Abstract
Most of our knowledge of extrasolar planets rests on precise radial-velocity measurements, either for direct detection or for confirmation of the planetary origin of photometric transit signals. This has limited our exploration of the parameter space of exoplanet hosts to solar- and later-type, sharp-lined stars. Here we extend the realm of stars with known planetary companions to include hot, fast-rotating stars. Planet-like transits have previously been reported in the lightcurve obtained by the SuperWASP survey of the A5 star HD15082 (WASP-33; V=8.3, v sin i = 86 km/sec). Here we report further photometry and time-series spectroscopy through three separate transits, which we use to confirm the existence of a gas giant planet with an orbital period of 1.22d in orbit around HD15082. From the photometry and the properties of the planet signal travelling through the spectral line profiles during the transit we directly derive the size of the planet, the inclination and obliquity of its orbital plane, and its retrograde orbital motion relative to the spin of the star. This kind of analysis opens the way to studying the formation of planets around a whole new class of young, early-type stars, hence under different physical conditions and generally in an earlier stage of formation than in sharp-lined late-type stars. The reflex orbital motion of the star caused by the transiting planet is small, yielding an upper mass limit of 4.1 Jupiter masses on the planet. We also find evidence of a third body of sub-stellar mass in the system, which may explain the unusual orbit of the transiting planet. In HD 15082, the stellar line profiles also show evidence of non-radial pulsations, clearly distinct from the planetary transit signal. This raises the intriguing possibility that tides raised by the close-in planet may excite or amplify the pulsations in such stars.
Highlights
More than 400 extrasolar planets have been discovered orbiting solar- and later-type stars, but very little is known so far about planets orbiting intermediate-mass main-sequence stars
Transit signals have been reported for main-sequence A– F-type stars, suggesting the possible presence of close-in planets (Christian et al 2006; Clarkson et al 2007; Lister et al 2007; Street et al 2007; Kane et al 2008), but measuring the small reflex motion that would confirm their planetary origin is generally not possible for these line-poor, fast-rotating stars
A partial transit was observed in the R band on 2006 November 13 using the CCD camera of the 0.95-m James Gregory Telescope (JGT) at the St Andrews University Observatory
Summary
More than 400 extrasolar planets have been discovered orbiting solar- and later-type stars, but very little is known so far about planets orbiting intermediate-mass main-sequence stars. We know very little yet about the frequency of giant-planet formation around earlier type stars, with their harder radiation fields, more distant snow lines and greater disc masses. The travelling spectral signature of a planet transiting the disc of the star during the light minimum yields information on the properties of the planet and its orbit. This complements or supplements that derived from standard photometric analyses, notably its size, retrograde orbit and non-alignment of the stellar and orbital spin axes
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