Friday 29 June 2012

Two new views of τ Boötis b.

τ Boötis is a binary star system roughly 50 light years from Earth in the constellation of Boötis; the system (or more accurately its primary star) is naked-eye visible from Earth on a dark night. The primary star (τ Boötis A) is a Yellow-White K-type dwarf star with 1.3 time the mass of the Sun and an effective surface temperature of 6360 K (compared to 5778 K for our Sun). This is orbited by a Red Dwarf companion (τ Boötis B) with a mass 40% of the Sun's at a distance of 240 AU (i.e. 240 times the average distance between the Earth and the Sun. In 1997 a Hot-Jupiter type planet was discovered orbiting τ Boötis A every 3.3 days at a distance of 0.0481 AU; this planet was named τ Boötis b.

The position of τ Boötis (here written as Tau Bootis) in the constellation of Boötes.

τ Boötis b does not transit (pass in front of) τ Boötis A when seen from Earth; it was detected by the tiny wobble that its gravity causes in the motion of the star. This means that only a rough estimation of the planets mass could be made, and nothing determined about its atmospheric chemistry. However techniques in planetary science have moved on since the 1990s, and it is now in theory possible to undertake a spectrographic analysis of the atmosphere of a non-transiting planet, and to determine some of the other properties of such a planet by measuring the Doppler Shifting of these spectra, particularly that produced by the gas Carbon Monoxide. on 27 June 2012 two papers were published on the online arXiv database at Cornell University Library detailing different analyses of data on the τ Boötis System obtained by the Very Large Telescope, located at the European Southern Observatory on Cerro Paranal, Chile.

In the first of these papers a team of scientists lead by Matteo Brogi of the Leiden Observatory at Leiden University, conclude that the orbit of τ Boötis b is inclined at an angle of 44.5° relative to the Earth, and that the planet has a mass 5.95 times that of Jupiter. They also conclude that the atmosphere of τ Boötis b lacks a thermal inversion; that is to say it cools steadily at higher altitudes rather than having cloud-trapping warmer and cooler layers as on Earth. Such thermal inversions have been detected on other Hot-Jupiter type planets; Brogi et al. theorize that τ Boötis b may have lost the heat absorbing compounds (such as water and methane) that cause such inversions due to high levels of ultra-violet irradiation from τ Boötis A.

Schematic representation of the orbit of τ Boötis b about τ Boötis A. The arrow points towards Earth. The star and the orbit are to scare; the planet has been magnified ×3 for clarity. Brogi et al. (2012).

In the second paper Florian RodlerMercedes Lopez-Morales and Ignasi Ribas, of the Institut de Ciències de l’Espai at the Universitat Autònoma de Barcelona, conclude that the orbit of τ Boötis b is inclined at an angle of 47° relative to the Earth, and that the planet has a mass 5.6 times that of Jupiter. They furthermore conclude the atmosphere has an average temperature in excess of 1800 K (compared to 287.2 K for Earth or 735 K for Venus).

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