ORCID Profile
0000-0002-1015-3268
Current Organisations
North-West University
,
University of Central Lancashire
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Publisher: Oxford University Press (OUP)
Date: 04-09-2020
Abstract: Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spectrum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of 2.8 ± 0.5 kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one δ Sct–roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic d ing in suppressing low-overtone p modes in Ap stars. We compute the magnetic d ing effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths ≲4 kG, while other low-overtone p modes are driven for polar field strengths up to ∼1.5 kG. We find that the high-order g modes commonly observed in γ Dor stars are heavily d ed by polar fields stronger than 1–4 kG, with the d ing being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as γ Dor stars. We use our helium-depleted models to calculate the δ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at ∼5 × 104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars.
Publisher: American Astronomical Society
Date: 09-2022
Abstract: Using TESS 20 s cadence data, we have discovered an unusual combination of pulsating stars in what we infer to be a binary system. The binary consists of a standard δ Scuti star with pulsations over the range 32–41 day −1 this is in a likely wide orbit with a hot subdwarf-B (sdB) star, which itself has a large- litude p-mode pulsation at 524 day −1 . We establish constraints on the period of the putative binary by using radial velocity measurements of the δ Scuti star and show that any sdB companion star must orbit with a period greater than approximately thirty days. Our identification of this sdB binary serves as an important addition to the relatively small number of sdB binaries known to have orbital periods longer than a few days. We model such a binary using MESA and find that this system could be formed through stable, nonconservative mass transfer from either a low-mass or an intermediate-mass progenitor, without undergoing a common-envelope phase.
Publisher: Springer Science and Business Media LLC
Date: 09-2011
DOI: 10.1038/NATURE10389
Abstract: Delta Scuti (δSct) stars are opacity-driven pulsators with masses of 1.5-2.5 M⊙, their pulsations resulting from the varying ionization of helium. In less massive stars such as the Sun, convection transports mass and energy through the outer 30 per cent of the star and excites a rich spectrum of resonant acoustic modes. Based on the solar ex le, with no firm theoretical basis, models predict that the convective envelope in δSct stars extends only about 1 per cent of the radius, but with sufficient energy to excite solar-like oscillations. This was not observed before the Kepler mission, so the presence of a convective envelope in the models has been questioned. Here we report the detection of solar-like oscillations in the δSct star HD187547, implying that surface convection operates efficiently in stars about twice as massive as the Sun, as the ad hoc models predicted.
Publisher: Oxford University Press (OUP)
Date: 10-03-2011
Publisher: Oxford University Press (OUP)
Date: 19-11-2012
Publisher: Oxford University Press (OUP)
Date: 14-01-2023
Abstract: The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation – the so-called hump and spike feature. We investigate whether the rotational modulation (spike) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the litudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anticorrelation between the spike litudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the spike feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in hump and spike stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age).
Publisher: EDP Sciences
Date: 03-2006
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 11-11-1998
Publisher: EDP Sciences
Date: 2015
Publisher: EDP Sciences
Date: 04-2014
Publisher: Oxford University Press (OUP)
Date: 09-05-2015
DOI: 10.1093/MNRAS/STV868
Publisher: Oxford University Press (OUP)
Date: 04-08-2015
Publisher: IOP Publishing
Date: 02-2010
DOI: 10.1086/650399
Publisher: American Astronomical Society
Date: 23-09-2015
Publisher: Oxford University Press (OUP)
Date: 29-03-2018
DOI: 10.1093/MNRAS/STY784
Publisher: Cambridge University Press (CUP)
Date: 08-2015
DOI: 10.1017/S1743921316006244
Abstract: Continuous and precise space-based photometry has made it possible to measure the orbital frequency modulation of pulsating stars in binary systems with extremely high precision over long time spans. We present the phase modulation (PM) method for finding binaries among pulsating stars. We demonstrate how the orbital elements of a pulsating binary star can be obtained analytically from photometry alone, without spectroscopic radial velocity measurement. Frequency modulation (FM) caused by binary orbital motion also manifests itself in the Fourier transform, as a multiplet with equal spacing of the orbital frequency. The orbital parameters can also be extracted by analysing the litudes and phases of the peaks in these multiplets. We derive analytically the theoretical relations between the multiplet properties and the orbital parameters, and present a method for determining these parameters, including the eccentricity and the argument of periapsis. This, too, is achievable with the photometry alone, without spectroscopic radial velocity measurements. We apply these two methods to Kepler mission data and demonstrate that the results are in good agreement with each other. These methods are used to search for invisible binary companions, including planets and invisible massive objects such as neutron stars and stellar-mass black holes.
Publisher: Oxford University Press (OUP)
Date: 08-08-2016
Publisher: Oxford University Press (OUP)
Date: 26-11-2010
Publisher: EDP Sciences
Date: 2017
Publisher: EDP Sciences
Date: 2015
Publisher: Oxford University Press (OUP)
Date: 10-11-2016
Publisher: EDP Sciences
Date: 2017
Publisher: American Astronomical Society
Date: 31-03-2010
Publisher: Oxford University Press (OUP)
Date: 22-09-2011
Publisher: Oxford University Press (OUP)
Date: 19-05-2015
DOI: 10.1093/MNRAS/STV875
Publisher: Oxford University Press (OUP)
Date: 31-05-2021
Abstract: We present the results of a systematic search for new rapidly oscillating Ap (roAp) stars using the 2-min cadence data collected by the Transiting Exoplanet Survey Satellite (TESS) during its Cycle 1 observations. We identify 12 new roAp stars. Amongst these stars we discover the roAp star with the longest pulsation period, another with the shortest rotation period, and six with multiperiodic variability. In addition to these new roAp stars, we present an analysis of 44 known roAp stars observed by TESS during Cycle 1, providing the first high-precision and homogeneous s le of a significant fraction of the known roAp stars. The TESS observations have shown that almost 60 per cent (33) of our s le of stars are multiperiodic, providing excellent cases to test models of roAp pulsations, and from which the most rewarding asteroseismic results can be gleaned. We report four cases of the occurrence of rotationally split frequency multiplets that imply different mode geometries for the same degree modes in the same star. This provides a conundrum in applying the oblique pulsator model to the roAp stars. Finally, we report the discovery of non-linear mode interactions in α Cir (TIC 402546736, HD 128898) around the harmonic of the principal mode – this is only the second case of such a phenomenon.
Publisher: Oxford University Press (OUP)
Date: 16-06-2017
Publisher: American Astronomical Society
Date: 07-07-2023
Abstract: We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars μ Lep and β Scl, and to the superficially normal star ν Cap. In the spectra of the six stars, the Hg ii line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of P orb = 26.12022 ± 0.00004 day.
Publisher: Oxford University Press (OUP)
Date: 13-06-2019
Abstract: We perform a search for rapidly oscillating Ap stars in the Kepler long-cadence data, where true oscillations above the Nyquist limit of 283.21 $\\mu$Hz can be reliably distinguished from aliases as a consequence of the barycentric time corrections applied to the Kepler data. We find evidence for rapid oscillations in six stars: KIC 6631188, KIC 7018170, KIC 10685175, KIC 11031749, KIC 11296437, and KIC 11409673, and identify each star as chemically peculiar through either pre-existing classifications or spectroscopic measurements. For each star, we identify the principal pulsation mode, and are able to observe several additional pulsation modes in KIC 7018170. We find that KIC 7018170 and KIC 11409673 both oscillate above their theoretical acoustic cut-off frequency, whilst KIC 11031749 oscillates at the cut-off frequency within uncertainty. All but KIC 11031749 exhibit strong litude modulation consistent with the oblique pulsator model, confirming their mode geometry and periods of rotation.
Publisher: Oxford University Press (OUP)
Date: 05-12-2018
Publisher: Oxford University Press (OUP)
Date: 13-10-2017
Publisher: Oxford University Press (OUP)
Date: 29-01-2015
Publisher: Oxford University Press (OUP)
Date: 21-04-2021
Abstract: Among the intermediate mass pulsating stars known as δ Sct stars is a subset of high- litude and predominantly radial-mode pulsators known as high- litude δ Sct (HADS) stars. From more than 2000 δ Sct stars observed by the Kepler space mission, only two HADS stars were detected. We investigate the more perplexing of these two HADS stars, KIC 5950759. We study its variability using ground- and space-based photometry, determine its atmospheric parameters from spectroscopy and perform asteroseismic modelling to constrain its mass and evolutionary stage. From spectroscopy, we find that KIC 5950759 is a metal-poor star, which is in agreement with the inferred metallicity needed to reproduce its pulsation mode frequencies from non-adiabatic pulsation models. Furthermore, we combine ground-based WASP and Kepler space photometry, and measure a linear change in period of order $\\dot{P}/P \\simeq 10^{-6}$ yr−1 for both the fundamental and first overtone radial modes across a time base of several years, which is at least two orders of magnitude larger than predicted by evolution models, and is the largest measured period change in a δ Sct star to date. Our analysis indicates that KIC 5950759 is a metal-poor HADS star near the short-lived contraction phase and the terminal-age main sequence, with its sub-solar metallicity making it a candidate SX Phe star. KIC 5950759 is a unique object among the thousands of known δ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.
Publisher: American Astronomical Society
Date: 21-10-2016
Publisher: Wiley
Date: 22-11-2010
Abstract: The Kepler space mission, successfully launched in March 2009, is providing continuous and high‐precision photometry of thousands of stars simultaneously. The uninterrupted time‐series of stars of all known pulsation types are a precious source for asteroseismic studies. The Kepler data do not provide information on the physical parameters, such as log g , metallicity, and v sin i , which are crucial for successful asteroseismic modelling. Additional ground‐based time‐series data are needed to characterize mode parameters in several types of pulsating stars. Therefore, ground‐based multi‐colour photometry and mid/high‐resolution spectroscopy are needed to complement the space data. We present ground‐based activities within KASC on selected asteroseismic Kepler targets of several pulsation types (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Publisher: Oxford University Press (OUP)
Date: 19-11-2014
Publisher: Oxford University Press (OUP)
Date: 07-10-2015
Publisher: Oxford University Press (OUP)
Date: 10-05-2013
DOI: 10.1093/MNRAS/STT587
Publisher: Oxford University Press (OUP)
Date: 13-03-2012
Publisher: Oxford University Press (OUP)
Date: 13-05-2016
Publisher: Oxford University Press (OUP)
Date: 16-01-2015
Publisher: Oxford University Press (OUP)
Date: 15-07-2016
Publisher: Oxford University Press (OUP)
Date: 17-11-2017
Publisher: Oxford University Press (OUP)
Date: 13-08-2014
Publisher: Oxford University Press (OUP)
Date: 30-04-2008
DOI: 10.1111/J.1365-2966.2008.13167.X
Abstract: We have used the Sydney University Stellar Interferometer to measure the angular diameter of α Cir. This is the first detailed interferometric study of a rapidly oscillating A (roAp) star, α Cir being the brightest member of its class. We used the new and more accurate Hipparcos parallax to determine the radius to be 1.967 ± 0.066 R⊙. We have constrained the bolometric flux from calibrated spectra to determine an effective temperature of 7420 ± 170 K. This is the first direct determination of the temperature of an roAp star. Our temperature is at the low end of previous estimates, which span over 1000 K and were based on either photometric indices or spectroscopic methods. In addition, we have analysed two high-quality spectra of α Cir, obtained at different rotational phases and we find evidence for the presence of spots. In both spectra we find nearly solar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn, while Co, Y, Nd and Eu are overabundant by about 1 dex. The results reported here provide important observational constraints for future studies of the atmospheric structure and pulsation of α Cir.
Publisher: Oxford University Press (OUP)
Date: 18-04-2011
Publisher: Oxford University Press (OUP)
Date: 21-06-2009
Publisher: Oxford University Press (OUP)
Date: 28-06-2012
Publisher: Oxford University Press (OUP)
Date: 28-03-2016
DOI: 10.1093/MNRAS/STW705
Publisher: EDP Sciences
Date: 2015
Publisher: American Astronomical Society
Date: 03-1998
DOI: 10.1086/305259
Publisher: Oxford University Press (OUP)
Date: 26-05-2011
Publisher: Oxford University Press (OUP)
Date: 20-05-2014
DOI: 10.1093/MNRAS/STU765
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Donald Kurtz.