Browsing by Author "O'Donoghue, D."
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Item Hot subdwarf stars in close-up view IV. Helium abundances and the 3He isotopic anomaly of subdwarf B stars(EDP Sciences, 2013) Geier, S.; Edelmann, H.; Kilkenny, David; O'Donoghue, D.; Marsh, T.R.; Copperwheat, C.Atmospheric parameters and helium abundances of 44 bright subdwarf B stars have been determined. More than half of our sample consists of newly discovered stars from the Edinburgh Cape survey. We showed that effective temperatures and surface gravities can be derived from high resolution echelle spectra with sufficient accuracy. Systematic uncertainties have been determined by comparing the parameters derived from the high resolution data with the ones derived from medium resolution spectra. Helium abundances have been measured with high accuracy. Besides the known correlation of helium abundance with temperature, two distinct sequences in helium abundance have been confirmed. Significant isotopic shifts of helium lines due to an enrichment in 3He have been found in the spectra of 8 subluminous B stars (sdBs). Most of these stars cluster in a small temperature range between 27 000K and 31 000K very similar to the known 3He-rich main sequence B stars, which also cluster in such a small strip, but at different temperatures. Both the helium sequences and the isotopic anomaly are discussed.Item Two new pulsating hot subdwarf stars from the Edinburgh-Cape survey(Oxford University Press, 2009) Kilkenny, David; O'Donoghue, D.; Crause, L.; Engelbrecht, C.; Hambly, N.; MacGillivray, H.We report the discovery of very rapid pulsations in two hot subdwarf stars from the Edinburgh- Cape blue object survey. The short periods, small amplitudes and multiperiodicity establish these stars as members of the class of rapidly-pulsating sdB stars. The spectrograms of both stars, however, show relatively strong He II 4686 and they are therefore more properly classified as sdOB. The light curve of EC 01541?1409 is dominated by two strong (?1 per cent) variations with frequencies near 7114 and 7870 ?Hz (periods near 140.6 and 127.1 s), though at least five frequencies are present with amplitudes above about 0.002 mag. The light curve of EC 22221?3152 appears to be generated by at least 10 frequencies in the range 5670-11850 ?Hz (about 175-85 s) with amplitudes between about 0.01 and 0.001 mag, including the first overtone of the strongest variation. Somewhat surprisingly, this number of frequencies is detectable in observing runs as short as 3 h, probably due to the fact that the detected frequencies are well-separated.