arXiv subjects

Daniel L. Holdsworth

Publications and source records attributed to Daniel L. Holdsworth.

At least 19 recordsLinked to original sources

arXiv2026

Demonstration of a Single-Laser-Diode-Pumped Ti:Sapphire Astrocomb on the Southern African Large Telescope

Astrocombs -- broadband lasers comprising thousands of narrow, uniformly spaced and atomically-referenced spectral lines -- offer gold-standard wavelength calibration for ground-based optical telescopes, with the potential for the cm/s precision needed for radial-velocity follow-ups of exoplanet candidates from photometric space missions like Kepler, TESS and PLATO. Current astrocombs, particularly those operating in the visible spectrum, are complex and expensive, putting them beyond the reach of many observatories. Here, we present the first on-instrument demonstration of a new and simple astrocomb concept, providing calibration light across nearly the entire 550 nm to 890 nm red channel of the Southern African Large Telescope's High Resolution Spectrograph (SALT-HRS). An octave-spanning supercontinuum generated in a silicon nitride waveguide by a GPS-referenced diode-pumped Ti:sapphire laser is filtered to 21 GHz before fibre delivery to the spectrograph. Using the astrocomb, we obtain the wavelength solution for one order of SALT-HRS and the wavelength-dependent line-spread function of the instrument. With potential for further extension into the blue spectral region, this uniquely simple architecture brings visible-to-infrared astrocomb technology within the reach of a wider range of astronomical observatories.

astro-ph.IM↗

arXiv2026

Rapidly oscillating Ap stars observed with TESS. The LAMOST Ap sample and 49 Cam

The rapidly oscillating chemically peculiar A-type (roAp) stars offer valuable insights into the internal physical processes of all stars, but their study is challenged by their rarity. The large-scale TESS surveys have allowed for the collection of data for a sizeable dataset of roAp stars. Nevertheless, asteroseismic data obtained with TESS and Gaia has not been explored to its full potential. We develop an algorithm capable of analysing large quantities of data to search for new roAp stars and increase the current sample. We focus on data products that have not been previously explored for the search of roAp stars, namely the 200-s FFIs and 20-s cadence light curves. 20-s and 200-s cadence light curves of target stars are retrieved from the MAST server and cleaned. DFTs are computed for each light curve which are used to pre-whiten the data to remove any low frequency signals. A final DFT is calculated which is used to classify stars as non-pulsating (NP), delta Scuti or roAp based on the remaining signal. We apply our algorithm to two independent datasets: i) ~2700 Ap stars spectroscopically classified with LAMOST and observed by TESS in the 200-s FFIs and ii) all TESS 20-s cadence light curves available for known or candidate roAp stars. These two samples have no overlap, i.e. none of the LAMOST stars have been observed with 20-s cadence. We identify four new roAp stars: TIC 312111544, TIC 252881095, TIC 46054683, and 49 Cam (TIC 393276640). We find evidence in TESS data that TIC 252881095 may be part of a binary system. If the tentative ~30-d orbital signal is confirmed, TIC 252881095 could be one of the shortest-period roAp binary currently known. Furthermore, the detection of high-frequency pulsations in 49 Cam is particularly relevant, as this well-known roAp candidate star is here confirmed to be roAp based on TESS 20-s cadence data.

astro-ph.SR↗

arXiv2025

Super-slowly rotating Ap (ssrAp) stars: New spectroscopic observations

The rotation periods of Ap stars range over five to six orders of magnitude. The origin of their differentiation remains unknown. We carry out a systematic study of the longest period Ap stars to gain insight into their properties. We analyse newly obtained spectra of a sample of super-slowly rotating Ap (ssrAp) star candidates identified by a TESS photometric survey to confirm that their projected equatorial velocity v sin i is consistent with (very) long rotation periods, to obtain a first determination of their magnetic fields, and to test their binarity. The value of v sin i in 16 of the 18 studied stars is low enough for them to have moderately to extremely long rotation periods. All stars but one are definitely magnetic; for five of them, the magnetic field was detected for the first time. Five new stars with resolved magnetically split lines were discovered. Five stars that were not previously known to be spectroscopic binaries show radial velocity variations; in one of them, lines from both components are observed.

astro-ph.SR↗

arXiv2024

HD 60435: The star that stopped pulsating

HD 60435 is a well-known rapidly oscillating (roAp) Ap star with a series of alternating even and odd degree modes, making it a prime asteroseismic target. It is also an oblique pulsator with rotational inclination, $i$, and magnetic/pulsation obliquity, $β$, such that both magnetic/pulsation poles are viewed over the rotation period, $P_{\rm rot} = 7.679696$ d, determined from rotational light variations. While some roAp stars have stable pulsation mode amplitudes over decades, HD 60435 is known to have amplitude variations on time scales as short as 1 d. We show from 5 yr of {\it TESS} observations that there is strong amplitude modulation on this short time scale with possible mode interactions. Most remarkably, HD 60435 stopped pulsating during the time span of the {\it TESS} observations. This is the first time that any pulsating star has been observed to cease pulsating entirely. That has implications for mode interaction, excitation and damping, and is relevant to the problem of why only some stars in many pulsation instability strips pulsate, while others do not. During a 24.45-d time span of the {\it TESS} data when there was mode stability for a dipole mode and a quadrupole mode, the oblique pulsator model constrained $i$ and $β$, which we used to model those modes with a magnetic pulsation model from which we determined a polar field strength of 4 kG, in good agreement with a known magnetic measurement. We modelled the frequency separations showing that they can constrain the global metallicity, something that is not possible from spectroscopy of the highly peculiar Ap atmosphere.

astro-ph.SR↗

arXiv2023

The GW Vir instability strip in the light of new observations of PG 1159 stars. Discovery of pulsations in the central star of Abell 72 and variability of RX J0122.9-7521

We present the results of new time series photometric observations of 29 pre-white dwarf stars of PG 1159 spectral type, carried out in the years 2014-2022. For the majority of stars, a median noise level in Fourier amplitude spectra of 0.5-1.0 mmag was achieved. This allowed the detection of pulsations in the central star of planetary nebula Abell 72, consistent with g-modes excited in GW Vir stars, and variability in RX J0122.9-7521 that could be due to pulsations, binarity or rotation. For the remaining stars from the sample that were not observed to vary, we placed upper limits for variability. After combination with literature data, our results place the fraction of pulsating PG 1159 stars within the GW Vir instability strip at 36%. An updated list of all known PG 1159 stars is provided, containing astrometric measurements from the recent Gaia DR3 data, as well as information on physical parameters, variability, and nitrogen content. Those data are used to calculate luminosities for all PG 1159 stars to place the whole sample on the theoretical Hertzsprung-Russell diagram for the first time in that way. The pulsating stars are discussed as a group, and arguments are given that the traditional separation of GW Vir pulsators in "DOV" and "PNNV" stars is misleading and should not be used.

astro-ph.SR↗

arXiv2021

Could the Magnetic Star HD 135348 Possess a Rigidly Rotating Magnetosphere?

We report the detection and characterization of a new magnetospheric star, HD 135348, based on photometric and spectropolarimetric observations. The TESS light curve of this star exhibited variations consistent with stars known to possess rigidly rotating magnetospheres (RRMs), so we obtained spectropolarimetric observations using the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) at four different rotational phases. From these observations, we calculated the longitudinal magnetic field of the star $\langle B_z \rangle$, as well as the Alfvén and Kepler radii, and deduced that this star contains a centrifugal magnetosphere. However, an archival spectrum does not exhibit the characteristic "double-horned" emission profile for H$α$ and the Brackett series that has been observed in many other RRM stars. This could be due to the insufficient rotational phase coverage of the available set of observations, as the spectra of these stars significantly vary with the star's rotation. Our analysis underscores the use of TESS in photometrically identifying magnetic star candidates for spectropolarimetric follow-up using ground-based instruments. We are evaluating the implementation of a machine learning classifier to search for more examples of RRM stars in TESS data.

astro-ph.SR↗

arXiv2021

Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181

We report the frequency analysis of a known roAp star, HD 86181 (TIC 469246567), with new inferences from TESS data. We derive the rotation frequency to be $ν_{rot}$ = 0.48753 $\pm$ 0.00001d$^{-1}$. The pulsation frequency spectrum is rich, consisting of two doublets and one quintuplet, which we interpret to be oblique pulsation multiplets from consecutive, high-overtone dipole, quadrupole and dipole modes. The central frequency of the quintuplet is 232.7701d$^{-1}$ (2.694 mHz). The phases of the sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show that the quadrupole mode is distorted. Following the oblique pulsator model, we calculate the rotation inclination, i, and magnetic obliquity, $β$, of this star, which provide detailed information about the pulsation geometry. The i and $β$ derived from the best fit of the pulsation amplitude and phase modulation to a theoretical model, including the magnetic field effect, slightly differ from those calculated for a pure quadrupole, indicating the contributions from l = 4, 6, 8, ... are small. Non-adiabatic models with different envelope convection conditions and physics configurations were considered for this star. It is shown that models with envelope convection almost fully suppressed can explain the excitation at the observed pulsation frequencies.

astro-ph.SR↗

arXiv2020

Very regular high-frequency pulsation modes in young intermediate-mass stars

Asteroseismology is a powerful tool for probing the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs. However, a large group of pulsating stars of intermediate mass--the so-called delta Scuti stars--have rich pulsation spectra for which systematic mode identification has not hitherto been possible. This arises because only a seemingly random subset of possible modes are excited, and because rapid rotation tends to spoil the regular patterns. Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, allowing definitive mode identification. Some of these stars have space motions that indicate they are members of known associations of young stars, and modelling of their pulsation spectra confirms that these stars are indeed young.

astro-ph.SR↗

arXiv2020

Long period Ap stars discovered with TESS data

The TESS space mission is producing superb data for asteroseismology, eclipsing binary stars, gyrochronology, and other fields of stellar astronomy where the data are variable light curves. We show that TESS data are excellent for astrophysical inference from peculiar stars that show variability. The Ap stars have the strongest magnetic fields of any main-sequence stars. Some have been shown to have rotation periods of months to centuries. The astrophysical cause of their slow rotation -- the braking mechanism -- is not known with certainty. These stars are rare: there are currently about 3 dozen with known periods. The magnetic Ap stars have long-lived spots that allow precise determination of their rotation periods. We show that most Ap stars with TESS data that show no low-frequency variability must have rotation periods longer than a TESS sector of 27 d. We find 60 Ap stars in the southern ecliptic hemisphere TESS data with no rotational variability, of which at most a few can be pole-on, and six likely have nearly aligned magnetic and rotation axes. Of the other 54, 31 were previously known to have long rotation periods or low projected equatorial velocities, which proves our technique; 23 are new discoveries. These are prime targets for long-term magnetic studies. We also find that 22 per cent of long-period Ap stars are roAp stars, versus only 3 per cent of the other Ap stars studied with TESS sector 1 and 2 data. This work demonstrates astrophysical inference from nonvariable stars -- "something for nothing".

astro-ph.SR↗

arXiv2019

Environmental effects on halo abundance and weak lensing peak statistics toward large underdense regions

The cosmic microwave background (CMB) contains an anomalous cold spot with a surrounding hot ring, known as the Cold Spot. Inoue & Silk (2006) proposed that this feature could be explained by postulating a supervoid: if such a large underdense region exists, then the growth of matter perturbing around the spot might differ from the average value in the Universe and the differences might affect weak lensing analysis of peak statistics. To investigate environmental effects on halo number count and peak statistics, we used a publicly available ray-tracing simulation for a box size of 2250$h^{-1}$Mpc on a side (Takahashi et al. 2017). We found that the number counts for massive haloes toward the largest underdense region in the simulation decreases and the corresponding significance of the difference, based on a cosmic average, is $\geq3σ$. On the basis of the results of peak statistics analysis, the number of high peaks decreases with the decrement of massive haloes, but the number of low peaks increases with the lack of matter in the line of sight. The highest significance of the decrement in peak counts in large underdense regions is $5σ$ in the total signal-to-noise ratio. Our result implies that environmental effects on halo abundance and weak lensing peak statistic can be used to probe the presence and properties of supervoids.

astro-ph.CO↗

arXiv2019

Adaptive elliptical aperture photometry: a software package for high-cadence ground-based photometry. I. Application to rapid oscillators observed from SAAO

Context. Modern space telescopes are currently providing high-precision light curves for a large fraction of the sky, such that many new variable stars are being discovered. However, some stars have periodic variability with periods of order minutes and require high-cadence photometry to probe the physical mechanisms responsible. A cadence of less than a minute is often required to remove Nyquist ambiguities and confirm rapid variability which forces observers to obtain high-cadence ground-based photometry. Aims. We aim to provide a modern software package to reduce ground-based photometric time series data and deliver optimised (differential) light curves. To produce high-quality light curves, which maximise the amplitude signal-to-noise ratio of short-period variability in a Fourier spectrum, we require adaptive elliptical aperture photometry as this represents a significant advantage compared to aperture photometry using circular apertures of fixed radii. Methods. The methodology of our code and its advantages are demonstrated using high-cadence ground-based photometry from the South African Astronomical Observatory (SAAO) of a confirmed rapidly oscillating Ap (roAp) star. Furthermore, we employ our software package to search for rapid oscillations in three candidate roAp stars. Results. We demonstrate that our pipeline represents a significant improvement in the quality of light curves, and we make it available to the community for use with different instruments and observatories. We search for and demonstrate the lack of high-frequency roAp pulsations to a limit of $\sim$1 mmag using B data in the three Ap stars HD 158596, HD 166542 and HD 181810. Conclusions. We demonstrate the significant improvement in the extraction of short-period variability caused by high-frequency pulsation modes, and discuss the implication of null detections in three Ap stars.

astro-ph.SR↗

arXiv2019

Six new rapidly oscillating Ap stars in the Kepler long-cadence data using super-Nyquist asteroseismology

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 μHz 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 cutoff frequency, whilst KIC 11031749 oscillates at the cutoff frequency within uncertainty. All but KIC 11031749 exhibit strong amplitude modulation consistent with the oblique pulsator model, confirming their mode geometry and periods of rotation.

astro-ph.SR↗

arXiv2018

K2 observations of the rapidly oscillating Ap star 33 Lib (HD 137949): new frequencies and unique non-linear interactions

We present the analysis of K2 short cadence data of the rapidly oscillating Ap (roAp) star, 33 Librae (HD 137949). The precision afforded to the K2 data allow us to identify at least 11 pulsation modes in this star, compared to the three previously reported. Reoccurring separations between these modes leads us to suggest a large frequency separation, $Δν$, of 78.9 $μ$Hz, twice that reported in the literature. Other frequency separations we detect may represent the small frequency separation, $δν$, but this is inconclusive at this stage due to magnetic perturbation of the frequencies. Due to the highly non-linear pulsation in 33 Lib, we identify harmonics to four times the principal frequency. Furthermore, we note a unique occurrence of non-linear interactions of the 11 identified modes. The frequency separations of the modes around the principal frequency are replicated around the first harmonic, with some interaction with the second harmonic also. Such a phenomenon has not been seen in roAp stars before. With revised stellar parameters, linear non-adiabatic modelling of 33 Lib shows that the pulsations are not greater than the acoustic cutoff frequency, and that the $κ$-mechanism can excite the observed modes. Our observations are consistent with 33 Lib having a rotation period much larger than 88 d as presented in the literature.

astro-ph.SR↗

arXiv2018

LCO observations of a super-critical distorted pulsation in the roAp star J0855 (TYC 2488-1241-1)

We report the results of a 60-hr photometric campaign of a rapidly oscillating Ap star, J0855 (TYC 2488-1241-1). We have utilised the multi-site Las Cumbres Observatory's (LCO) 0.4-m telescopes to obtain short cadence B-band observations of an roAp star previously lacking detailed study. Our observations confirm the rotation period presented in the discovery paper of this star ($P_{\rm rot}=3.0918$ d), and reveal the star to be pulsating in a distorted mode. The B data show this star to be among the highest amplitude roAp stars, with a peak-to-peak amplitude of 24 mmag. Modelling of the pulsation frequency at 197.2714 d$^{-1}$ (2283 $μ$Hz; P=7.30 min) shows that this star belongs to the subgroup of super-critical pulsators, where the observed frequencies are above the theoretical acoustic cutoff frequency. From the modelling, we deduce that the star's rotation axis is inclination angle of about $30^\circ$ to the line-of-sight, with an angle of obliquity of the magnetic axis to the rotation axis of either $40^\circ$ or $24^\circ$ depending on whether the pulsation mode is dipole or quadrupole, respectively.

astro-ph.SR↗

arXiv2018

Suppressed phase variations in a high amplitude rapidly oscillating Ap star pulsating in a distorted quadrupole mode

We present the results of a multisite photometric observing campaign on the rapidly oscillating Ap (roAp) star 2MASS 16400299-0737293 (J1640; $V=12.7$). We analyse photometric $B$ data to show the star pulsates at a frequency of $151.93$ d$^{-1}$ ($1758.45 μ$Hz; $P=9.5$ min) with a peak-to-peak amplitude of 20.68 mmag, making it one of the highest amplitude roAp stars. No further pulsation modes are detected. The stellar rotation period is measured at $3.6747\pm0.0005$ d, and we show that rotational modulation due to spots is in anti-phase between broadband and $B$ observations. Analysis and modelling of the pulsation reveals this star to be pulsating in a distorted quadrupole mode, but with a strong spherically symmetric component. The pulsational phase variation in this star is suppressed, leading to the conclusion that the contribution of $\ell>2$ components dictate the shape of phase variations in roAp stars that pulsate in quadrupole modes. This is only the fourth time such a strong pulsation phase suppression has been observed, leading us to question the mechanisms at work in these stars. We classify J1640 as an A7 Vp SrEu(Cr) star through analysis of classification resolution spectra.

astro-ph.SR↗

arXiv2017

Whole Earth Telescope discovery of a strongly distorted quadrupole pulsation in the largest amplitude rapidly oscillating Ap star

We present a new analysis of the rapidly oscillating Ap (roAp) star, 2MASS J$19400781-4420093$ (J1940; $V=13.1$). The star was discovered using SuperWASP broadband photometry to have a frequency of 176.39 d$^{-1}$ (2041.55 $μ$Hz; $P = 8.2$ min; Holdsworth et al. 2014a) and is shown here to have a peak-to-peak amplitude of 34 mmag. J1940 has been observed during three seasons at the South African Astronomical Observatory, and has been the target of a Whole Earth Telescope campaign. The observations reveal that J1940 pulsates in a distorted quadrupole mode with unusual pulsational phase variations. A higher signal-to-noise ratio spectrum has been obtained since J1940's first announcement, which allows us to classify the star as A7 Vp Eu(Cr). The observing campaigns presented here reveal no pulsations other than the initially detected frequency. We model the pulsation in J1940 and conclude that the pulsation is distorted by a magnetic field of strength $1.5$ kG. A difference in the times of rotational maximum light and pulsation maximum suggests a significant offset between the spots and pulsation axis, as can be seen in roAp stars.

astro-ph.SR↗

arXiv2017

A seven-planet resonant chain in TRAPPIST-1

The TRAPPIST-1 system is the first transiting planet system found orbiting an ultra-cool dwarf star. At least seven planets similar to Earth in radius and in mass were previously found to transit this host star. Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 d orbital period for the outermost planet, TRAPPIST-1h, which was unconstrained until now. This value matches our theoretical expectations based on Laplace relations and places TRAPPIST-1h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1h has a radius of 0.727 Earth radii and an equilibrium temperature of 173 K. We have also measured the rotational period of the star at 3.3 d and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf.

astro-ph.EP↗

arXiv2017

Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away. Indeed, the transiting configuration of these planets with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible in-depth studies of their atmospheric properties with current and future astronomical facilities. Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope. Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces.

astro-ph.EP↗