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Balona L.A.,South African Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2014

We consider the high mode density reported in the δ Scuti star HD 50844 observed by CoRoT. Using simulations, we find that extracting frequencies down to a given false alarm probability by means of successive pre-whitening leads to a gross overestimate of the number of frequencies in a star. This is due to blending of the peaks in the periodogram due to the finite duration of the time series. Pre-whitening is equivalent to adding a frequency to the data which is carefully chosen to interfere destructively with a given frequency in the data. Since the frequency extracted from a blended peak is not quite correct, the interference is not destructive with the result that many additional fictitious frequencies are added to the data. In data with very high signal-to-noise ratio, such as the CoRoT data, these spurious frequencies are highly significant. Continuous pre-whitening thus causes a cascade of spurious frequencies which leads to a much larger estimate of the mode density than is actually the case. The results reported for HD 50844 are consistent with this effect. Direct comparison of the power in the raw periodogram in this star with that in δ Scuti stars observed by Kepler shows that HD 50844 has a typical mode density. © 2014 The Author.


Balona L.A.,South African Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2014

Kepler photometry of A stars shows that a considerable fraction (about 19 per cent) have a peculiar feature in the periodogram. This feature consists of a broad peak, thought to be due to differential rotation in a spotted star, and a sharp peak at slightly higher frequency. The pattern clearly involves some widespread stellar property and the sharp peak implies a strictly coherent periodicity. We investigate the possibility that the periodicity is due to rotation, pulsation or an orbital effect.We argue that neither rotation nor pulsation can provide a suitable, testable, explanation. We suggest that the sharp feature could be due to a planet in synchronous orbit around the rapidly rotating, spotted A star, not necessarily in transit. Spectroscopic observations of sufficient precision are required to falsify this hypothesis. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.


Balona L.A.,South African Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2014

A method is presented whereby the orbital parameters of a pulsating star in a binary ormultiple system can be determined using the time delay due to the changing distance of the pulsating star. The method differs from previously published methods in that a direct periodogram of the orbital frequency is derived using all possible information to extract the time delay from the stellar pulsations. The method is easy to use provided the pulsation frequencies are known. The method is tested using various simulations and applied to two stars which have been previously analysed in the literature. Application to 34 Kepler objects of interest which are also δ Sct variables resulted in the detection of five stars which are binaries. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.


Miszalski B.,South African Astronomical Observatory | Mikolajewska J.,Nicolaus Copernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2014

We introduce the first results from an ongoing, systematic survey for new symbiotic stars selected from the AAO/UKST SuperCOSMOS Ha Survey. The survey aims to identify and characterize the fainter population of symbiotic stars under-represented in extant catalogues. The accreting white dwarfs (WDs) in symbiotic stars, fuelled by their red giant donors with high mass-loss rate winds, make them promising candidates for Type Ia supernovae. Several candidates were observed spectroscopically with the Southern African Large Telescope. A total of 12 bona fide and 3 possible symbiotic stars were identified. The most remarkable example is a rare carbon-rich symbiotic star that displays coronal [Fe X] λ 6375 emission, suggesting it may be a supersoft X-ray source with a massive WD. Several other emission line objectswith near-infrared colours similar to symbiotic stars are listed in an appendix, including six B[e] stars, four planetary nebulae (PNe), two possible Be stars, one [WC9] Wolf-Rayet (WR) central star of a PN and one WC9WR star. These initial discoveries will help shape and refine the candidate selection criteria that we expect will uncover several more symbiotic stars as the survey progresses. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Balona L.A.,South African Astronomical Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2013

We analyse over three years of Kepler photometry for the two roAp stars KIC 10483436 and KIC 10195926. Our aim is to detect possible unexpected low frequencies and to determine the frequency and amplitude variations of the roAp pulsations. Both stars have frequencies close to, but not exactly equal to, half the frequency of rotation with highly variable amplitudes. The origin of this variation is at present not known. In KIC 10483436, the main roAp pulsation is highly variable in frequency, but none of the other modes vary in this way. © 2013 The Author published by Oxford University Press on behalf of the Royal Astronomical Society.

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