H. L. Worters

South African Astronomical Observatory, Kaapstad, Western Cape, South Africa

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Publications (26)83.85 Total impact

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    ABSTRACT: Results for Zone 3 of the Edinburgh-Cape (EC) Blue Object survey are presented. This zone covers that part of the South Galactic Cap between 40° and 50° from the Galactic plane and south of about −12 $_{.}^{\circ}$3 of declination. EC Zone 3 contains 53 UK Schmidt Telescope fields covering about 1400 deg2 in which we find some 534 blue objects, including hot subdwarfs (∼39 per cent), white dwarfs (∼21 per cent), cataclysmic variables (∼2 per cent) and some star-like galaxies (∼9 per cent). A further 178 cooler stars observed in the survey, including low-metallicity F- and G-type stars, are also listed. Both low-dispersion spectroscopic classification and UBV photometry are presented for almost all of the hot objects and either spectroscopy or photometry (sometimes both) for the cooler ones.
    No preview · Article · Oct 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Stellar rotation depends on different parameters. The range of values of these parameters causes the dispersion in the rotation period distributions observed in young stellar clusters/associations. We focus our investigation on the effects of different circumstellar environments on stellar rotation. More specifically, we are searching in stellar Associations for visual triple systems where all stellar parameters are similar, with the only exceptions of the unknown initial rotation period, and of the circum-stellar environment, in the sense that one of the two about equal-mass components has a close-by third 'perturber' component. In the present study we analyse the 35-Myr old visual triple system TYC 9300-0891-1AB + TYC 9300-0529-1 in the young Octans stellar association consisting of three equal-mass K0V components. We collected from the literature all information that allowed us to infer that the three components are actually physically bound forming a triple system and are members of the Octans Association. We collected broad-band photometric timeseries in two observation seasons. We discovered that all the components are variable, magnetically active, and from periodogram analysis we found the unresolved components TYC 9300-0891-1AB to have a rotation period P = 1.383d and TYC 9300-0529-1 a rotation period P = 1.634d. TYC 9300-0891-1A, TYC 9300-0891-1B, and TYC 9300-0529-1 have same masses, ages, and initial chemical compositions. The relatively small 16% rotation period difference measured by us indicates that all components had similar initial rotation periods and disc lifetimes, and the separation of 157AU between the component A and the 'perturber' component B (or vice-versa) has been sufficiently large to prevent any significant perturbation/shortening of the accretion-disc lifetime.
    Full-text · Article · Jun 2015 · New Astronomy
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    Full-text · Article · Mar 2015 · Monthly Notices of the Royal Astronomical Society
  • H. L. Worters · M. T. Rushton
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    ABSTRACT: We present optical spectroscopy of the recurrent nova RS Ophiuchi with time resolutions of similar to 6, similar to 5 and similar to 2 min. In contrast to previous high time resolution studies of the system, changes in the emission from H alpha are detected on time-scales as short as 2 min. Future high time resolution spectroscopy at a higher spectral resolution will be invaluable in determining the nature of the variation.
    No preview · Article · Jun 2014 · Monthly Notices of the Royal Astronomical Society
  • D. Kilkenny · H. L. Worters
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    ABSTRACT: Twenty-eight new timings of maxima of the radially pulsating extreme helium (EHe) star BX Cir (=LSS3184) obtained during 2005-2013 are reported. These results almost double the baseline of observations of the pulsations to 20 yr, determine the period decrease rate at dP/dn = -3.47 × 10-10 d, and better define the ephemeris for pulsation maximum. No evidence for ephemeris terms higher than quadratic is found. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
    No preview · Article · Jan 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Two identical new instruments, the Sutherland High-speed Optical Cameras (SHOC), have been developed for use on the South African Astronomical Observatory’s (SAAO) 1.9, 1.0 and 0.75 m telescopes at Sutherland. The SHOC systems are fast-frame-rate, accurately-timed, high-quality, visible-wavelength imagers. Each system consists of a camera, global positioning system (GPS), control computer and peripherals. The primary component is an Andor iXon X3 888 UVB camera, utilizing a 1024 × 1024 pixel, frame-transfer, thermoelectrically-cooled, back-illuminated CCD. One of SHOC’s most important features is that it can achieve frame rates of between 1 and 20 frames/s during normal operation (dependent on binning and subframing) with microsecond timing accuracy on each frame (achieved using frame-by-frame GPS triggering). Frame rates can be increased further, and fainter targets observed, by making use of SHOC’s electron-multiplying (EM) modes. SHOC is therefore ideally suited to time domain astronomy where high frame rates and extremely accurate timing are critical. Here, we present details of the instrument components, characteristics measured during commissioning, science demonstrations, and development plans. Attention is specifically given to exploration of the signal-to-noise (S/N) parameter space as a function of EM and conventional modes. These results enable observers to optimize instrumental settings for their observations and clearly demonstrate the advantages and potential pitfalls of the EM modes.
    No preview · Article · Aug 2013 · Publications of the Astronomical Society of the Pacific
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    ABSTRACT: On 2013-06-24.063 UT, the Southern African Large Telescope (SALT; Buckley et al. 2006; O’Donoghue et al. 2006) observed MASTER OT J142023.5-485540 (RA=14h 20m 23.5s; Dec=-48d 55m 40s). This outbursting CV was discovered by MASTER-ICATE on 2013-06-08.048 UT and followup observations were requested in ATEL #5144.
    No preview · Article · Jul 2013
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    ABSTRACT: Optical and near-infrared observations of novae give us useful information for understanding the diversity of nova eruptions. Classical nova V1723 Aql was discovered by F. Kabashima and K. Nishiyama on 2010 September 11. We have conducted photometric and spectroscopic observations of V1723 Aql in both optical and near-infrared (NIR) wavelength regions since its discovery. The V-band decline time by 2 mag after the maximum, t2, was ~12 d. The apparent Fe II emission lines were also seen in the optical spectra. The R c- and I c-band light curves exhibited rapid declines (0.16 mag d-1 in R c) 20 days after the visual maximum, while the NIR (J, H, and K s) showed slow decline rates (~0.07 mag d-1). This rapid reddening suggests that dust particles formed during the very early phase of the expansion in V1723 Aql.
    No preview · Article · Jul 2013 · Proceedings of the International Astronomical Union
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    ABSTRACT: We have photometrically monitored RE J0317-853, a rapidly rotating (P = 725 s), highly magnetic (300 MG < B < 800 MG), massive (M ≍ 1.3M⊙) white dwarf, since 1994. The O-C analysis reveals we have detected changes in the arrival time of maximum flux for RE J0317-853, consistent with that expected from the orbital motion of the wide binary pair of RE J0317-853 and LB 9802.
    No preview · Article · Jan 2013
  • H. L. Worters · G. E. Bromage · D. A. H. Buckley
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    ABSTRACT: High time-resolution photometric observations of M-dwarfs have been made with the Southern African Large Telescope (SALT), and we report here the first flares detected with SALT. These observations form part of a longer term study of dMe flares on post-T-Tauri and early main sequence stars using SAAO telescopes and SALT. One of the aims of the SALT study involves investigating potential rapid structural variation in these stellar flares.
    No preview · Article · Dec 2011
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    ABSTRACT: U Scorpii (U Sco) is a recurrent nova which has been observed in outburst on 10 occasions, most recently in 2010. We present near-infrared (near-IR) and optical spectroscopy of the 2010 outburst of U Sco. The reddening of U Sco is found to be E(B − V) = 0.14 ± 0.12, consistent with previous determinations, from simultaneous optical and near-IR observations. The spectra show the evolution of the linewidths and profiles to be consistent with previous outbursts. Velocities are found to be up to 14 000 km s−1 in broad components and up to 1800 km s−1 in narrow-line components, which become visible around day 8 due to changes in the optical depth. From the spectra we derive a helium abundance of N(He)/N(H) = 0.073 ± 0.031 from the most reliable lines available; this is lower than most other estimates and indicates that the secondary is not helium-rich, as previous studies have suggested.
    Full-text · Article · Sep 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The eruption of the recurrent nova U Scorpii on 2010 January 28 is now the all-time best observed nova event. We report 36,776 magnitudes throughout its 67 day eruption, for an average of one measure every 2.6 minutes. This unique and unprecedented coverage is the first time that a nova has had any substantial amount of fast photometry. With this, two new phenomena have been discovered: the fast flares in the early light curve seen from days 9-15 (which have no proposed explanation) and the optical dips seen out of eclipse from days 41-61 (likely caused by raised rims of the accretion disk occulting the bright inner regions of the disk as seen over specific orbital phases). The expanding shell and wind cleared enough from days 12-15 so that the inner binary system became visible, resulting in the sudden onset of eclipses and the turn-on of the supersoft X-ray source. On day 15, a strong asymmetry in the out-of-eclipse light points to the existence of the accretion stream. The normal optical flickering restarts on day 24.5. For days 15-26, eclipse mapping shows that the optical source is spherically symmetric with a radius of 4.1 R ☉. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R ☉. For days 41-67, the optical source is a center-bright disk of radius 2.2 R ☉. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.
    Full-text · Article · Aug 2011 · The Astrophysical Journal
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    ABSTRACT: We report the detection of short-period variations in the stars HD 69013 and HD 96237. These stars possess large overabundances of rare earth elements and global magnetic fields, thus belong to the class of chemically peculiar Ap stars of the main sequence. Pulsations were found from analysis of high time resolution spectra obtained with the European Southern Observatory (ESO) Very Large Telescope using a cross-correlation method for wide spectral bands, from lines belonging to rare earth elements and from the Hα core. Pulsation amplitudes reach more than 200 m s−1 for some lines in HD 69013 with a period of 11.4 min and about 100 m s−1 in HD 96237 with periods near 13.6 min. The pulsations have also been detected in photometric observations obtained at the South African Astronomical Observatory.
    Full-text · Article · Feb 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report the detection of short period variations in the stars HD69013 and HD96237. These stars possess large overabundances of rare earth elements and global magnetic fields, thus belong to the class of chemically peculiar Ap stars of the main sequence. Pulsations were found from analysis of high time resolution spectra obtained with the ESO Very Large Telescope using a cross correlation method for wide spectral bands, from lines belonging to rare earth elements and from the H alpha core. Pulsation amplitudes reach more than 200 m/s for some lines in HD69013 with a period of 11.4 min and about 100m/s in HD96237 with periods near 13.6 min. The pulsations have also been detected in photometric observations obtained at the South African Astronomical Observatory. Comment: submitted to MNRAS
    Full-text · Article · Sep 2010
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    ABSTRACT: We present near-IR observations of the 2010 outburst of U Sco. JHK photometry is presented on ten consecutive days starting from 0.59 days after outburst. Such photometry can gainfully be integrated into a larger database of other multi-wavelength data which aim to comprehensively study the evolution of U Sco. Early near-IR spectra, starting from 0.56 days after outburst, are presented and their general characteristics discussed. Early in the eruption, we see very broad wings in several spectral lines, with tails extending up to ~10000km/s along the line of sight; it is unexpected to have a nova with ejection velocities equal to those usually thought to be exclusive to supernovae. From recombination analysis, we estimate an upper limit of 10^-4.64[+0.92.-0.74]Msun for the ejected mass.
    Full-text · Article · Aug 2010 · Monthly Notices of the Royal Astronomical Society
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    D. Kilkenny · C. Koen · H. Worters
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    ABSTRACT: HE 0230−4323 is a hot sdB star in a binary system. An earlier work demonstrated that the light curve of the system shows a strong (∼4 per cent) reflection effect and also appears to exhibit photometric variations of the type associated with the slowly pulsating class of sdB star (multiple periods in the range ∼1–2 h). In this paper, we show that HE 0230−4323 is, in fact, a rapidly pulsating sdB with at least five frequencies between 3227 and 3532 μHz (periods between 310 and 283 s). The long periods previously claimed were the result of undersampling the light curve at a time interval very close to that of the short periods. The interpretation of the very slow variation (∼0.45 d) as a reflection effect in a close binary is unaffected by these new results.
    Preview · Article · Mar 2010 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We carried out observations, with five different instruments ranging in aperture from 0.4m to 10m, of the satellites of Uranus during that planet's 2007 Equinox. Our observations covered specific intervals of time when mutual eclipses and occultations were predicted. The observations were carried out in the near-infrared part of the spectrum to mitigate the glare from the planet. Frames were acquired at rates > 1/min. Following modelling and subtraction of the planetary source from these frames, differential aperture photometry was carried out on the satellite pairs involved in the predicted events. In all cases but one, nearby bright satellites were used as reference sources. We have obtained fifteen individual lightcurves, eight of which show a clear drop in the flux from the satellite pair, indicating that a mutual event took place. Three of these involve the faint satellite Miranda. All eight lightcurves were model-fitted to yield best estimates of the time of maximum flux drop and the impact parameter. In three cases best-fit albedo ratios were also derived. We used these estimates to generate intersatellite astrometric positions with typical formal uncertainties of <0.01 arcsec, several times better than conventional astrometry of these satellites. The statistics of our estimated event midtimes show a systematic lag, with the observations later than predictions. In addition, lightcurves of two partial eclipses of Miranda show no statistically significant evidence of a light drop, at variance with the predictions. These indicate that new information about the Uranian satellite system is contained in observations of mutual events acquired here and by other groups.
    Full-text · Article · Mar 2009 · Astronomy and Astrophysics
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    ABSTRACT: We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around 4.7 μm, we determine the excitation conditions in the line-forming region. We find 12C/13C = 3.5+2.0−1.5, consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of 2.2 × 10−6≤MCO≤ 2.7 × 10−6 M⊙ of CO ejecta outside the dust, forming a high-velocity wind of 500 ± 80 km s−1. We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.
    Full-text · Article · Feb 2009 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present follow-up observations of pulsating subdwarf B (sdB) stars as part of our efforts to resolve the pulsation spectra for use in asteroseismological analyses. This paper reports on multisite campaigns of the pulsating sdB stars PG 1618+563B and PG 0048+091. Data were obtained from observatories placed around the globe for coverage from all longitudes. For PG 1618+563B, our five-site campaign uncovered a dichotomy of pulsation states. Early during the campaign the amplitudes and phases (and perhaps frequencies) were quite variable, while data obtained late in the campaign were able to fully resolve five stable pulsation frequencies. For PG 0048+091, our five-site campaign uncovered a plethora of frequencies with short pulsation lifetimes. We find them to have observed properties consistent with stochastically excited oscillations, an unexpected result for subdwarf B stars. We discuss our findings and their impact on subdwarf B asteroseismology.
    Full-text · Article · Dec 2008 · The Astrophysical Journal
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    H. L. Worters · S. P. S. Eyres · G. E. Bromage · J. P. Osborne
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    ABSTRACT: The latest outburst of the recurrent nova RS Oph occurred in 2006 February. Photometric data presented here show evidence of the resumption of optical flickering, indicating re-establishment of accretion by day 241 of the outburst. Magnitude variations of up to 0.32 mag in the V-band and 0.14 mag in B on time-scales of 600-7000 s are detected. Over the two week observational period we also detect a 0.5 mag decline in the mean brightness, from V ≈ 11.4 to V ≈ 11.9, and record B ≈ 12.9 mag. Limits on the mass accretion rate of ˜ 10^{-10} ≤ dot{M}_{acc} ≤ 10^{-9} M_{sun} yr^{-1} are calculated, which span the range of accretion rates modelled for direct wind accretion and Roche lobe overflow mechanisms. The current accretion rate presents difficulties in explaining the observed recurrence interval by thermonuclear runaway, implying low average accretion rates immediately post-outburst.
    Full-text · Article · Nov 2008