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Interacting binary stars & astronomical instrumentation

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Vik Dhillon, Stuart Littlefair, Dave Sahman, Martin McAllister, Liam Hardy, Martin Dyer, Somsawat Rattanasoon

Interacting Binary Stars

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Some of the most interesting binary stars are close pairings, where one compact component (typically a white dwarf, neutron star or black hole) is accreting mass from the other component (typically a white dwarf, brown dwarf or main-sequence star). These interacting binary stars (which include Cataclysmic Variables (CVs) and X-ray Binaries) are responsible for some of the most exotic phenomena observed in the Universe, such as Type Ia Supernovae, short Gamma-Ray bursts, millisecond pulsars and micro-quasars. At Sheffield, we investigate these objects using a series of high-speed cameras and robotic telescopes that we have developed, as described below.

Astronomical Instrumentation

The ancient Greeks believed the Universe to be perfect and unchanging, whereas we now know that astronomical objects vary in brightness on timescales ranging from milliseconds to billions of years. Although astronomy has made great strides in recent years, the study of the most rapidly varying phenonema (on timescales of milliseconds to seconds) has been largely ignored. To address this situation, we have built a series of high-speed cameras (ULTRACAM, ULTRASPEC, HiPERCAM) and mounted them on the world's largest telescopes to study astronomical objects which eclipse, transit, occult, flicker, flare, pulsate, oscillate, erupt, outburst or explode, thereby opening up a new region of observational parameter space for discovery.

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ULTRACAM is an ultra-fast, triple-beam imaging photometer, which we use at the 4.2-m William Herschel Telescope (WHT) on La Palma, the 8.2-m Very Large Telescope (VLT) in Chile, and the 3.5m New Technology Telescope (NTT) in Chile. The instrument was built by a consortium involving the Universities of Sheffield and Warwick, and the UK Astronomy Technology Centre (UKATC), Edinburgh. ULTRACAM currently leads the world in the field of high-speed optical astrophysics and has been used to study white dwarfs, brown dwarfs, red dwarfs, pulsars, black-hole/neutron-star X-ray binaries, gamma-ray bursts, cataclysmic variables, eclipsing binary stars, extrasolar planets, active galactic nuclei, asteroseismology and occultations by Solar System objects (Titan, Pluto and Kuiper Belt objects).

ULTRASPEC is a high-speed camera employing a frame-transfer electron-multiplying CCD (EMCCD) and the data acquisition system of ULTRACAM. The project is a collaboration between the Universities of Sheffield, Warwick and the UKATC. ULTRASPEC is permanently mounted on the 2.4m Thai National Telescope (TNT), providing Sheffield and Warwick with 25 nights per year of time to pursue a programme of research into high-speed astrophysics.

HiPERCAM is our latest high-speed camera for the study of rapid variability in the Universe. The project is funded by a 3.5 MEuro European Research Council Advanced Grant. HiPERCAM will be a major advance on ULTRACAM and ULTRASPEC. It will be able to image simultaneously in 5 optical channels - u' g' r' i' z' - at rates of over 1000 frames per second. The detectors will have a FoV of 10' and will employ custom-made frame-transfer CCDs, with 4 low noise outputs (2.5e-) and ~180K cooling giving essentially zero dark current. The two reddest CCDs will be deep-depletion devices with anti-etaloning, providing high quantum efficiencies across the optical spectrum with no fringing. The instrument will also incorporate scintillation noise correction via the conjugate-plane photometry technique. HiPERCAM will see first light on the WHT in 2017, and we then hope to mount it on the largest telescope in the world: the 10.4m Gran Telescopio de Canarias (GTC) on La Palma.