Wednesday, 11 January 2012

The strange debris disk of 99 Herculis.

99 Herculis is a binary star system 51 light years from Earth in the constellation of Hercules. It comprises an F-type white dwarf star (99 Herculis A) with a mass 0.94 times that of our sun and a smaller K-type orange dwarf star (99 Herculis B) with a mass 0.49 times that of the sun, orbiting about their mutual centre of gravity on highly elliptical orbits. A debris disk (a ring of material orbiting a star system, similar to the Asteroid Belt or Kuiper Belt in our own solar system; not necessarily actually made up of debris) was detected about this system by the European Space Agency's Herschel Space Telescope in 2010.

Image of the 99 Herculis system from the W.M. Keck Observatory's NIRC2 instrument.

On 9 January 2011 a team lead by Grant Kennedy of the Institute of Astronomy at the University of Cambridge published a paper on the online arXiv database at Cornell University Library in which they detail the results of a new study of the 99 Herculis system, using data from the Herschel Space Telescope. plus new data gathered by the NIRC2 instrument at the W.M. Keck Observatory in Hawaii and the Spitzer Space Telescope.

This showed that the disk has a diameter of approximately 250 AU (that is to say 250 times the diameter of Earth's orbit, and does not orbit in the same plane as the two stars, but rather at an angle of about 90°. The ring has a total mass of about 10 times that of the Earth.

A model of the 99 Herculis system. The dotted rings are the orbits of the two stars, the grey circles the two stars in their current position; the lower is 99 Herculis A, the higher 99 Herculis B. The dotted line marked 'pericenter' is the line through the two points at which the stars are at their maximum distance from the center of the system, when the two stars are at their maximum distance they are 16.5 AU apart, or 16.5 times the distance at which the Earth orbits the Sun. The dashed line marked 'line of nodes' represents the line in which the debris ring orbits the system; the full extent of this is not shown, as the diameter is 250 AU. The scale bar represents 16.5 AU or 1.06" (1.06 arc-seconds) in the sky; the full circle of the sky (assuming you could see it without the Earth in the way) would be 360° (360 degrees); each degree is divided into 60' (60 minutes) and each minute into 60" (60 seconds).

Kennedy et al. also considered how the system could have formed. It is possible that the ring could have formed by the collision of two objects, though they would have to have been very large objects, since rings formed in this way tend to be unstable and lose mass over time. Therefore Kennedy et al. favor a model in which the ring formed at the same time as the stars, and gained its unusual alignment due to gravitational interaction with other stars in a dense cluster of star formation.

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