| Observation data EpochJ2000 EquinoxJ2000 | |
|---|---|
| Constellation | Fornax |
| Right ascension | 03h 35m 28.65156s[2] |
| Declination | −25° 44′ 21.7656″[2] |
| Apparent magnitude (V) | 17.0 - 20.5[3] |
| Characteristics | |
| Spectral type | D + M4.5V[4] |
| Variable type | AM Her +eclipses[5] |
| Astrometry | |
| Radial velocity (Rv) | 150[6] km/s |
| Proper motion (μ) | RA: 18.137[2]mas/yr Dec.: −1.470[2]mas/yr |
| Parallax (π) | 4.1736±0.0742 mas[2] |
| Distance | 780 ± 10 ly (240 ± 4 pc) |
| Orbit[7] | |
| Period (P) | 0.087865437 d (126.526229 min) |
| Semi-major axis (a) | 3.67×10−3 AU (5.49×105 km) |
| Inclination (i) | 81° |
| Details[8] | |
| White dwarf | |
| Mass | 0.71 M☉ |
| Radius | 0.011 R☉ |
| M dwarf | |
| Mass | 0.14 M☉ |
| Radius | 0.20 R☉ |
| Other designations | |
| EXO 033319-2554.2[6] | |
| Database references | |
| SIMBAD | data |
UZ Fornacis (abbreviated asUZ For) is abinary star in theconstellation ofFornax. It appears exceedingly faint with a maximum apparent magnitude 17.0.[3] Its distance, as measured byGaia using theparallax method, is about 780 light-years (240 parsecs).[2]
The system consists of two stars, awhite dwarf and ared dwarf, in close orbit around each other. It is hypothesized that there are also two planets orbiting the central stars.[4]
The system is most commonly referred to as UZ Fornacis, which is itsvariable star designation. TheGeneral Catalogue of Variable Stars describes it as "E+XM", meaning it is aneclipsing binary system consisting of a low-mass star with anX-ray-emitting companion.[9] In the past the system has also been referred to using the designationEXO 033319–2554.2, which refers to its coordinates on the celestial sphere, as well as theEXOSAT satellite that detected it.[6]
UZ Fornacis is acataclysmic variable. The two stars, a white dwarf and red dwarf, orbit each other every 127 minutes.[7] The stars' orbit is inclined about 81 degrees away from the plane-of-sky, so the systemeclipses. The eclipsing nature of this system was first discovered in 1987.[10] At the time, it was the 14thAM Herculis star known and only the third system known to eclipse.[11]
In systems like UZ Fornacis, matter is siphoned off the red dwarf and towards the white dwarf. However, unlike typical cataclysmic variable where this matter forms anaccretion disk, the white dwarf is highly magnetic and has a strongmagnetic field. This magnetic field channels the matter into loops that eventually accrete onto the white dwarf. When this happens, the matter emitscyclotron radiation and soft X-rays.[7] Due to the activity of the red dwarf, sometimes more mass gets transferred and X-ray flare-ups occur.[12]
Matter flows onto a spot on the white dwarf, at a rate of1×10−4 to 1 grams per square centimeter per second.[13] The white dwarf's magnetism also locks its rotation so it matches the orbit.[13]
The brightness of UZ Fornacis varies rapidly and somewhat unpredictably. The two stars in the system eclipse each other regularly. The eclipses last for about380 s, with the initial drop in brightness and return to maximum brightness each taking about3 s. The eclipselight curves do not all have the same shape, some being more or less flat-bottomed while others show a smooth variation in brightness, and some are asymmetrical. The times of the eclipses vary, possibly due to substellar companions.[4] Outside of the eclipses, the brightness varies during the orbit depending on the visibility of an accretion spot on the white dwarf.[14]
The brightness also varies over a period of years due to differences in the rate of accretion onto the white dwarf from the red dwarf. This can generally be seen as a bright state and a faint state, although the magnitudes of each state vary. For example, UZ Fornacis has been observed between magnitudes 15.9 and 16.75 at different times in the bright state.[15] The system also shows rapid "flickering" on a timescale of minutes, common in cataclysmic variable systems.[4]
Investigations in 2010 and 2011 found that theorbital period of the two stars in UZ Fornacis varied cyclically. Researchers attributed this to two possiblegas giant sized planets around the two stars, perturbing their orbits and causing the orbital period to vary.[16]
As of 2019, there is not enough information to explain all of the period variations,[4] since the planets would have to be in eccentric orbits to fit the data, and that would cause the orbits to be dynamically unstable. It is possible that there are even more planets causing additional perturbation, or some physical effect such as theApplegate mechanism is responsible for the eclipse timing variations.[4]
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| c | 10.00 MJ | 5.7 | 5355 | 0.69 | — | — |
| d(unconfirmed) | 3.22MJ | 3.0 | 2124 | 0.45 | — | — |
