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A.E. Lynas-Gray 《Astrophysics and Space Science》2004,291(3-4):197-203
Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (T e ff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research. 相似文献
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A. E. Lynas-Gray C. Rodríguez-López D. Kilkenny 《Astrophysics and Space Science》2010,329(1-2):225-229
SDSS J160043.6+074802.9 appears to be a binary consisting of a subwarf-O (sdO) star and a late-type Main Sequence companion; the sdO is the only known pulsator in this class of stars and some modes have pulsation amplitude and frequency variations. Surface gravity and helium abundance determinations in the literature do not agree within quoted error limits; these appear to have unidentified systematic errors which, once accounted for, should show that surface gravity and helium abundance determinations to date are not as discordant as is currently supposed. Non-linear pulsation effects are proposed as a possible interpretation of pulsation amplitude and frequency variations observed in some modes. 相似文献
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C. Rodríguez-López A. E. Lynas-Gray D. Kilkenny J. MacDonald A. Moya C. Koen P. A. Woudt D. J. Wium B. Oruru E. Zietsman 《Monthly notices of the Royal Astronomical Society》2010,401(1):23-34
A spectroscopic analysis of Sloan Digital Sky Survey (SDSS) J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields T eff = 70 000 ± 5000 K and log g = 5.25 ± 0.30 , together with a most likely type of K3 V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. Therefore, it seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having log g > 5.3 to be more likely to be unstable and capable of driving pulsation in the observed frequency range. 相似文献
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Z. Han Ph. Podsiadlowski A. E. Lynas-Gray 《Monthly notices of the Royal Astronomical Society》2007,380(3):1098-1118
The discovery of a flux excess in the far-ultraviolet (FUV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the FUV excess of elliptical galaxies based on the binary model developed by Han et al. for the formation of hot subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces most of the properties of elliptical galaxies with a UV excess: the range of observed UV excesses, both in (1550 − V ) and (2000 − V ) , and their evolution with redshift. We also present colour–colour diagrams for use as diagnostic tools in the study of elliptical galaxies. The model has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals. 相似文献
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A. A. Gatti J. E. Drew R. D. Oudmaijer T. R. Marsh & A. E. Lynas-Gray 《Monthly notices of the Royal Astronomical Society》1998,301(2):33L-37L
Using the Planetary Camera on board the Hubble Space Telescope , we have measured the projected separation of the binary components in the nucleus of the planetary nebula Abell 35 to be larger than 0.08 arcsec but less than 0.14 arcsec. The system has been imaged in three filters centred at 2950, 3350 and 5785 Å. The white dwarf primary star responsible for ionizing the nebula is half as bright as its companion in the 2950-Å filter, causing the source to be visibly elongated. The 3350-Å setting, on the other hand, shows no elongation as a result of the more extreme flux ratio. The F300W data allow the determinination of the projected separation of the binary. At the minimum distance of 160 pc to the system, our result corresponds to 18 ± 5 au. This outcome is consistent with the wind accretion induced rapid rotation hypothesis, but cannot be reconciled with the binary having emerged from a common-envelope phase. 相似文献
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D. Kilkenny R. S. Stobie D. O'Donoghue C. Koen N. Hambly H. MacGillivray A. E. Lynas-Gray 《Monthly notices of the Royal Astronomical Society》2006,367(4):1603-1608
We report the discovery of very rapid pulsations in three sdB stars from the Edinburgh–Cape blue object survey. The short periods, small amplitudes and multi-periodicity clearly establish these stars as members of the EC 14026 class. EC 11583−2708 has pulsation periods near 149, 144 and 114 s, though evidence is presented that the 149-s period is resolved into two periods at 148.87 and 148.55 s by the full photoelectric data set. The amplitudes of the detected variations are in the range 0.002–0.006 mag. The light variation of EC 20338−1925 is dominated by a period near 147 s with a very large amplitude for a variable sdB star (0.025 mag), though four other frequencies are detected with periods near 168, 151, 141 and 135 s and amplitudes in the range 0.002–0.005 mag. The third star, EC 09582−1137, displays a light curve which is virtually a textbook example of frequency beating, being produced by two pulsations of almost equal amplitude (∼0.008 mag) and periods near 136.0 and 151.2 s. 相似文献
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D. J. Wright A. E. Lynas-Gray D. Kilkenny P. L. Cottrell R. R. Shobbrook C. Koen F. W. van Wyk P. M. Kilmartin P. Martinez A. C. Gilmore 《Monthly notices of the Royal Astronomical Society》2006,369(4):2049-2058
We present results from a three-site photometric and high-resolution spectroscopic campaign on the hottest known extreme helium star V2076 Oph (HD 160641). A core programme of intensive observations covered two weeks and a much lower sampling rate extended over another two months. Despite the fact that the data seem to indicate periodicity near half a day (though the light curves are clearly not formed by a single periodicity), conventional Fourier analysis of the data fails to reveal coherent frequencies. Furthermore, we are unable to recover frequencies which were apparently clear in an earlier campaign on the star. Evidence of monotonic pulsation amplitude changes is seen at the higher frequencies from a wavelet analysis, but more data are needed before this study can be extended to lower frequencies. The application of linear stochastic differential equation (LSDE) methods indicates that the observed light variations could be a result of random variations giving the appearance of intermittent periodicity. High-resolution spectroscopic observations were obtained during the campaign and additional observations were made three years later. Complex line profile variations were observed. It is proposed that the different behaviour of the emission line studied may indicate it is associated with a stellar wind or resident circumstellar material. The frequencies that are extracted from the velocity data do not conform to those identified in the current or previous photometric campaigns. 相似文献
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O'Donoghue Koen Lynas-Gray Kilkenny & van Wyk 《Monthly notices of the Royal Astronomical Society》1998,296(2):306-316
We report the discovery of rapid oscillations in the sdB star PG 1047+003. The oscillations are multiperiodic, with nine periods in the range 104–162 s. The optical spectrum of PG 1047+003 is consistent with that of a single sdB star. Line profile fitting yields an effective temperature of 35 000±1000 K and log g =5.9±0.1, although optical photometry and IUE spectrophotometry may indicate a cooler effective temperature. These properties demonstrate that PG 1047+003 is an EC 14026 star, a recently discovered class of sdB pulsators. Optical and infrared photometry constrains any cool companion to the sdB star to be a main-sequence star of spectral type M0 or later. With V =13.47 and a relatively rich pulsation spectrum, PG 1047+003 is an attractive target for an intensive photometric campaign to extract more periods from the light curve which, along with a suitable grid of pulsation models, will probe the interior structure of the star. 相似文献