GJ 3991
Star in the constellation Hercules
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GJ 3991 (also known as Gliese 3991 and G 203-47) is a binary star system located 24.78 light-years (7.60 parsecs) away in the constellation Hercules. Due to their small separation, the two stars were initially detected from the radial velocity changes of GJ 3991, making the system a spectroscopic binary. It consists of a red dwarf star with 20–30% the mass of the Sun, and a white dwarf star roughly 50% the mass of the sun. The two components orbit each other in a tight orbit only 0.11 astronomical units from each other, with an orbital period of only 14.91 days. This system is the nearest example of a post-common-envelope binary.[9]
| Observation data Epoch J2000.0 Equinox ICRS | |
|---|---|
| Constellation | Hercules[1] |
| Right ascension | 17h 09m 31.544s[2] |
| Declination | +43° 40′ 52.77″[2] |
| Apparent magnitude (V) | 13.671[3] |
| Characteristics | |
| Spectral type | M3.5V[4] |
| Apparent magnitude (B) | 13.46[5] |
| Apparent magnitude (R) | 11.511[5] |
| Apparent magnitude (J) | 7.380[5] |
| Apparent magnitude (H) | 6.76[5] |
| Apparent magnitude (K) | 6.485[5] |
| Astrometry | |
| Radial velocity (Rv) | −27.87[6] km/s |
| Proper motion (μ) | RA: +332.032 mas/yr[2] Dec.: −274.501 mas/yr[2] |
| Parallax (π) | 131.5996±0.4285 mas[2] |
| Distance | 24.78 ± 0.08 ly (7.60 ± 0.02 pc) |
| Absolute magnitude (MV) | 12.41[7] |
| Orbit[8] | |
| Period (P) | 14.909777±0.00001 days[9] |
| Semi-major axis (a) | 0.015+0.01 −0.05" (0.1102 AU) |
| Eccentricity (e) | 0.068±0.004 |
| Argument of periastron (ω) (primary) | 175.0±3.0° |
| Semi-amplitude (K1) (primary) | 50.6±0.2 km/s |
| Details | |
| GJ 3991 A | |
| Mass | 0.2714+0.0066 −0.0065[10] M☉ |
| Radius | 0.286±0.011[10] R☉ |
| Luminosity | 0.0085±0.0001[11] L☉ |
| Temperature | 3279+65 −63[10] K |
| GJ 3991 B | |
| Mass | ≳0.5[11] M☉ |
| Surface gravity (log g) | 8.0+1.00 −0.13[11] cgs |
| Temperature | 5286+788 −91[11] K |
| Rotation | >100 days[11] |
| Age | (as white dwarf) ~4[11] Gyr |
| Other designations | |
| GJ 3991, HIP 83945, G 203-47, WD 1708+437, USNO 752[5] | |
| Database references | |
| SIMBAD | data |
| ARICNS | data |
White dwarf
GJ 3991 B was first identified in 1997 by astronomers I. N. Reid and J. E. Gizis through significant radial velocity variations, although were unable to identify the nature of the secondary object.[12] In 1998, another group of astronomers was able to determine the secondary as a cold white dwarf star,[8] the compact remnant that remains after a low-mass star such as the Sun is no longer able to fuse elements for energy. It was directly detected for the first time, and spectroscopically confirmed as a white dwarf, in 2026.[11] GJ 3991 B is the 9th nearest white dwarf, after Sirius B, Procyon B, Van Maanen 2, Gliese 440, 40 Eridani B, Stein 2051 B, G 240-72, and LP 658-2.[13] Despite the close orbit of the white dwarf, it is not tidally locked, with a slow rotation period likely exceeding 100 days.[11]
Some sources call the component stars A & B,[12][13] while others call them Aa & Ab.[8]