HD 81817

HD 81817 is a possible binary star system^[6] with two brown dwarf companions^[4][7] in the northern circumpolar constellation of Draco. It has an orange hue and is visible to the naked eye with an apparent visual magnitude of 4.28.^[1] The system is located at a distance of approximately 990 light years from the Sun based on parallax,^[2] and is drifting closer with a radial velocity of −7 km/s.^[1] It is a member of the IC 2391 moving group.^[8]

HD 81817

Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Draco[1]
Right ascension	09h 37m 05.28778s[2]
Declination	+81° 19′ 34.9711″[2]
Apparent magnitude (V)	4.28[1]
Characteristics
Spectral type	K3III + ? + DA2.8[3]
B−V color index	+1.488±0.001[1]
Astrometry
Radial velocity (Rv)	−6.98±0.10[1] km/s
Proper motion (μ)	RA: −16.62[2] mas/yr Dec.: −16.41[2] mas/yr
Parallax (π)	3.28±0.15 mas[2]
Distance	990 ± 50 ly (300 ± 10 pc)
Absolute magnitude (MV)	−3.14[1]
Details[4]
A
Mass	4.3±0.5 M☉
Radius	83.8±7.8 R☉
Luminosity	1,822.9 L☉
Temperature	4,140±55 K
Metallicity [Fe/H]	−0.17±0.1 dex
Rotation	801 d
Rotational velocity (v sin i)	4.7±0.1 km/s
Age	150±40 Myr
Other designations
BD+81°302, FK5 910, HD 81817, HIP 47193, HR 3751, SAO 1551, WD 0930+815[5]
Database references
SIMBAD	data

The primary component of this system is an aging giant star with a stellar classification of K3III.^[3] The star's chromosphere is of the type called "hybrid", displaying a cool stellar wind in combination with hot emission lines.^[9] It appears to be the source for the X-ray emission coming from this system.^[10] The star is 150 million years old with 4.3 times the mass of the Sun.^[4] With the supply of hydrogen at its core exhausted, the star has expanded to 83.8 times the Sun's radius.^[4] It is radiating 1,823 times the luminosity of the Sun from its photosphere at an effective temperature of 4,140 K.^[4]

A possible companion star was discovered in 1984 based upon its ultraviolet spectrum. The distribution of the far ultraviolet flux matches that of a white dwarf star of class DA.^[6] A 2020 study finds it unlikely that there is a white dwarf companion; instead claiming the radial velocity variations are caused by a substellar object HD 81817 b, probably a brown dwarf, and possibly another substellar object. HD 81817 b would have a minimum mass of 27.1 M_J and orbit at 3.3 AU with a period of 1047.1 days and an eccentricity of 0.17. Gaia DR2 astrometry also suggested a companion with a mass of about 124 M_J (with a high margin of error) orbiting at 2.67 AU, consistent with the radial velocity detection. If the latter mass estimate is correct, this object would be a low-mass star, probably a red dwarf.^[4] Further observations through 2022 have confirmed that the companion is indeed a brown dwarf, in addition to the detection of a second brown dwarf on a closer orbit.^[7] By some definitions, these objects could be considered supermassive planets, similar to those around Nu Ophiuchi.^{[citation needed]}

The HD 81817 planetary system^[7]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
c	≥22.609+1.859 −1.876 MJ	2.325+0.087 −0.095	1.706±0.006	0.095+0.058 −0.044	—	—
b	24.128+9.747 −0.691 MJ	3.233+0.125 −0.131	2.796+0.037 −0.027	0.097+0.090 −0.057	95.594+29.637 −35.307°	—

See also

• List of exoplanets discovered in 2020 - HD 81817 b