely older than the Solar System. It has a low metallicity of −0.4±0.1 dex, which translates to an iron abundance 32–50% that of the Sun.[2]
It is the largest host star to a confirmed exoplanet included in the NASA Exoplanet Archive as of September 2024.[6] However, some definitions[8] may classify the planet as a brown dwarf instead due to its high mass.
Much like many AGB stars,[9] HD 18438 is suspected to be a variable star, its apparent magnitude fluctuating between 5.43 and 5.49.[3]
Binary system
editThe star has been noted to be part of a double star since at least 1932, receiving the designation ADS 2294 in the Aitken Double Star Catalogue,[1][10] with a 9th-magnitude F7IV[3] star later designated TYC 4516-2148-1.[1] Due to its similarities to HD 18438 in distance and proper motion, this star is thought to be a physical binary companion to HD 18438. Located at a wide separation of 1,100 AU from the primary star, TYC 4516-2148-1 is slightly more massive than the Sun, but is 2.5 times larger and 7.5 times as luminous. The star is similar to HD 18438 in that it is somewhat more massive than the Sun and is past the main-sequence stage, but its relationship with HD 18438 is poorly understood.[2]
Substellar companion
editIn 2023, a team of South Korean astronomers reported the discovery of a substellar object, named HD 18438 b, which could either be described as a massive gas giant exoplanet or a low-mass brown dwarf. It is about 8% larger than the planet Jupiter and at least 21 times as massive, which places it above the deuterium burning limit (~13 MJ[8]). It revolves around the star in a roughly circular (eccentricity 0.1) orbit with a semi-major axis of 2.1 AU (310,000,000 km) once every 803 days (2.20 years).[2]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | ≥21 ± 1 MJ | 2.1 ± 0.1 | 803 ± 5 | 0.1 ± 0.1 | — | ~1.08[11] RJ |
See also
editReferences
edit- ^ a b c d e f g h i j k l m n o "HD 18438". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 27 September 2024.
- ^ a b c d e f g h i j Lee, Byeong-Cheol; et al. (30 December 2023). "A Search for Exoplanets around Northern Circumpolar Stars. VII. Detection of Planetary Companion Orbiting the Largest Host Star HD 18438". Journal of the Korean Astronomical Society. 56 (1): 35–40. arXiv:2303.08357. Bibcode:2023JKAS...56...35L. doi:10.5303/JKAS.2023.56.1.35.
- ^ a b c d Černis, K.; Meištas, E.; Straižys, V.; Jasevičius, V. (1989). "Photoelectric photometry of bright stars in the vicinity of the North Celestial Pole". Vilnius Astronomijos Observatorijos Biuletenis. 84: 9–20. Bibcode:1989VilOB..84....9C.
- ^ a b Bidelman, William P. (1958). "Spectral Classification of Visual Binaries Having Primaries Above the Main Sequence". Publications of the Astronomical Society of the Pacific. 70. IOP Publishing: 168. doi:10.1086/127203. ISSN 0004-6280.
- ^ "TYC 4516-2148-1". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 27 September 2024.
- ^ a b "Planetary Systems". NASA Exoplanet Archive. NASA Exoplanet Science Institute. Retrieved 27 September 2024.
- ^ Eggen, Olin J. (1992). "Asymptotic giant branch stars near the sun". The Astronomical Journal. 104. American Astronomical Society: 275. doi:10.1086/116239. ISSN 0004-6256.
- ^ a b "Working Group on Extrasolar Planets: Definition of a "Planet"". IAU position statement. 28 February 2003. Archived from the original on 16 December 2014. Retrieved 28 April 2014.
- ^ Groenewegen, M. A. T. (2022). "A WISE view on extreme AGB stars". Astronomy & Astrophysics. 659. EDP Sciences: A145. arXiv:2203.09875. doi:10.1051/0004-6361/202142648. ISSN 0004-6361.
- ^ Robert Grant Aitken; Eric Doolittle (1932). New General Catalogue of Double Stars within 120° of the North Pole. Carnegie Institution of Washington. Bibcode:1932ngcd.book.....A.
- ^ "Exoplanet-catalog". Exoplanet Exploration: Planets Beyond our Solar System. Archived from the original on 4 June 2023. Retrieved 31 October 2023.