V723 Monocerotis
Variable star in Monoceros
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V723 Monocerotis is a binary star in the constellation Monoceros. It is an ellipsoidal variable about 1,500 light-years distant, too faint to be seen to the naked eye, with a variable apparent magnitude between 8.21 and 8.42.
| Observation data Epoch J2000.0 Equinox ICRS | |
|---|---|
| Constellation | Monoceros |
| Right ascension | 06h 29m 04.659s[2] |
| Declination | −05° 34′ 20.23″[2] |
| Apparent magnitude (V) | 8.21 - 8.42[3] |
| Characteristics | |
| Spectral type | G0II[4] |
| Variable type | Ellipsoidal[5] |
| Astrometry | |
| Proper motion (μ) | RA: −1.347 mas/yr[2] Dec.: 16.140 mas/yr[2] |
| Parallax (π) | 2.1748±0.0331 mas[2] |
| Distance | 1,500 ± 20 ly (460 ± 7 pc) |
| Absolute magnitude (MV) | +0.93[6] |
| Orbit[1] | |
| Period (P) | 59.9398 d |
| Eccentricity (e) | 0 (fixed) |
| Inclination (i) | 87.0+1.7 −1.4° |
| Argument of periastron (ω) (secondary) | 0 (fixed)° |
| Details[5] | |
| Stripped giant star | |
| Mass | 0.44±0.06 M☉ |
| Radius | 22.5±1.0 R☉ |
| Luminosity | 105 L☉ |
| Surface gravity (log g) | 1.3 cgs |
| Temperature | 3,800±100 K |
| Metallicity [Fe/H] | −0.5 dex |
| Rotational velocity (v sin i) | 15±2 km/s |
| Age | 5.4+5.1 −2.6[1] Gyr |
| Subgiant | |
| Mass | 2.8±0.3 M☉ |
| Radius | 8.3±0.4 R☉ |
| Luminosity | 70 L☉ |
| Surface gravity (log g) | ~3 cgs |
| Temperature | 5,800±200 K |
| Metallicity [Fe/H] | −0.5 dex |
| Rotational velocity (v sin i) | 70±10 km/s |
| Other designations | |
| V723 Mon, BD−05 1649, HD 45762, HIP 30891, SAO 133321, PPM 189220[7] | |
| Database references | |
| SIMBAD | data |
It was proposed in 2021 to be a binary system including a lower mass gap black hole candidate nicknamed "The Unicorn".[1] Located 1,500 light years from Earth, it would be the closest known black hole, and among the smallest ever found[8][9] with a mass 3 times the mass of the Sun, corresponding to a Schwarzschild radius of 9 kilometers.[10][11]
Follow-up work in 2022 argued that V723 Monocerotis does not contain a black hole, but is a mass-transfer binary containing a red giant that has been stripped of much of its mass, and a subgiant star.[5] The primary red giant star has 25 times the Sun's radius but an unusually low mass of 0.44 solar masses due to mass transfer to its partner. The secondary star is 2.8 times more massive than the Sun and 8.3 times larger, a subgiant approaching the base of the red giant branch. The primary is more than a 100 times as luminous as the Sun, while the secondary is about 70 times as luminous.[5]