HAT-P-38

HAT-P-38, formally named Horna, is a star located in the northern constellation Triangulum. It has an apparent magnitude of 12.51,^[2] making it readily visible in amateur telescopes but not to the naked eye. The object is located relatively far at a distance of 821 light-years based on Gaia DR3 parallax measurements, but it is drifting closer with a spectroscopic radial velocity of −19.85 km/s. ^[1]

HAT-P-38/Horna

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Triangulum
Right ascension	02h 21m 31.98035s[1]
Declination	+32° 14′ 46.0933″[1]
Apparent magnitude (V)	12.51±0.02[2]
Characteristics
Spectral type	G5[3]
B−V color index	+0.83[2]
Astrometry
Radial velocity (Rv)	−19.85±0.73[1] km/s
Proper motion (μ)	RA: +47.671 mas/yr[1] Dec.: −21.594 mas/yr[1]
Parallax (π)	3.9747 ± 0.0134 mas[1]
Distance	821 ± 3 ly (251.6 ± 0.8 pc)
Details
Mass	0.886±0.044[4] M☉
Radius	1.01+0.07 −0.05[5] R☉
Luminosity	0.6772+0.007 −0.008[1] L☉
Surface gravity (log g)	4.46±0.08[6] cgs
Temperature	5,330±100[4] K
Metallicity [Fe/H]	+0.06±0.10[7] dex
Rotational velocity (v sin i)	0.4±0.5[4] km/s
Age	10.1+3.9 −4.8[7] Gyr
Other designations
Horna, TOI-3681, TIC 285272237, GSC 02314-00559
Database references
SIMBAD	data
Exoplanet Archive	data

HAT-P-38 has a stellar classification of G5,^[3] indicating that it is a G-type star. It has 88.6% the mass of the Sun^[4] and 101% the radius of the Sun.^[5] It radiates 67.72% the luminosity of the Sun^[1] from its photosphere at an effective temperature of 5,330 K,^[4] giving it a yelllowish-orange hue. HAT-P-38 is slightly metal enriched with an iron abundance 115% that of the Sun's.^[7] It is estimated to be approximately 10.1 billion years old,^[7] which is more than twice the age of the Sun. It spins modestly with a projected rotational velocity of 0.4 km/s.^[4]

Planetary system

In 2012, a hot Saturn was detected on a tight 4-day orbit via the transit method; the planet has a similar mass and radius to Saturn.^[4] A 2017 paper comparing HAT-P-38b and WASP-67b found that there was water vapor in the planet's atmosphere.^[8] HAT-P-38b had a higher water vapor abundance compared to the latter, indicating that the planet is free of clouds or hazes in its upper atmosphere.^[9]

In 2019, the system was selected for the second NameExoWorlds campaign for Finland as part of the IAU's 100th anniversary. The approved name for the star is Horna, which is hell or the underworld from Finnic mythology..^[10]

The HAT-P-38 planetary system^[4]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b (Hiisi)	0.267±0.020 MJ	0.05231+0.00085 −0.00089[7]	4.6403288±0.0000011[11]	<0.055[7]	88.3±0.7°	0.825+0.092 −0.063 RJ

References

1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
2. Zacharias, N.; Finch, C. T.; Girard, T. M.; Henden, A.; Bartlett, J. L.; Monet, D. G.; Zacharias, M. I. (January 14, 2013). "The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)". The Astronomical Journal. 145 (2): 44. arXiv:1212.6182. Bibcode:2013AJ....145...44Z. doi:10.1088/0004-6256/145/2/44. ISSN 0004-6256. S2CID 119299381.
3. Luo, A. -L.; Zhao, Y. -H.; Zhao, G. (August 2018). "VizieR Online Data Catalog: LAMOST DR4 catalogs (Luo+, 2018)". VizieR Online Data Catalog. 5153: V/153. Bibcode:2018yCat.5153....0L.
4. Sato, Bun'ei; et al. (October 25, 2012). "HAT-P-38b: A Saturn-Mass Planet Transiting a Late G Star". Publications of the Astronomical Society of Japan. 64 (5): 97. arXiv:1201.5075. Bibcode:2012PASJ...64...97S. doi:10.1093/pasj/64.5.97. ISSN 2053-051X. S2CID 119186118.
5. Stassun, Keivan G.; et al. (9 September 2019). "The Revised TESS Input Catalog and Candidate Target List". The Astronomical Journal. 158 (4): 138. arXiv:1905.10694. Bibcode:2019AJ....158..138S. doi:10.3847/1538-3881/ab3467. eISSN 1538-3881. hdl:1721.1/124721. S2CID 166227927.
6. Magrini, L.; Danielski, C.; Bossini, D.; Rainer, M.; Turrini, D.; Benatti, S.; Brucalassi, A.; Tsantaki, M.; Delgado Mena, E.; Sanna, N.; Biazzo, K.; Campante, T. L.; Van der Swaelmen, M.; Sousa, S. G.; Hełminiak, K. G.; Neitzel, A. W.; Adibekyan, V.; Bruno, G.; Casali, G. (July 2022). "Ariel stellar characterisation: I. Homogeneous stellar parameters of 187 FGK planet host stars: Description and validation of the method". Astronomy & Astrophysics. 663: A161. arXiv:2204.08825. Bibcode:2022A&A...663A.161M. doi:10.1051/0004-6361/202243405. ISSN 0004-6361. S2CID 248240168.
7. Bonomo, A. S.; et al. (June 2017). "The GAPS Programme with HARPS-N at TNG: XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets⋆". Astronomy & Astrophysics. 602: A107. arXiv:1704.00373. Bibcode:2017A&A...602A.107B. doi:10.1051/0004-6361/201629882. ISSN 0004-6361. S2CID 118923163.
8. Bruno, Giovanni; Lewis, Nikole K.; Stevenson, Kevin B.; Filippazzo, Joseph; Hill, Matthew; Fraine, Jonathan D.; Wakeford, Hannah R.; Deming, Drake; Kilpatrick, Brian; Line, Michael R.; Morley, Caroline V.; Collins, Karen A.; Conti, Dennis M.; Garlitz, Joseph; Rodriguez, Joseph E. (February 1, 2018). "A Comparative Study of WASP-67 b and HAT-P-38 b from WFC3 Data". The Astronomical Journal. 155 (2): 55. arXiv:1712.03384. Bibcode:2018AJ....155...55B. doi:10.3847/1538-3881/aaa0c7. ISSN 0004-6256.
9. Wall, Mike (6 June 2017). "Surprise! Giant Exoplanet 'Cousins' Have Different Atmospheres". Space.com.
10. "Approved names". NameExoworlds. Retrieved 2020-01-02.
11. Kokori, A.; et al. (February 1, 2022). "ExoClock Project. II. A Large-scale Integrated Study with 180 Updated Exoplanet Ephemerides". The Astrophysical Journal Supplement Series. 258 (2): 40. arXiv:2204.08825. Bibcode:2022ApJS..258...40K. doi:10.3847/1538-4365/ac3a10. ISSN 0067-0049. S2CID 248240168.