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.^[2] The system is located at a distance of approximately 990 light years from the Sun based on parallax,^[1] and is drifting closer with a radial velocity of −7 km/s.^[2] It is a member of the IC 2391 moving group.^[8]

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 80817 b, probably a brown dwarf, and possibly another substellar object. HD 80817 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]}