Publications (38)112.76 Total impact
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Article: SDSS J160043.6+074802.9: a very rapid sdO pulsator
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ABSTRACT: We report the serendipitous discovery of the Sloan Digital Sky Survey (SDSS) star SDSS J160043.6+074802.9 to be a very rapid pulsator. The variation is dominated by a frequency near 8380 μHz (period = 119.33 s) with a large amplitude (0.04 mag) and its first harmonic at 16760 μHz (59.66 s; 0.005 mag). In between these frequencies, we find at least another eight variations with periods between 62 and 118 s and amplitudes between about 0.007 and 0.003 mag; weaker oscillations might also be present. Preliminary spectrograms from the performance verification phase of the Southern African Large Telescope indicate that SDSS J160043.6+074802.9 is a spectroscopic binary consisting of an sdO star and a late-type main-sequence companion. This makes it the first unambiguous detection of such an sdO star to pulsate, and certainly the first found to exhibit multifrequency variations.Monthly Notices of the Royal Astronomical Society 09/2006; 371(3):1497 - 1502. · 4.90 Impact Factor -
Article: Three new pulsating sdB stars from the Edinburgh–Cape survey
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ABSTRACT: 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.Monthly Notices of the Royal Astronomical Society 02/2006; 367(4):1603 - 1608. · 4.90 Impact Factor -
Article: The DA+dMe eclipsing binary EC13471-1258: its cup runneth over...just
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ABSTRACT: EC13471-1258 is a detached eclipsing binary with Porb = 3h37m, comprising a DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf lasting 14 min, and a large amplitude ellipsoidal variation are seen in the light curve. Flares from the dMe star occur regularly. Each star contributes roughly equal amounts of light at 5500 Ang. HST STIS spectra show strong Ly alpha with weak metal lines, and yield Teff = 14220 K, log g = 8.34, Z = 1/30th solar, K = 138 km/s and V sin i = 400 km/s for the white dwarf. Optical spectra yield the spectral type (M3.5-4.0), Teff = 3100 K, Z = solar, K = 266 km/s and V sin i = 140 km/s for the dMe star. The H alpha emission line comprises 2 or more components and implies that very weak mass transfer is occurring. The dynamical solution also implies that the dMe star just fills its Roche lobe. Accurate masses and radii for each star were derived: the dMe values favour the Clemens et al. (1998) mass-radius relation. The large rotational velocity of the white dwarf (400 km/s) suggests that the system has transferred mass in the past so that it is presently a hibernating cataclysmic variable. The metallicity contrast between the component stars provides an opportunity for tests of diffusion theory. Comment: 25 pages, 18 figures, accepted for publication in MNRAS07/2003; -
Article: The EC 14026 stars – XIII. EC 05217–3914 and KUV 0442+1416
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ABSTRACT: We report the discovery of short-period oscillations in the sdB stars EC 05217–3914 and KUV 0442+1416, establishing them as members of the EC 14026 stars, a recently discovered class of sdB pulsators. The pulsation periods and atmospheric parameters for both stars are determined. The results are: periods of 218, 216 and 213 s, Teff = 31 300 ± 250 K and log g= 5.76 ± 0.06 for EC 05217–3914; periods of 231, 216 and 184 s, Teff = 30 900 ± 400 K and log g= 5.72 ± 0.1 for KUV 0442+1416. Considerable substructure in the periods is apparent in both stars when several nights of data are combined. KUV 0442+1416 is significantly reddened. The evolutionary status of the 13 well-studied EC 14026 stars is examined. All stars except PG 1605+072 appear to be core He burning. The positions of the pulsators and the apparently constant stars are plotted in the log g–Teff plane. Apparently constant stars mingle with the pulsators so that there is no well-defined instability strip. This poses a significant challenge to the Fe driving mechanism of Charpinet et al. Five out of the 13 pulsators are obvious binaries.Monthly Notices of the Royal Astronomical Society 04/2002; 306(1):213 - 222. · 4.90 Impact Factor -
Article: Cataclysmic variables in the Edinburgh–Cape Blue Object Survey
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ABSTRACT: The discovery of 15 new cataclysmic variables in the Edinburgh–Cape Blue Object Survey is announced. Photometric and spectroscopic observations are presented including high-speed photometry and time-resolved spectroscopy. On the basis of these data the systems are classified as dwarf novae and nova-like variables. In addition, two of them are of uncertain subtypes because of their peculiar behaviour.Monthly Notices of the Royal Astronomical Society 04/2002; 325(1):89 - 110. · 4.90 Impact Factor -
Article: Indirect imaging of the accretion stream in eclipsing polars IV: V895 Cen
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ABSTRACT: We present spectroscopic and high speed photometric data of the eclipsing polar V895 Cen. We find that the eclipsed component is consistent with it being the accretion regions on the white dwarf. This is in contrast to Stobie et al who concluded that the eclipsed component was not the white dwarf. Further, we find no evidence for an accretion disc in our data. From our Doppler tomography results, we find that the white dwarf has M geq 0.7 Msun. Our indirect imaging of the accretion stream suggests that the stream is brightest close to the white dwarf. When we observed V895 Cen in its highest accretion state, emission is concentrated along field lines leading to the upper pole. There is no evidence for enhanced emission at the magnetic coupling region. Comment: 8 pages, 7 figures, accepted for publication in MNRAS12/2001; -
Article: Model atmosphere and kinematical analyses of early‐type stars from the Edinburgh–Cape Survey
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ABSTRACT: We present high-resolution spectroscopic observations of 21 B-type stars, selected from the Edinburgh–Cape Blue Object Survey. Model atmosphere analyses confirm that 14 of these stars are young, main-sequence B-type objects with Population I chemical compositions. The remaining seven are found to be evolved objects, including subdwarfs, horizontal branch and post-AGB objects. A kinematical analysis shows that all 14 young main-sequence stars could have formed in the disc and subsequently been ejected into the halo. These results are combined with the analysis of a previous subsample of stars taken from the Survey. Of the complete sample, 31 have been found to be young, main-sequence objects, with formation in the disc, and subsequent ejection into the halo, again being found to be a plausible scenario.Monthly Notices of the Royal Astronomical Society 05/2001; 324(3):747 - 756. · 4.90 Impact Factor -
Article: Metallicity estimates for A-, F-, and G-type stars from the Edinburgh-Cape blue object survey
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ABSTRACT: The Edinburgh-Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit B~18.0) over the entire high-Galactic-latitude southern sky, then studied with broadband UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disk populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 candidate metal-deficient dwarfs, subgiants, giants, and horizontal-branch stars with photoelectric colours redder than (B-V)o = 0.3, and with available spectroscopy. Radial velocities, accurate to ~10-15 km/s, are presented for all of these stars. Spectroscopic metallicity estimates for these stars are obtained using a recently re-calibrated relation between Ca II K-line strength and (B-V)o colour. The identification of metal-poor stars from this colour-selection technique is remarkably efficient, and competitive with previous survey methods. An additional sample of 186 EC stars with photoelectric colours in the range -0.4 < (B-V)o < 0.3, comprised primarily of field horizontal-branch stars and other, higher-gravity, A- and B-type stars, is also analysed. Estimates of the physical parameters Teff, log g, and [Fe/H] are obtained for cooler members of this subsample, and a number of candidate RR Lyrae variables are identified. Comment: Includes 10 figures, 6 tables, to be published in the Monthly Notices of the Royal Astronomical Society11/2000; -
Article: EC 11481-2303: A Peculiar Hot High-Gravity Pre-White Dwarf
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ABSTRACT: We report the discovery of a peculiar new hot high-gravity predegenerate star, EC 11481-2303 from the Edinburgh-Cape Blue Object Survey, based upon the analysis of far-UV International Ultraviolet Explorer (IUE) echelle and low-resolution spectra and an optical spectrum. The IUE spectra reveal the presence of far-UV photospheric/circumstellar high-ionization absorption features due to N V λλ1238, 1242, Si IV λλ1393, 1402, C IV λλ1548, 1550, and He II λ1640. The mean velocity of these features is -17.5 km s-1. In addition, there is low-ionization interstellar absorption at an average velocity of -5.6 km s-1. The interstellar lines are strong and imply a distance of more than 500 pc. The optical spectrum is dominated by Stark-broadened Balmer absorption, a sharp He II λ4686 feature, and the absence of Balmer members higher than H8. A best-fit model atmosphere to the optical data yields Teff=41,790 K and logg=5.84, suggesting an extended horizontal branch sdOB subdwarf. However, synthetic spectral fits to the far-UV spectrum do not provide a consistent fit to both the Lyα region and the far-UV continuum of EC 11481-2303. It is possible that EC 11481-2303 is an unresolved binary. Follow-up ground-based and far-UV studies are encouraged to discern the possible nature of any binary companion.Publications of the Astronomical Society of the Pacific 02/2000; 112:354-358. · 3.58 Impact Factor -
Article: A study of optically featureless objects from the EC Survey: searching for new southern BL Lac objects
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ABSTRACT: In an attempt to discover new southern BL Lac objects, 14 optically featureless objects from the Edinburgh–Cape Survey were selected. Optical polarimetry and spectroscopy, radio and IR observations were carried out in order to improve their classifications. The 14 objects were examined according to special criteria that are described. Their UBV and JHK colour–colour distributions, spectrograms, radio observations and visible-region polarimetry were utilized to conclude that the selected objects are not BL Lacs. Most of them are apparently not extragalactic objects. It is suggested that four of the 14 candidates are DC white dwarfs, one is a QSO, three are DA white dwarfs or sdB subdwarfs, one is a cataclysmic variable, one is a DAO/sdO and another is possibly a DA+dM binary (composite system with a ‘primary’ hot white dwarf and a ‘secondary’ M-type main-sequence dwarf) or a cataclysmic variable. One object is likely to be a subdwarf, while two remain unclassifiable.Monthly Notices of the Royal Astronomical Society 11/1999; 309(4):1043 - 1050. · 4.90 Impact Factor -
Article: Rotational velocities of B-type stars from the Edinburgh-Cape survey
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ABSTRACT: The projected rotational velocity distribution for a sample of 34 high Galactic latitude B-type stars from the Edinburgh-Cape Faint Blue Object Survey is presented to investigate the evolutionary status of the group as a whole. Statistical analyses of the distribution show it to be similar to that expected if the sample contained mainly normal Population i early B-type stars, although a contamination of up to 20 % by evolved stars cannot be ruled out. This implies that a large fraction of the sample consists of normal Population i B-type stars similar to those found in the Galactic disk. Possible mechanisms explaining the presence of these stars in the halo are briefly discussed.Astronomy and Astrophysics 09/1998; 338:85-90. · 4.59 Impact Factor -
Article: The EC 14026 Stars
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ABSTRACT: The EC 14026 stars are a new class of pulsating sdB stars. All members of the class are multiperiodic pulsators with periods in the range 90--500 s and amplitudes in the range ~ 0.001 to 0.3 mag. Atmospheric parameter determinations indicate T_eff ~ 35 000 K and log g ~ 5.9, although at least one star has significantly lower gravity and is cooler than most of the others. A brief review of the properties of this class of stars is given.Baltic Astronomy. 02/1998; 7:313-317. -
Article: High-resolution spectroscopic observations of B-type stars from the Edinburgh-Cape survey
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ABSTRACT: High-resolution spectroscopy has been obtained for 25 high-latitude stars identified from the Edinburgh-Cape faint blue object survey as having B-type spectra. Five objects are found to be subluminous (subdwarf or horizontal branch), chemically peculiar, or later than B-type. We present model atmosphere analyses for the other 20 objects, and conclude that 17 stars exhibit stellar properties typical of young B-type dwarfs. Photospheric abundances determined for a subset of stars were also found to be consistent with a Population I composition. Furthermore, we believe EC 05229-6058 to be an evolved object currently on the post-asymptotic giant branch phase, whilst EC 20411-2704 and 11074-2912 are consistent with being zero-age horizontal branch and post-blue horizontal branch objects respectively. A kinematic analysis of the normal stars implies that all could have formed in, and have been subsequently ejected from, the Galactic disc.Monthly Notices of the Royal Astronomical Society 08/1997; 290:422-430. · 4.90 Impact Factor -
Article: The Edinburgh-Cape Blue Object Survey - II. Zone 1 - the North Galactic CAP
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ABSTRACT: Results for Zone 1 of the Edinburgh-Cape (EC) Blue Object Survey are presented. This zone covers that part of the North Galactic Cap more than ~30 deg from the Galactic plane and south of about -12^ deg_.3 declination (although a few fields north of this declination are included). The zone effectively complements the Palomar-Green Survey in the North Galactic Cap, although the EC Survey should be more complete to a fainter limit (B=16.5 mag) and to somewhat redder stars (U-B bluer than about -0.4). Zone 1 covers approximately 1560 deg^2 and contains 675 blue objects for which we list equatorial coordinates accurate to ~1 arcsec, UBV photoelectric photometry, and spectral types determined from moderate-dispersion (100 A mm^-1) spectrograms.Monthly Notices of the Royal Astronomical Society 05/1997; 287:867-893. · 4.90 Impact Factor -
Article: A new class of rapidly pulsating star - I. EC 14026-2647, the class prototype
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ABSTRACT: We report the discovery of small-amplitude, very rapid light variations in the sdB binary, EC 14026-2647. The variations, probably due to pulsation of the sdB star, have an apparently stable main period near 144s with a semi- amplitude of ~0.012 mag. There is good evidence for a period near 134s with a semi-amplitude of ~0.004 mag; the latter might be variable. We regard this star as the prototype for a new class of variable which we refer to as EC 14026 stars.Monthly Notices of the Royal Astronomical Society 02/1997; 285:640-644. · 4.90 Impact Factor -
Article: A new class of rapidly pulsating star - IV. Oscillations in EC 20117-4014 and atmospheric analyses
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ABSTRACT: A new class of pulsating stars, the EC 14026 stars, has recently been announced. We report the discovery of rapid oscillations in EC 20117-4014. The oscillations are multiperiodic with three periods detected so far: 137.3, 142.1 and 158.7s. The peak-to-peak amplitude of these oscillations is ~1 per cent. Along with the spectrum and UBV colours, this implies that EC 20117-4014 is another member of the new class. Interestingly, there is evidence for period/phase changes in the main 137.3-s period, as no linear ephemeris can be found that will fit the collection of light curves we have obtained. We interpret the oscillations as pulsations of an sdB star in a binary with an F/G companion, as indicated by the optical spectra. In order to derive atmospheric parameters of the component stars, low- (~3.5A) and intermediate- (~1A) resolution optical spectroscopy is presented for the first four EC 14026 stars to be discovered: EC 14026-2647, EC 10228-0905, EC 20117-4014 and PB 8783. Their spectra are modelled as the combination of sdB and F/G stars using a grid of high-gravity model atmospheres to represent the former, and observed spectra to represent the latter. The spectral types of the late-type companions are estimated to be in the range early F to early G. Atmospheric parameters for the sdB components of the four stars are derived and found to be similar: log g~6.0 and T_eff~35 000K. If the late-type companions are on or near the main sequence, their distances agree with those estimated for the sdB components, assuming that the masses of the latter are ~0.5Msolar. The evolutionary status of the four stars strongly suggests that the sdB component has suffered substantial mass loss by Roche lobe overflow, along the lines originally proposed by Mengel et al. This hypothesis can be tested by using the orbital time delay in the pulsations to measure the binary period and separation. The discovery of pulsations in sdB stars is important: asteroseismological studies to probe their structure and evolutionary time-scale can now be contemplated.Monthly Notices of the Royal Astronomical Society 02/1997; 285:657-672. · 4.90 Impact Factor -
Article: A new class of rapidly pulsating star - III. Oscillations in EC 10228-0905 and pulsation analysis
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ABSTRACT: EC 10228-0905 is the third pulsating sdB+ star to be announced. High-speed photometry reveals three periods present in the range P=139-152s with semi-amplitudes of less than 10mmag. All periods show evidence of variable amplitude. Pulsation models have been calculated for sdB stars with log g~6.0 and T_eff~35 000K. These all have radial and non-radial modes with periods in the observed range. No unique mode identification is possible at present, but it is clear that at least some of the modes are non-radial. No stars have been predicted to pulsate in this region of the Hertzsprung-Russell (HR) diagram and the cause of the pulsation is not yet understood.Monthly Notices of the Royal Astronomical Society 02/1997; 285:651-656. · 4.90 Impact Factor -
Article: EUVE J1429-38.0: an eclipsing polar
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ABSTRACT: Photometric, polarimetric and spectroscopic observations are presented for EUVE J1429-38.0. Spectroscopically, the system is dominated by the M2 secondary star but with the emission lines characteristic of a polar in its low state. The photometric data reveal the system to be an eclipsing binary, P_orb=4.765h, with a strong ellipsoidal variation. The eclipse of the hot compact source occurs substantially before inferior conjunction as determined by the ellipsoidal variation. This, together with the eclipse ingress and egress detail, leads to the conclusion that the compact light source eclipsed is not coincident with the white dwarf, but may be associated with the accretion stream.Monthly Notices of the Royal Astronomical Society 11/1996; 283:L127-L132. · 4.90 Impact Factor -
Article: EC 20058-5234, a low-amplitude pulsating DB white dwarf
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ABSTRACT: EC 20058-5234 is the eighth pulsating helium white dwarf to be discovered. At least eight periodicities have been observed in about 52h of high-speed photometry, the most prominent being at 281 and 257s. Many of the periodicities appear to be permanent features of the light curve, but the amplitudes vary: the strongest mode has been observed at amplitudes between 6 and 13 mmag. The star is exceptional in the group of DBVs because of both its short periods and its low amplitudes.Monthly Notices of the Royal Astronomical Society 11/1995; 277:913-919. · 4.90 Impact Factor -
Article: Apparently normal, high-latitude B stars from the Edinburgh-Cape survey - II
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ABSTRACT: Low-dispersion spectrograms and UBV photometry are used to carry out a simple analysis of a second sample of apparently normal, high galactic latitude B stars detected in the Edinburgh-Cape faint blue object survey. The sample is bigger than that in Paper I and contains more faint stars. Of the 30 stars, eight are classified as `weak-line' stars and appear very similar to stars in the blue horizontal branch of globular clusters. A further eight stars are sdB stars (including two possible binaries), and 11 stars appear to be normal B stars. Nine of these have distances from the Galactic plane in the approximate range 1<= z<= 4 kpc, and two are probably much more distant. These results, and those of Paper I, indicate that B stars at large distances from the Galactic plane ( z<~ 5 kpc) must be very rare. Three stars do not fall into obvious categories; two of them are helium-rich and all may be hot subdwarfs.Monthly Notices of the Royal Astronomical Society 09/1995; 276:906-914. · 4.90 Impact Factor
Top co-authors
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Institutions
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1998–2006
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South African Astronomical Observatory
Cape Town, Province of the Western Cape, South Africa
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2000
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Villanova University
Norristown, PA, USA
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1997
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Queen's University Belfast
Belfast, NIR, United Kingdom
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1993–1997
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University of Cape Town
- Department of Astronomy
Cape Town, Province of the Western Cape, South Africa
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1994
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The University of Arizona
Tucson, AZ, USA
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