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HR 858

HR 858

Position of HR 858 in Fornax
Observation data
Epoch J2000.00      Equinox J2000.00
Constellation Fornax
Right ascension 02h 51m 56.246s[1]
Declination −30° 48′ 52.26″[1]
Apparent magnitude (V) 6.382[2]
Characteristics
Evolutionary stage F6V[3] + M[4]
Spectral type Main sequence
B−V color index +0.484[5]
Astrometry
Radial velocity (Rv)+9.77±0.12[1] km/s
Proper motion (μ) RA: 123.435(25) mas/yr[1]
Dec.: 105.996(37) mas/yr[1]
Parallax (π)31.5418 ± 0.0352 mas[1]
Distance103.4 ± 0.1 ly
(31.70 ± 0.04 pc)
Absolute magnitude (MV)+3.82[5]
Details[6]
A
Mass1.204±0.052 M☉
Radius1.258±0.019 R☉
Luminosity2.31±0.12 L☉
Surface gravity (log g)4.30±0.06 cgs
Temperature6,354±70 K
Metallicity [Fe/H]0.025±0.050 dex
Rotation6.7±1.3 d
Rotational velocity (v sin i)7.5±0.2 km/s
Age2.0±0.6 Gyr
B
Radius0.17±0.04[4] R☉
Temperature2,800±300[4] K
Other designations
CD−31°1148, GC 3443, HD 17926, HIP 13363, HR 858, SAO 193951, TOI-396, TIC 178155732[7]
Database references
SIMBADdata

HR 858 (also known as HD 17926 or TOI-396) is a star with a planetary system located 103 light-years from the Sun in the southern constellation of Fornax. It has a yellow-white hue and is visible to the naked eye, but it is a challenge to see with an apparent visual magnitude of 6.4.[2] The star is drifting further away with a radial velocity of 10 km/s.[1] It has an absolute magnitude of +3.82.[5]

This object is a slightly-evolved F-type main-sequence star with a stellar classification of F6V,[3] which indicates it is generating energy through core hydrogen fusion. It is roughly two[8] billion years old and is spinning with a projected rotational velocity of 8.3 km/s. The star has 1.1 times the mass of the Sun and 1.3 times the Sun's radius. It is radiating 2.3 times the luminosity of the Sun from its photosphere at an effective temperature of 6,201 K.[4]

A faint co-moving stellar companion, designated component B, at an angular separation of 8.4″. This corresponds to a projected separation of 270 AU. It is a red dwarf star.[4]

Planetary system

In May 2019, HR 858 was announced to have at least 3 exoplanets as observed by the transit method with the Transiting Exoplanet Survey Satellite. All three are orbiting close to the host star and are close in size, each about twice the radius of the Earth. Described as super-Earths by their discovery paper,[4] measurements of their masses suggest that in terms of composition they may be better described as sub-Neptunes.[6] Planets 'b' and 'c' may be in a 3:5 mean-motion resonance.[4]

Further research measured the masses of the planets b and d using accurate radial velocities, giving masses of 3.55 and 7.1 ME as well as planetary densities of 2.44 and 4.9 g/cm3. The system displays significant transit timing variations. The mass of planet c could not be measured using radial velocities, but it is constrained to be less than 3.8 ME, and a not very reliable value of 2.24 ME was measured using TTVs.[6]

The HR 858 planetary system[6][a]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b 3.55+0.94
−0.96 M🜨 		0.04888±0.00066 3.585287+0.000009
−0.000012 		0 85.98+0.26
−0.25° 		2.004+0.045
−0.047 R🜨
c <3.8 M🜨 0.06870±0.00092 5.973865+0.000015
−0.000016 		0 86.59+0.15
−0.14° 		1.979+0.054
−0.051 R🜨
d 7.1±1.6 M🜨 0.1046±0.0014 11.230511+0.000043
−0.000045 		0 87.72+0.10
−0.11° 		2.001+0.063
−0.064 R🜨
Artist's impression of known planets in the HR 858 system and their size comparison with Earth

Notes

  1. ^ In the 2024 model of the system used here the orbits are assumed to be circular. The 2019 discovery paper finds upper limits on the planets' eccentricities of 0.30, 0.19, and 0.28, respectively.[4]

References

  1. ^ a b c d e f 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. ^ a b Høg, E.; et al. (March 2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862.
  3. ^ a b Gray, R. O.; et al. (July 2006). "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample". The Astronomical Journal. 132 (1): 161–170. arXiv:astro-ph/0603770. Bibcode:2006AJ....132..161G. doi:10.1086/504637. S2CID 119476992.
  4. ^ a b c d e f g h Vanderburg, Andrew; et al. (2019). "TESS Spots a Compact System of Super-Earths around the Naked-Eye Star HR 858". The Astrophysical Journal. 881 (1): L19. arXiv:1905.05193. Bibcode:2019ApJ...881L..19V. doi:10.3847/2041-8213/ab322d. S2CID 153311715.
  5. ^ a b c Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644.
  6. ^ a b c d Bonfanti, A.; Amateis, I.; et al. (November 2024). "Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396". Astronomy & Astrophysics. arXiv:2411.14911.
  7. ^ "HR 858". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-12-04.
  8. ^ Pace, G. (March 2013). "Chromospheric activity as age indicator. An L-shaped chromospheric-activity versus age diagram". Astronomy & Astrophysics. 551: 4. arXiv:1301.5651. Bibcode:2013A&A...551L...8P. doi:10.1051/0004-6361/201220364. S2CID 56420519. L8.
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