HD 219828

HD 219828

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Pegasus
Right ascension	23^h 18^m 46.734156^s[1]
Declination	+18° 38′ 44.619358″[1]
Apparent magnitude (V)	8.04[2]
Characteristics
Spectral type	G0IV[2]
B−V color index	0.654[2]
Astrometry
Radial velocity (Rv)	−24.07±0.12[1] km/s
Proper motion (μ)	RA: −5.010±0.028 mas/yr[1] Dec.: 5.758±0.022 mas/yr[1]
Parallax (π)	13.7872 ± 0.0278 mas[1]
Distance	236.6 ± 0.5 ly (72.5 ± 0.1 pc)
Absolute magnitude (MV)	3.58[2]
Details[2]
Mass	1.23±0.10 M☉
Radius	1.69 R☉
Luminosity	3.08 L☉
Surface gravity (log g)	4.08±0.10 cgs
Temperature	5,891±18 K
Metallicity [Fe/H]	+0.19±0.03 dex
Rotational velocity (v sin i)	2.9 km/s
Age	4.6±0.7 Gyr
Other designations
BD17 4896, HIP 115100, SAO 108536, GSC 01716-01182[3]
Database references
SIMBAD	data

HD 219828 is a star with two exoplanetary companions in the constellation of Pegasus. With an apparent visual magnitude of 8.04,^[2] it is an eighth magnitude star that is too dim to be readily visible to the naked eye. The star is located at a distance of approximately 237 light years from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −24 km/s.^[1]

The stellar classification of this star is G0IV, matching a subgiant star that is ending the hydrogen fusion in its core. It is a metal-rich star that has slightly evolved off the main sequence. HD 219828 is older than two billion years with a low level of chromospheric activity and is spinning with a projected rotational velocity of 2.9 km/s. It has 1.23 times the mass of the Sun with radius estimates ranging from 1.47 to 1.69 the Sun's girth. The star is radiating three times the luminosity of the Sun from its slightly enlarged photosphere at an effective temperature of 5,891 K.^[2]

Planetary system

In 2007, a Neptune-mass planet was found orbiting the star by Melo. According to the author, assuming an Earth-like rocky composition,^[4] the planet could have a radius 2.2 times that of Earth. This could be demonstrated if a transit were observed, though, as authors state, it would be a difficult task.^[5] At this point, it was suggested that a best fit orbital solution hints the presence of an additional planetary companion (so far unconfirmed) with 70% of Jupiter's mass, likely orbital separation of 0.68 Astronomical units and eccentric orbit (e=0.3). However, in 2016, refined analysis of the radial velocity data confirmed the presence of an outer planet (or possibly a brown dwarf) larger and more eccentric than was originally believed. With a high eccentricity and a high mass ratio between the two planets, the system is quite unique.^[2] The planet c is a potential target for Gaia astrometry or atmospheric characterization using direct imaging or high-resolution spectroscopy, and in 2022 its inclination and true mass were measured using Gaia astrometry.^[6]

The HD 219828 planetary system^[2]^[6]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	≥21.0±1.4 M🜨	0.045	3.834887±0.000096	0.059±0.036	—	—
c	16.054+3.520 −1.270 MJ	5.657+0.228 −0.242	4789.9+90.7 −80.7	0.811±0.004	122.816+11.012 −69.974°	—

See also

• HD 159868
• List of extrasolar planets

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. Santos, N. C.; et al. (2016). "An extreme planetary system around HD219828. One long-period super Jupiter to a hot-neptune host star". Astronomy & Astrophysics. 592: A13. arXiv:1605.06942. Bibcode:2016A&A...592A..13S. doi:10.1051/0004-6361/201628374. S2CID 96428891.
3. "HD 219828". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-12-19.
4. Valencia, Diana; et al. (2006). "Internal structure of massive terrestrial planets". Icarus (Submitted manuscript). 181 (2): 545–554. arXiv:astro-ph/0511150. Bibcode:2006Icar..181..545V. doi:10.1016/j.icarus.2005.11.021. S2CID 118946944.
5. Melo, C.; et al. (2007). "A new Neptune-mass planet orbiting HD 219828". Astronomy and Astrophysics. 467 (2): 721–727. arXiv:astro-ph/0702459. Bibcode:2007A&A...467..721M. doi:10.1051/0004-6361:20066845. S2CID 13524822.
6. Feng, Fabo; Butler, R. Paul; et al. (August 2022). "3D Selection of 167 Substellar Companions to Nearby Stars". The Astrophysical Journal Supplement Series. 262 (21): 21. arXiv:2208.12720. Bibcode:2022ApJS..262...21F. doi:10.3847/1538-4365/ac7e57. S2CID 251864022.

External links

• "HD 219828". Extrasolar Planets Encyclopaedia. Archived from the original on July 2, 2007.