GJ 3991

GJ 3991 (also known as Gliese 3991 and G 203-47) is a binary star system located 24.2 light-years away in the constellation Hercules. 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 14.71 days. Due to their small separation, the two objects have never been visually resolved and are merely predicted from the radial velocity changes of GJ 3991 A, making the system a spectroscopic binary.

Gliese 3991
Observation data
Epoch J2000.0      Equinox 2015.5
Constellation Hercules
GJ 3991 A
Right ascension  17h 09m 32.018s[1]
Declination +43° 40′ 48.52″
Apparent magnitude (V) 13.671
GJ 3991 B
Right ascension  17h 09m 32.018s
Declination +43° 40′ 48.52″
Apparent magnitude (V) >15
Characteristics
GJ 3991 A
Spectral type M3.5V
Apparent magnitude (B) 13.46
Apparent magnitude (R) 11.511
Apparent magnitude (J) 7.380
Apparent magnitude (H) 6.76
Apparent magnitude (K) 6.485
GJ 3991 B
Evolutionary stage White dwarf
Spectral type D?12
Astrometry
Radial velocity (Rv)−9.7±0.2 km/s
Total velocity17.971±0.199 km/s
Proper motion (μ) RA: 332.59±1.01[1] mas/yr
Dec.: −271.83±1.11 mas/yr
Parallax (π)134.597 ± 0.489[1] mas
Distance24.23 ± 0.09 ly
(7.43 ± 0.03 pc)
Absolute magnitude (MV)14.316
Orbit[2]
PrimaryGJ 3991 B
CompanionGJ 3991 A
Period (P)14.7136±0.0005 days
(0.0402836±0.0000014 yr)
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
Mass0.20 M☉
Temperature3250±50 K
Metallicity1.584+0.235
−0.205 Fe/☉
Metallicity [Fe/H]0.20±0.06 dex
GJ 3991 B
Mass0.50 M☉
Temperature~4900 K
Age>6? Gyr
Other designations
G 203-47, Gliese 3991, HIP 83945, USNO 752
GJ 3991 A: G 203-47 A
GJ 3991 B: G 203-47 B, WD 1708+437
Database references
SIMBADdata

White dwarf

GJ 3991 B was first identified in 1997 by astronomers I. N. Reid and J. E. Gizis through significant radial velocity variations visible through GJ 3991 A, although were unable to identify the nature of the secondary object.[3] In 1998, another group of astronomers was able to determine the secondary as a cold white dwarf star, the compact remnant that remains after a low-mass star such as our Sun is no longer able to fuse elements for energy. GJ 3991 B is the 9th nearest white dwarf, after Sirius B, Procyon B, Van Maanen 2, LP 145-141, 40 Eridani B, Stein 2051 B, G 240-72, and Gliese 223.2. Among these, GJ 3991 B is not only the coldest, but the only one in a short-period orbit with another star. GJ 3991 B is probably over 6 billion years old, making it the oldest among these objects as well.[4]

It is unknown exactly how GJ 3991 A and B are in such a tight orbit, as normally an object massive enough to create GJ 3991 B so early in the universe's history would destroy any objects orbiting less than 1-2 AU from it. GJ 3991 A, however, orbits only 0.11 AU from the white dwarf. It is possible that it migrated inwards after GJ 3991 B's red giant phase from a much further orbit.

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See also

References
  1. Lindegren, L.; et al. (25 April 2018). "Gaia Data Release 2: The astrometric solution". Astronomy & Astrophysics. A2: 616. arXiv:1804.09366. Bibcode:2018A&A...616A...2L. doi:10.1051/0004-6361/201832727.
  2. Delfosse, X.; Forveille, T.; Beuzit, J.-L.; Udry, S.; Mayor, M.; Perrier, C. (1 December 1998). "New neighbours. I. 13 new companions to nearby M dwarfs". Astronomy & Astrophysics. 334: 897. arXiv:astro-ph/9812008. Bibcode:1999A&A...344..897D.
  3. Reid, I. Neill; Gizis, John E. (June 1997). "Low-Mass Binaries and the Stellar Luminosity Function". The Astronomical Journal. 113: 2246. Bibcode:1997AJ....113.2246R. doi:10.1086/118436. ISSN 0004-6256.
  4. Bergeron, Pierre. "Synthetic Colors and Evolutionary Sequences of Hydrogen- and Helium-Atmosphere White Dwarfs". Retrieved 22 June 2018.
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