HU Aquarii (abbreviated HU Aqr) is an

eclipsing binary A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

system approximately 620

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away from the Sun, forming a

cataclysmic variable In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...

of AM Herculis-type. The two stars orbit each other every 2.08 hours and the ultra-short binary system includes an eclipsing

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

and

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave since 2009, gaining a cult following. T ...

Eclipse timing variations

The HU Aquarii binary system exhibits variations in the timing of the eclipses. Schwarz et al. (2009) note that the variations are too large to be caused by the Applegate mechanism, and are within the expected range of magnetic braking but 30 times too large to be caused by

gravitational radiation Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...

alone. As an alternative explanation, they suggested that the variations might be caused by an object in orbit around the binary, causing it to move back and forth along the line-of-sight to the system. In March 2011, Qian et al. claimed the presence of two planets, though the proposed orbits were subsequently shown to be unstable and on orbits determined from a more thorough and precise analysis of the observational data. Indeed, it seems likely that the signal that was interpreted as being the result of planetary companions might be instead the result of not well-studied interactions between the white dwarf and its red dwarf companion: as of 2014 all proposed orbital solutions have been refuted and it appears that the full set of timing measurements is inconsistent with any 1, 2 or 3-planet system due to poor fits to the data and unstable orbital configurations. With new data gathered by 2018, the existence of planetary system remains elusive, as no model fitting all observations can be made.

References

{{DEFAULTSORT:HU Aquarii Aquarius (constellation) Eclipsing binaries Hypothetical planetary systems Aquarii, HU White dwarfs M-type main-sequence stars Polars (cataclysmic variable stars) J21075818-0517404

Eclipse timing variations

See also

References