HU Aquarii#Eclipse timing variations
{{short description|Star in the constellation Aquarius}}
{{Starbox begin
| name = HU Aquarii
}}
{{Starbox image
| image = 250px
| caption = The visual band light curve of HU Aquarii, adapted from Shengbang et al. (2016)
}}
{{Starbox observe
| epoch = J2000
| constell = Aquarius
| ra = {{RA|21|07|58.19445}}{{cite DR2|6911950900211768704}}
| dec = {{DEC|-05|17|40.5577}}
| appmag_v = +15.3
}}
{{Starbox character
| class = DAm / M4.5V
| variable = AM Herculis
}}
{{Starbox astrometry
| radial_v =
| parallax = 5.2354
| p_error = 0.0457
}}
{{Starbox orbit
| period_unitless = 2.08 hours
| axis_unitless = 0.0032 au
| eccentricity = 0.0
| inclination = 85
}}
{{Starbox detail
| mass = 0.88/0.2
| radius = 0.01/0.22
| luminosity = 0.0022/0.0052
| temperature = 12,500/3,400
}}
{{Starbox catalog
| names = HU Aqr, RBS 1724, RE J210755-051621, 1RXS J210758.5-051744, 1AXG J210759-0518, 2RE J210755-051630, RE J2107-051, EUVE J2108-05.2, 2RE J2107-051, RX J2107.9-0518, AAVSO 2102-05, GSC 05200-00849, RE J2107-05, RX J2107.9-0517, 2MASS J21075818-0517404[http://simbad.u-strasbg.fr/simbad/sim-id?Ident=V*+HU+Aqr V* HU Aqr -- CV of AM Her type (polar)]
}}
{{Starbox reference
| Simbad = HU+Aqr
}}
{{Starbox end}}HU Aquarii (abbreviated HU Aqr) is an eclipsing binary system approximately 620 light-years away from the Sun, forming a cataclysmic variable of AM Herculis-type. The two stars orbit each other every 2.08 hours{{cite journal | arxiv=astro-ph/0005435| author=Howell| display-authors=etal| title=An Exploration of the Paradigm for the 2-3 Hour Period Gap in Cataclysmic Variables| journal=The Astrophysical Journal|volume=550|issue=2| pages=897–918| date=2002| bibcode=2002MNRAS.336.1129B|doi = 10.1046/j.1365-8711.2002.05841.x | doi-access=free| s2cid=609324}} and the ultra-short binary system includes an eclipsing white dwarf and red dwarf.
In 1993, Axel Schwope et al. discovered that the star, then called RXJ2107.9-0518, was both an eclipsing binary and a cataclysmic variable. The star had been flagged as a probable AM Herculis star from data collected during the ROSAT All Sky Survey. It was given its variable star designation, HU Aquarii, in 1995.
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 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.{{cite journal | arxiv=0901.4902| author=Schwarz| display-authors=etal| title=Hunting high and low: XMM monitoring of the eclipsing polar HU Aquarii| journal=Astronomy and Astrophysics|volume=496|issue=3| pages=833–840| date=2009| bibcode=2009A&A...496..833S|doi = 10.1051/0004-6361/200811485 | s2cid=14243402}} In March 2011, Qian et al. claimed the presence of two planets,{{cite journal|author1=Qian, S.-B. |author2=Liu, L. |author3=Liao, W.-P. |author4=Li, L.-J. |author5=Zhu, L.-Y. |author6=Dai, Z.-B. |author7=He, J.-J. |author8=Zhao, E.-G. |author9=Zhang, J. |author10=Li, K. |title=Detection of a planetary system orbiting the eclipsing polar HU Aqr|date=2011|journal=Monthly Notices of the Royal Astronomical Society|volume=414|issue=1|pages=L16–L20|doi=10.1111/j.1745-3933.2011.01045.x|doi-access=free |bibcode=2011MNRAS.414L..16Q|arxiv=1103.2005|s2cid=119165158 }} though the proposed orbits were subsequently shown to be unstable{{cite journal|last=Horner|first=Jonathan|author2=Marshall, J. P. |author3=Wittenmyer, Robert A. |author4= Tinney, C. G. |title=A dynamical analysis of the proposed HU Aquarii planetary system|journal=Monthly Notices of the Royal Astronomical Society: Letters|date=September 2011|volume=416|issue=1|pages=L11–L15|doi=10.1111/j.1745-3933.2011.01087.x|doi-access=free |bibcode = 2011MNRAS.416L..11H |arxiv = 1106.0777 |s2cid=59135141}} and on orbits determined from a more thorough and precise analysis of the observational data.{{cite journal|last=Wittenmyer|first=Robert|author2=Horner, J. |author3=Marshall, J. P. |author4=Butters, O. W. |author5= Tinney, C. G. |title=Revisiting the proposed planetary system orbiting the eclipsing polar HU Aquarii|journal=Monthly Notices of the Royal Astronomical Society |date=February 2012 |bibcode=2012MNRAS.419.3258W |doi=10.1111/j.1365-2966.2011.19966.x |arxiv=1110.2542 |volume=419 |issue=4|pages=3258–3267|doi-access=free |s2cid=49524325}} 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.{{cite journal|title=Testing the planetary models of HU Aquarii|author1=M. Bours |author2=T. Marsh |author3=E. Breedt |author4=C. Copperwheat |author5=V. Dhillon |author6=A. Leckngam |author7=S. Littlefair |author8=S. Parsons |author9=A. Prasit |date=2014|arxiv=1409.3586 |doi=10.1093/mnras/stu1879 |volume=445 |issue=2 |journal=Monthly Notices of the Royal Astronomical Society |pages=1924–1931|doi-access=free |bibcode = 2014MNRAS.445.1924B |s2cid=53630677 }} With new data gathered by 2018, the existence of planetary system remains elusive, as no model fitting all observations can be made.{{citation|arxiv=1810.12886|title=On the ephemeris of the eclipsing polar HU Aquarii. II: New eclipse epochs obtained 2014–2018|year=2018|doi=10.1002/asna.201813526|last1=Schwope|first1=A. D.|last2=Thinius|first2=B. D.|journal=Astronomische Nachrichten|volume=339|issue=7–8|pages=540–544|bibcode=2018AN....339..540S|s2cid=96446931}}
See also
References
{{reflist| refs=
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{{Sky|21|07|58.29|-|05|17|39.4}}
{{Stars of Aquarius}}
{{DEFAULTSORT:HU Aquarii}}
Category:Aquarius (constellation)
Category:Hypothetical planetary systems
Category:M-type main-sequence stars