HD 166620
Star in the constellation Hercules
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HD 166620 or HR 6806 is a solitary, orange, main sequence, and Sun-like (K2 V) star located thirty-six light-years away,[8] in the constellation Hercules. The star is smaller than the Sun, with around 79% of the solar mass and radius, and 35% of the solar luminosity.[9] It appears to be rotating slowly with an estimated period of 42 days. In 1988, it was noticed that the star had an inactive chromosphere, with a surface magnetic field strength of only 1,500 G.[10] From 1990 activity in the chromosphere increased, in line with a 16 year stellar cycle previously observed. But, sometime after 1994 (exact date unknown because of a data collection gap between 1995 and 2004) chromospheric activity greatly reduced, and has stayed flat for more than 16 years. As of 2022,[update] the star appears to have entered the equivalent of a Maunder minimum.[8] The star is around six billion years of age.[11]
 | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Hercules[1] |
| Right ascension | 18h 09m 37.41621s[2] |
| Declination | +38° 27′ 27.9980″[2] |
| Apparent magnitude (V) | 6.40[3] |
| Characteristics | |
| Spectral type | K2 V[4] |
| U−B color index | +0.585[3] |
| B−V color index | +0.875[3] |
| Astrometry | |
| Radial velocity (Rv) | −19.51±0.12[2] km/s |
| Proper motion (μ) | RA: −316.454 mas/yr[2] Dec.: −468.348 mas/yr[2] |
| Parallax (π) | 90.1234±0.0156 mas[2] |
| Distance | 36.190 ± 0.006 ly (11.096 ± 0.002 pc) |
| Absolute magnitude (MV) | 6.17[5] |
| Details[6] | |
| Mass | 0.76+0.032 −0.019 M☉ |
| Radius | 0.77+0.007 −0.006 R☉ |
| Luminosity | 0.36+0.02 −0.01 L☉ |
| Surface gravity (log g) | 4.55+0.02 −0.01 cgs |
| Temperature | 4,989±48 K |
| Metallicity [Fe/H] | −0.21±0.04 dex |
| Rotation | 42.4 d |
| Rotational velocity (v sin i) | <2.0 km/s |
| Age | 10.09+2.73 −3.76 Gyr |
| Other designations | |
| BD+38°3095, GJ 706, HD 166620, HIP 88972, HR 6806, SAO 66700, LHS 3363[7] | |
| Database references | |
| SIMBAD | data |
There was suspected to be a nearby very cool, and very dim, T9 to Y brown dwarf companion, WISE J180901.07+383805.4, at an angular separation of 769″, which would have corresponded to a projected separation of 8460 AU at the distance of HD 166620. However, with further observation it was found to be bluer than at first thought and more typical of a slightly brighter T7 dwarf, which would place it at a much greater distance of 91 ly (28 pc)—ruling out a physical association. This is confirmed by the differing proper motion of the star and this object.[12]
This star has been targeted by planet searches using the radial velocity method, but as of 2023 no evidence of a planetary companion has been found. Observations rule out the existence of planets down to super-Earth masses with orbital periods less than 2,800 days; at periods less than 10 days, planets less massive than Earth would be detectable.[6]