Thursday, 29 December 2011

Planets in the NY Virginis system.

NY Virginis is an eclipsing binary system roughly 2000 light years from the Earth in the constellation of Virgo. The name NY Virginis implies the 258th variable star in the constellation of Virgo. The system is also known as PG1336-018, where the PG stands for 'Palomar Green'; it was discovered during the Palomar Green survey.

The system is made up of two stars referred to as M₁ and M₂. M₁ has a mass of approximately 46% of that of our sun, but is over six times as hot (33 000 K as opposed to 5578 K for our sun). It is an 'Extreme Horizontal Branch' blue-white subdwarf-star, where the 'Extreme Horizontal Branch' refers to the position on a graph where colour/temperature is plotted against temperature. Stars of this type have run out of hydrogen in their core and expanded to form Red Giant stars, in which helium is fused in the core and hydrogen in the outer layers, then lost these outer layers for some reason (probably in this case interaction with M₂), exposing the helium-fusing core.

Orbiting this at a distance of slightly under 5 million km is M₂, a Red Dwarf star with 14% of the mass of our sun and a temperature of about 3000 K. Red Dwarfs are small, cool stars which do not get particularly hot due to their low mass, but subsequently burn for much longer than larger stars.

This month a study of the system by a team lead by SB Qian of the Yunnan Observatory was published in a paper on the arXiv database at Cornell University Library. Qian et al. combined data from an earlier study of the system by the South African Astronomical Observatory with new data collected by the Jorge Sahade Telescope in Argentina and the Yunnan Observatory.

This study was able to detect irregularities in the orbit of the two stars, which they used to calculate the existence of a planet with a mass of 2.3 times that of Jupiter, orbiting at a distance of 3.3 AU, that is to say 3.3 times the distance at which the Earth orbits the Sun, or twice the distance at which Mars orbits, every 2900 days. Qian et al. refer to this planet as M₃, but it is referred to elsewhere as NY Virginis b, or NY Vir b. This is inaccurate; using conventional numbering for the system M₁ would be NY Virginis A, M₂ would be NY Virginis B and M₃ would be NY Virginis c; since all bodies in the system are lettered, stars with a capitol and planets with a lower case letter.

The inclusion of M₃ in the model still does not completely resolve the irregularities in the two stars orbits. Qian et al. therefore infer a second planet, with a mass of about 2.5 times that of Jupiter, orbiting roughly once every 15 Earth years.

An artist's impression of a binary system with two planets. By scientific illustrator Mark A. Garlick.

Qian et al. also attempt to model the history of the NY Virginis system. As noted above, M₂ orbits M₁ at a distance of only 5 million km, but it cannot have been this close throughout the history of the system. Qian et al. calculate that if M₁ had originally had a mass the same as our sun (which is an arbitrary figure, but works for modeling the evolution of the system), then it would have grown till its radius was roughly 0.5 AU - half the radius of Earth's orbit, or slightly greater than the orbit of Mercury. In this model if M₂ had originally orbited M₁ at a distance of 0.8 AU (a slightly greater distance than that at which Venus orbits the sun) then once M₁ reached a radius of 0.5 AU, then M₂ would have started to tear away the outer atmosphere of M₁. This would have slowed down M₂ in its orbit, causing it to spiral inwards towards M₁, stabilizing in its current orbit once the outer atmosphere of M₁ was used up.

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