Friday, 7 June 2013

The planets of NN Serpentis.

The NN Serpentis system comprises a young binary pair of stars 1670 light years from Earth in the constellation of Serpens. The pair comprises a White Dwarf star with 53.5% of the Sun's mass and a Red Dwarf star with 11% of the Sun's mass orbiting at a distance of 1.5 AU (1.5 times the distance at which the Earth orbit's the Sun, or slightly under the radius of Mars's orbit). The pair form an eclipsing binary; they orbit one-another edge on when seen from Earth, so that they regularly eclipse one another every 3.12 hours. The system is probably only about a million years old, and it is likely that the main star was originally considerably larger, and shed matter under the smaller stars influence. This would have formed a common envelope around the two stars, with some of the mass eventually falling back, some settling on the smaller star and some being lost into space.

In 2009 a team of scientists led by Shengbang Qian of the Yunnan Astronomical Observatory and the Graduate University of the Chinese Academy of Sciences proposed that the system was orbited every 7.56 year, by a large planet (or possibly small Brown Dwarf), with a mass 1.4% of the Sun's at a distance of under 3.29 AU, in a paper published in The Astrophysical Journal.

A follow up study led by Klaus Beuermann of the Institut für Astrophysik at Georg-August-Universität published in the journal Astronomy & Astrophysics in 2010, based upon data from the MONET/North 1.2-m telescope at McDonald Observatory suggested that this was incorrect, and that the system was in fact orbited by two smaller (though still substantial) planets, but was not able to provide a confident estimate of the size and orbits of these. 

In a paper published on the online arXiv database at Cornell University Library on 28 May 2013, and in the journal Astronomy and Astrophysics on 29 May, Klaus Beuermann, along with Stefan Dreizler and Frederic Hessman, also of the Institut für Astrophysik at Georg-August-Universität, present the results of a more long term study with the MONET/North telescope, which resolves the masses and orbits of the planets of NN Serpentis.

Beuermann et al. conclude that the inner planet has a mass 1.74 times that of Jupiter, and orbits the primary star at a distance of 3.358 AU every 7.647 years, while the outer planet has a mass 6.96 times that of Jupiter, and orbits at a distance of 5.389 AU every 15.482 years. Furthermore the two planets appear to be locked into an orbital resonance, so that the inner planet completes two orbits for every single orbit of the outer planet.

The orbits of the planets of NN Serpentis. The locations of the periapses (point at which the planets are closes to the center of the system) are marked ‘P’, the solid dots indicate conjugation, and the open circles opposition. Orbital motion is counter-clockwise. Beuemann et al. (2013).


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