HU Aquarii
Eclipsing binary with disputed eclipse timing variations.
HU Aquarii, also known as HU Aqr, is a binary star system located about 623 light-years from Earth. It belongs to the AM Herculis class of cataclysmic variables and is an eclipsing binary, meaning one star periodically blocks the light of the other. The two stars—a white dwarf and a red dwarf—circle each other in just 2.08 hours, an extremely short orbital period.
The system was first identified as a probable AM Herculis star from data gathered by the ROSAT All Sky Survey. In 1993, Axel Schwope and colleagues confirmed it was both an eclipsing binary and a cataclysmic variable, at the time calling it RXJ2107.9-0518. It received its official variable star designation, HU Aquarii, in 1995.
The timing of the system’s eclipses varies, and these fluctuations have been studied extensively. Schwarz et al. (2009) found the variations too large to be explained by the Applegate mechanism, consistent with magnetic braking but about 30 times greater than what gravitational radiation alone would produce. They proposed that an orbiting object might be pulling the binary back and forth along our line of sight. In 2011, Qian et al. suggested two planets were responsible, but those orbits were later shown to be unstable when analyzed more thoroughly. As of 2014, all proposed planetary solutions had been ruled out, and the full set of timing data could not be fit by any one-, two-, or three-planet system due to poor fits and unstable configurations. By 2018, with additional observations, no model matching all the data had been found, and the existence of planets remains unconfirmed. The observed variations may instead stem from poorly understood interactions between the white dwarf and its red dwarf companion.
- Distance
- approximately 623 light-years
- Orbital period
- 2.08 hours
- Type
- eclipsing binary, cataclysmic variable (AM Herculis-type)
- Components
- white dwarf and red dwarf
- Discovery year
- 1993
- Discoverers
- Axel Schwope et al.
- Variable star designation year
- 1995
Lore & Background
HU Aquarii was first identified as a probable AM Herculis star from data collected during the ROSAT All Sky Survey. In 1993, Axel Schwope et al. discovered that the star, then called RXJ2107.9-0518, was both an eclipsing binary and a cataclysmic variable. It received its variable star designation, HU Aquarii, in 1995.
The system exhibits variations in the timing of its eclipses. Schwarz et al. (2009) noted that these variations are too large to be caused by the Applegate mechanism, are within the expected range of magnetic braking, but are 30 times too large to be caused by gravitational radiation alone. They suggested an alternative explanation: an object in orbit around the binary, causing it to move back and forth along the line-of-sight.
In March 2011, Qian et al. claimed the presence of two planets, but the proposed orbits were subsequently shown to be unstable. As of 2014, all proposed orbital solutions had been refuted, and the full set of timing measurements appeared inconsistent with any 1, 2, or 3-planet system due to poor fits and unstable configurations. By 2018, with new data, the existence of a planetary system remained elusive, as no model fitting all observations could be made.
Reader's Guide
HU Aquarii holds significance as a test case for detecting exoplanets via eclipse timing variations in cataclysmic variable binaries. The system's eclipse timing variations were initially interpreted as evidence for planetary companions, but subsequent analyses revealed that the proposed orbits were unstable and the data inconsistent with any multi-planet model. The controversy highlights the difficulty of distinguishing genuine planetary signals from intrinsic binary interactions, such as poorly understood magnetic interactions between the white dwarf and red dwarf. As of 2018, no model could fit all observations, leaving the existence of planets unconfirmed. The system thus serves as a cautionary example in the search for circumbinary planets, emphasizing the need for long-term, high-precision timing data and careful modeling of stellar processes. Its legacy is one of unresolved debate, underscoring the complexity of interpreting timing variations in close binaries.
Did You Know?
- HU Aquarii is an eclipsing binary system approximately 623 light-years away.
- The two stars orbit each other every 2.08 hours.
- As of 2018, no planetary model could fit all eclipse timing observations.
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