arXiv subjects

Daniel L. Holdsworth

Publications and source records attributed to Daniel L. Holdsworth.

25 records · Page 2Linked to original sources

arXiv2017

Three new pulsating sdB stars discovered with SuperWASP

We present an analysis of three new pulsating subdwarf B stars discovered in the SuperWASP archive. Two of the stars, J1938+5609 and J0902-0720, are p-mode pulsators; J1938+5609 shows a pulsation at 231.62 d$^{-1}$ ($P=373$ s; 2681 $μ$Hz) with an amplitude of 4 mmag, whereas J0902-0720 pulsates at frequencies 636.74 d$^{-1}$ ($P=136$ s; 7370 $μ$Hz) and 615.34 d$^{-1}$ ($P=140$ s; 7122 $μ$Hz), with amplitudes 7.27 and 1.53 mmag, respectively. The third star, J2344-3427, is a hybrid pulsator with a p-mode frequency at 223.16 d$^{-1}$ ($P=387$ s; 2583 $μ$Hz) and a corresponding amplitude of 1.5 mmag, and g modes in the frequency range $8.68-28.56$ d$^{-1}$ ($P=3025-9954$ s; $100-331$ $μ$Hz) and amplitudes between 0.76 and 1.17 mmag. Spectroscopic results place J1938+5609 and J2344-3427 among the long-period or hybrid pulsators, suggesting there may be further modes in these stars below our detection limits, with J0902-0720 placed firmly amongst the p-mode pulsators.

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arXiv2016

HD 24355 observed by the Kepler K2 mission: A rapidly oscillating Ap star pulsating in a distorted quadrupole mode

We present an analysis of the first Kepler K2 mission observations of a rapidly oscillating Ap (roAp) star, HD 24355 ($V=9.65$). The star was discovered in SuperWASP broadband photometry with a frequency of 224.31 d$^{-1}$, (2596.18 $μ$Hz; $P = 6.4$ min) and an amplitude of 1.51 mmag, with later spectroscopic analysis of low-resolution spectra showing HD 24355 to be an A5 Vp SrEu star. The high precision K2 data allow us to identify 13 rotationally split sidelobes to the main pulsation frequency of HD 24355. This number of sidelobes combined with an unusual rotational phase variation show this star to be the most distorted quadrupole roAp pulsator yet observed. In modelling this star, we are able to reproduce well the amplitude modulation of the pulsation, and find a close match to the unusual phase variations. We show this star to have a pulsation frequency higher than the critical cut-off frequency. This is currently the only roAp star observed with the Kepler spacecraft in Short Cadence mode that has a photometric amplitude detectable from the ground, thus allowing comparison between the mmag amplitude ground-based targets and the $μ$mag spaced-based discoveries. No further pulsation modes are identified in the K2 data, showing this star to be a single-mode pulsator.

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arXiv2016

Amplitude modulation in $δ$ Sct stars: statistics from an ensemble study of Kepler targets

We present the results of a search for amplitude modulation of pulsation mode frequencies in 983 $δ$ Sct stars, which have effective temperatures between 6400 $\leq T_{\rm eff} \leq$ 10 000 K in the Kepler Input Catalogue and were continuously observed by the Kepler Space Telescope for 4 yr. We demonstrate the diversity in pulsational behaviour observed, in particular nonlinearity, which is predicted for $δ$ Sct stars. We analyse and discuss examples of $δ$ Sct stars with constant amplitudes and phases; those that exhibit amplitude modulation caused by beating of close-frequency pulsation modes; those that exhibit pure amplitude modulation (with no associated phase variation); those that exhibit phase modulation caused by binarity; and those that exhibit amplitude modulation caused by nonlinearity. Using models and examples of individual stars, we demonstrate that observations of the changes in amplitude and phase of pulsation modes can be used to distinguish among the different scenarios. We find that 603 $δ$ Sct stars (61.3 per cent) exhibit at least one pulsation mode that varies significantly in amplitude over 4 yr. Conversely, many $δ$ Sct stars have constant pulsation amplitudes so short-length observations can be used to determine precise frequencies, amplitudes and phases for the most coherent and periodic $δ$ Sct stars. It is shown that amplitude modulation is not restricted to a small region on the HR diagram, therefore not necessarily dependent on stellar parameters such as $T_{\rm eff}$ or $\log\,g$. Our catalogue of 983 $δ$ Sct stars will be useful for comparisons to similar stars observed by K2 and TESS, because the length of the 4-yr Kepler data set will not be surpassed for some time.

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arXiv2015

Combining WASP and Kepler data: the case of the $δ$ Sct star KIC 7106205

Ground-based photometric observations from WASP have been calibrated, scaled and combined with Kepler observations of the $δ$ Sct star KIC 7106205, allowing us to extend the time base of the study of the unexplained amplitude and frequency variation of a single pressure mode at $ν=13.3942$ d$^{-1}$ by 2 yr. Analysis of the combined data sets, spanning 6 yr, show that the amplitude modulation in KIC 7106205 has a much larger range than a previous study of the Kepler data alone indicated. The single pressure mode decreased from $11.70 \pm 0.05$ mmag in 2007, to $5.87 \pm 0.03$ mmag in 2009, and to $0.58 \pm 0.06$ mmag in 2013. Observations of the decrease in mode amplitude have now been extended back 2 yr before the launch of Kepler. With observations over a longer time span, we have been able to further investigate the decrease in mode amplitude in KIC 7106205 to address the question of mode amplitude stability in $δ$ Sct stars. This study highlights the usefulness of the WASP data set for extending studies of some Kepler variable stars.

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arXiv2014

KIC 7582608: A new Kepler roAp star with frequency variability

We analyse the fifth roAp star reported in the Kepler field, KIC 7582608, discovered with the SuperWASP project. The object shows a high frequency pulsation at 181.7324 d$^{-1}$ (P=7.9 min) with an amplitude of 1.45 mmag, and low frequency rotational modulation corresponding to a period of 20.4339 d with an amplitude of 7.64 mmag. Spectral analysis confirms the Ap nature of the target, with characteristic lines of Eu II, Nd III and Pr III present. The spectra are not greatly affected by broadening, which is consistent with the long rotational period found from photometry. From our spectral observations we derive a lower limit on the mean magnetic field modulus of = 3.05$\pm$0.23 kG. Long Cadence Kepler observations show a frequency quintuplet split by the rotational period of the star, typical for an oblique pulsator. We suggest the star is a quadrupole pulsator with a geometry such that $i\sim66^\circ$ and $β\sim33^\circ$. We detect frequency variations of the pulsation in both the WASP and Kepler data sets on many time scales. Linear, non-adiabatic stability modelling allows us to constrain a region on the HR diagram where the pulsations are unstable, an area consistent with observations.

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arXiv2014

High frequency A-type pulsators discovered using SuperWASP

We present the results of a survey using the WASP archive to search for high frequency pulsations in F-, A- and B-type stars. Over 1.5 million targets have been searched for pulsations with amplitudes greater than 0.5 millimagnitude. We identify over 350 stars which pulsate with periods less than 30 min. Spectroscopic follow-up of selected targets has enabled us to confirm 10 new rapidly oscillating Ap stars, 13 pulsating Am stars and the fastest known $δ$ Scuti star. We also observe stars which show pulsations in both the high-frequency domain and in the low-frequency $δ$ Scuti range. This work shows the power of the WASP photometric survey to find variable stars with amplitudes well below the nominal photometric precision per observation.

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arXiv2013

Rapidly varying A-type stars in the SuperWASP archive

The searches for transiting exoplanets have produced a vast amount of time-resolved photometric data of many millions of stars. One of the leading ground-based surveys is the SuperWASP project. We present the initial results of a survey of over 1.5 million A-type stars in the search for high frequency pulsations using SuperWASP photometry. We are able to detect pulsations down to the 0.5 mmag level in the broad-band photometry. This has enabled the discovery of several rapidly oscillating Ap stars and over 200 delta Scuti stars with frequencies above 50 d$^{-1}$, and at least one pulsating sdB star. Such a large number of results allows us to statistically study the frequency overlap between roAp and delta Scuti stars and probe to higher frequency regimes with existing data.

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