| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Draco |
| Right ascension | 09h 37m 05.28778s[1] |
| Declination | +81° 19′ 34.9711″[1] |
| Apparent magnitude (V) | 4.28[2] |
| Characteristics | |
| Spectral type | K3III + ? + DA2.8[3] |
| B−V color index | +1.488±0.001[2] |
| Astrometry | |
| Radial velocity (Rv) | −6.98±0.10[2] km/s |
| Proper motion (μ) | RA: −16.62[1] mas/yr Dec.: −16.41[1] mas/yr |
| Parallax (π) | 3.28 ± 0.15 mas[1] |
| Distance | 990 ± 50 ly (300 ± 10 pc) |
| Absolute magnitude (MV) | −3.14[2] |
| 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 | |
| Database references | |
| SIMBAD | data |
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 MJ 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 MJ (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]
| 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° |
— |
vanLeeuwen2007 was invoked but never defined (see the help page).Anderson2012 was invoked but never defined (see the help page).Holberg2013 was invoked but never defined (see the help page).Bang2020 was invoked but never defined (see the help page).SIMBAD was invoked but never defined (see the help page).Reimers1984 was invoked but never defined (see the help page).Feng2022 was invoked but never defined (see the help page).Montes2001 was invoked but never defined (see the help page).Ayres2005 was invoked but never defined (see the help page).Bilíková2010 was invoked but never defined (see the help page).