H. L. Worters

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

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Publications (22)66.41 Total impact

  • 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.
    01/2014; 439(2).
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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.
    Publications of the Astronomical Society of the Pacific 08/2013; 125(930):976-988. · 3.69 Impact Factor
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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.
    The Astronomer's Telegram. 07/2013;
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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.
    01/2013;
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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.
    Proceedings of the International Astronomical Union 01/2013; 7(S281):121-123.
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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.
    12/2011;
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    ABSTRACT: U Scorpii is a recurrent nova which has been observed in outburst on 10 occasions, most recently in 2010. We present near-infrared and optical spectroscopy of the 2010 outburst of U Sco. The reddening of U Sco is found to be $E(B-V) = 0.14\pm0.12$, consistent with previous determinations, from simultaneous optical and near-IR observations. The spectra show the evolution of the line widths and profiles to be consistent with previous outbursts. Velocities are found to be up to 14000\,kms$^{-1}$ in broad components and up to 1800\,kms$^{-1}$ 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\pm0.031$ 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.
    Monthly Notices of the Royal Astronomical Society 09/2011; 419. · 5.52 Impact Factor
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    ABSTRACT: The eruption of the recurrent nova U Scorpii on 28 January 2010 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 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_sun. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R_sun. For days 41-67, the optical source is a center-bright disk of radius 2.2 R_sun. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.
    The Astrophysical Journal 08/2011; 742(2). · 6.73 Impact Factor
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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.
    Monthly Notices of the Royal Astronomical Society 02/2011; 411(2):978 - 982. · 5.52 Impact Factor
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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
    09/2010;
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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. Comment: Accepted by MNRAS
    Monthly Notices of the Royal Astronomical Society 08/2010; · 5.52 Impact Factor
  • 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 ~1-2h). In this paper, we show that HE 0230-4323 is, in fact, a rapidly pulsating sdB with at least five frequencies between 3227 and 3532muHz (periods between 310 and 283s). 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 (~0.45d) as a reflection effect in a close binary is unaffected by these new results.
    Monthly Notices of the Royal Astronomical Society 01/2010; 404:376-380. · 5.52 Impact Factor
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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.
    Astronomy and Astrophysics 03/2009; · 5.08 Impact Factor
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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.
    Monthly Notices of the Royal Astronomical Society 02/2009; 393(1):108 - 112. · 5.52 Impact Factor
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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.
    The Astrophysical Journal 12/2008; 664(1):518. · 6.73 Impact Factor
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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.
    11/2008; 401:223.
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    ABSTRACT: This paper presents details of the instrument calibration system employed on the SALT. It is designed to inject light into the Spherical Aberration Corrector at about the position of the primary mirror caustic, thereby simulating the same degree of vignetting as experienced by celestial objects. A light-shaping diffuser screen, coupled with Fresnel lenses, modifies the beam to increase efficiency and attempt to illuminate the detectors in the same manner as a uniform sky. Light is conveyed by means of liquid light guides from either QTH flat field lamps or a choice of hollow cathode (CuAr, ThAr) and penray (Ar, Hg, Xe, Ne) lamps, used for wavelength calibration. Changing entrance pupil effects are accounted for by employing a moving exit pupil baffle, which can simulate the pupil geometry of a specific track.
    Proc SPIE 08/2008;
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    ABSTRACT: Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a major optical sub-system. We present the results obtained so far, and discuss what remains to be done.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    07/2008;
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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 V band and 0.14 mag in B band 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 11.9, and record B≈ 12.9 mag. Limits on the mass accretion rate of are calculated, which span the range of accretion rates modelled for direct wind accretion and Roche lobe overflow mechanisms. The current accretion rates make it difficult for thermonuclear runaway models to explain the observed recurrence interval, and this implies average accretion rates are typically higher than seen immediately post-outburst.
    Monthly Notices of the Royal Astronomical Society 08/2007; 379(4):1557 - 1561. · 5.52 Impact Factor
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    ABSTRACT: We present results from a two-week, single-site photometric campaign on the slowly pulsating sdB star, EC 21324−1346. Nine frequencies are detected between about 333 and 125 μHz (periods between 3000 and 8000 s) with amplitudes ranging from 0.003 to 0.001 mag; large for this class of star. Comparison with sparser, earlier data indicates that some of these oscillations are persistent, though at least one shows clear evidence of substantial amplitude variation.
    Monthly Notices of the Royal Astronomical Society 02/2007; 375(4):1325 - 1328. · 5.52 Impact Factor