15 Sagittae

15 Sagittae (15 Sge) is a star in the northern constellation Sagitta, located around 58 light years away from the Sun. It is visible to the naked eye as a faint, yellow-hued star with an apparent visual magnitude of 5.80.^[1] Considered a solar analog, it was the target of the first radial velocity survey from Lick Observatory, which found a drift due to a companion.^[7] In 2002, the cause of this was found to be brown dwarf companion B via direct imaging.^[8]

15 Sagittae

15 Sagittae and its companion Credit: Keck Observatory
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Sagitta[1]
Right ascension	20h 04m 06.22091s[2]
Declination	+17° 04′ 12.6774″[2]
Apparent magnitude (V)	5.80[1]
Characteristics
Evolutionary stage	main sequence
Spectral type	G0V + L4[3]
B−V color index	0.600±0.005[1]
Astrometry
Radial velocity (Rv)	4.57±0.1[4] km/s
Proper motion (μ)	RA: −387.590[2] mas/yr Dec.: −419.542[2] mas/yr
Parallax (π)	56.426±0.069 mas[2]
Distance	57.80 ± 0.07 ly (17.72 ± 0.02 pc)
Absolute magnitude (MV)	4.55[1]
Orbit[3]
Period (P)	73.3+2.2 −3.9 yr
Semi-major axis (a)	18.3+0.4 −0.5 AU
Eccentricity (e)	0.50±0.01
Inclination (i)	97.3+0.4 −0.5°
Details[3]
15 Sge A
Mass	1.08±0.04 M☉
Radius	1.115±0.021 R☉
Luminosity	1.338±0.032 L☉
Surface gravity (log g)	4.42±0.06 cgs
Temperature	5,883±59 K
Metallicity [Fe/H]	0.05±0.07 dex
Rotation	17.5±2.3
Rotational velocity (v sin i)	4.42±0.06 km/s
Age	2.5±1.8 Gyr
15 Sge B
Mass	68.7+2.4 −3.1 MJup
Radius	0.82+0.10 −0.07[5] RJup
Luminosity	0.000075±0.000016 L☉
Temperature	1,510–1,850 K
Other designations
15 Sge, BD+16°4121, GJ 779, HD 190406, HIP 98819, HR 7672, SAO 105635, LFT 1517, LHS 3515, LTT 15872, Wolf 866[6]
Database references
SIMBAD	data

The companion is a high-mass substellar brown dwarf of spectral class L4 ± 1.5, only a few Jupiter masses below the limit for stars, in a long-period orbit around the primary star. Imaged by the Keck telescope, it was the first brown dwarf candidate orbiting a sun-like star detected via imaging and is currently the only known companion brown dwarf which both has a significant radial velocity trend on the primary that has also been imaged.

The brown dwarf was originally thought to have a semi-major axis of 14 AU and a circular orbit viewed from pole-on,^[9] but ten more years of observations found that the brown dwarf's orbit is viewed from nearly edge-on, is significantly eccentric and appeared to be moving in a circular orbit when first discovered, but is now approaching the primary as viewed from Earth.^[3]