roAp Stars Research Articles

Long-period Ap stars discovered with TESS data

The TESS space mission’s primary goal is to search for exoplanets around bright, nearby stars. Because of the high-precision photometry required for the main mission, it is also producing superb data for asteroseismology, eclipsing binary stars, gyrochronology, indeed any field of stellar astronomy where the data are variable light curves. In this work we show that the TESS data are excellent for astrophysical inference from peculiar stars that show no variability. Ap stars have the strongest magnetic fields of any main-sequence star. Some Ap stars have also been shown to have rotation periods of months, years, decades, and even centuries. The astrophysical cause of their slow rotation – the braking mechanism – is not known with certainty. These stars are rare: there are currently about three dozen with known periods. Magnetic Ap stars have long-lived spots that allow precise determination of their rotation periods. We argue and show that most Ap stars with TESS data that show no low-frequency variability must have rotation periods longer than, at least, a TESS sector of 27 d. From this 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 very low projected equatorial velocities, which proves our technique; 23 are new discoveries. These are now prime targets for long-term magnetic studies. We also find that 12 of the 54 (22%) long-period Ap stars are roAp stars, versus only 3% (29 out of 960) of the other Ap stars studied with TESS in Sectors 1–13, showing that the roAp phenomenon is correlated with rotation, although this correlation is not necessarily causal. In addition to probing rotation in Ap stars, these constant stars are also excellent targets to characterise the instrumental behaviour of the TESS cameras, as well as for the CHEOPS and PLATO missions. This work demonstrates astrophysical inference from nonvariable stars – we can get “something for nothing”.

The 10.5 year rotation period of the strongly magnetic rapidly oscillating Ap star HD 166473

How magnetic fields contribute to the differentiation of the rotation rates of the Ap stars and affect the occurrence of non-radial pulsation in some of them are important open questions. Valuable insight can be gained into these questions by studying some of the most extreme examples of the processes at play. The super-slowly rotating rapidly oscillating Ap (roAp) star HD 166473 is such an example. We performed the first accurate determination of its rotation period, Prot = (3836 ± 30) d, from the analysis of 56 measurements of the mean magnetic field modulus ⟨B⟩ based on high-resolution spectra acquired between 1992 and 2019 at various observatories and with various instrumental configurations. We complemented this analysis with the consideration of an inhomogeneous set of 21 determinations of the mean longitudinal magnetic field ⟨Bz⟩ spanning the same time interval. This makes HD 166473 one of only four Ap stars with a period longer than 10 years for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of ⟨B⟩ and of ⟨Bz⟩ are well approximated by cosine waves. The magnetic field of HD 166473 only seems to deviate slightly from axisymmetry, but it definitely involves a considerable non-dipolar component. Among the stars with rotation periods longer than 1000 d for which magnetic field measurements with full phase coverage are available, HD 166473 has the strongest field. Its magnetic field is also one of the strongest known among roAp stars. Overall, the magnetic properties of HD 166473 do not seem fundamentally distinct from those of the faster-rotating Ap stars. However, considering as a group the eight Ap stars that have accurately determined periods longer than 1000 d and whose magnetic variations have been characterised over a full cycle suggests that the angles between their magnetic and rotation axes tend to be systematically large.

HD 42659: the only known roAp star in a spectroscopic binary observed with B photometry, TESS, and SALT

ABSTRACT We present a multi-instrument analysis of the rapidly oscillating Ap (roAp) star HD 42659. We have obtained B photometric data for this star and use these data, in conjunction with the Transiting Exoplanet Survey Satellite (TESS) observations, to analyse the high-frequency pulsation in detail. We find a triplet that is split by the rotation frequency of the star (νrot = 0.3756 d−1; Prot = 2.66 d) and present both distorted dipole and distorted quadrupole mode models. We show that the pulsation frequency, 150.9898 d−1 (Ppuls = 9.54 min), is greater than the acoustic cut-off frequency. We utilize 27 high-resolution ($R\simeq 65\, 000$), high signal-to-noise ratio (∼120) spectra to provide new orbital parameters for this, the only known roAp star to be in a short-period binary (Porb = 93.266 d). We find the system to be more eccentric than previously thought, with e = 0.317, and suggest the companion is a mid-F to early-K star. We find no significant trend in the average pulsation mode amplitude with time, as measured by TESS, implying that the companion does not have an effect on the pulsation in this roAp star. We suggest further photometric observations of this star, and further studies to find more roAp stars in close binaries to characterize how binarity may affect the detection of roAp pulsations.

Adaptive elliptical aperture photometry: A software package for high-cadence ground-based photometry

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 on the order of 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 employed 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 ∼1 mmag usingBdata 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.

Statistical analysis of roAp, He-weak, and He-rich stars

Abstract To enlarge our data base of chemically peculiar stars, we compiled published data concerning the He-weak and He-rich stars observed by high-resolution spectroscopy techniques during last decades. Twenty He-weak and 28 He-rich stars have been added to the data base. We have also distinguished roAp stars from stars previously identified as Ap stars. To deepen our knowledge on statistical overview of the abundance anomalies versus the physical parameters of stars, we compared our data with previous compilations. We applied statistical tests on our data and found interesting correlations for effective temperature and surface gravity for all type of stars and a few correlations for projected rotation velocity only for He-rich stars. Because of the lack of the data, we could not check whether being a member of binary system is affecting on chemical peculiarities of those stars.

Rotation and pulsation in Ap stars: first light results from TESS sectors 1 and 2

We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these, and the singly-periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor 6 smaller than that in the $B$ filter which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 micromag, respectively, in two of the best characterised non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.

High frequencies in TESS A–F main-sequence stars

Abstract The driving mechanism for high-frequency oscillations in some chemically peculiar Ap stars, the rapidly oscillating Ap stars (roAp stars), is not understood. The Transiting Exoplanet Survey Satellite mission (TESS) data provide an ideal opportunity to extend the number of roAp stars that might provide further clues to address this problem. From an examination of over 18 000 stars in TESS sectors 1–7, we have discovered high-frequency pulsations in 14 A–F stars, of which only 3 are classified as Ap stars. In addition to these new discoveries, we discuss the frequencies in nine previously known roAp stars. In one of these stars, HD 60435, we confirm a previous finding that the pulsations have lifetimes of only a few days. In another known roAp star, HD 6532, the relative amplitudes of the rotationally modulated sidelobes, which are generally used to estimate the inclination of the magnetic axis relative to the rotational axis, are significantly different from those found in ground-based B-band photometric observations. We also discuss four δ Scuti stars that appear to have independent frequencies similar to those of roAp stars.

Pulsation in Intermediate-Mass Stars

A new perspective of pulsation in stars within the δ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all δ Scuti stars contain low frequencies. Because γ Doradus stars co-exist with δ Sct stars in the same region of the instability strip, it follows that γ Dor stars are driven by the same mechanism as δ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot γ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6300–8300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same mass - temperature - luminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere.

The search for roAp stars: null results and new candidates from Strömgren-Crawford photometry

The rapidly oscillating Ap (roAp) stars exhibit pulsational photometric and/or radial velocity variations on time scales of several minutes, which are essential to test current pulsation models as well as our assumptions of atmospheric structure characteristics. In addition, their chemical peculiarity makes them very interesting for probing stellar formation and evolution in the presence of a global magnetic field. To date, a limited number of only 61 roAp stars is known to show photometric variability. On the other hand, a literature survey yields 619 unique stars that have unsuccessfully been searched for variability of this kind. Strömgren-Crawford uvbyβ photometry of stars from both subgroups was used to investigate whether there is a selection bias for the investigated stars. We also present new photometric measurements (202 hours on 59 different nights) of 55 roAp candidates. We did not detect any new roAp star. Although our detection limits are comparable to other surveys, we also did not find pulsations in the known roAp star HD 12098, which may be a consequence of temporal amplitude changes. On the other hand, we do find some evidence for photometric variability of beta CrB at its spectroscopically derived pulsation period. From the uvbyβ photometry we conclude that the blue border of the roAp instability strip appears observationally well defined, whereas the red border is rather poorly known and studied. Within these boundaries, a total of 4646 candidates were identified which appear worth investigating for short-term pulsational variability.

K2 space photometry reveals rotational modulation and stellar pulsations in chemically peculiar A and B stars

Context. The physics of magnetic hot stars and how a large-scale magnetic field affects their interior properties is largely unknown. Few studies have combined high-quality observations and modelling of magnetic pulsating stars, known as magneto-asteroseismology, primarily because of the dearth of detected pulsations in stars with a confirmed and well-characterised large-scale magnetic field. Aims. We aim to characterise observational signatures of rotation and pulsation in chemically peculiar candidate magnetic stars using photometry from the K2 space mission. Thus, we identify the best candidate targets for ground-based, optical spectropolarimetric follow-up observations to confirm the presence of a large-scale magnetic field. Methods. We employed customised reduction and detrending tools to process the K2 photometry into optimised light curves for a variability analysis. We searched for the periodic photometric signatures of rotational modulation caused by surface abundance inhomogeneities in 56 chemically peculiar A and B stars. Furthermore, we searched for intrinsic variability caused by pulsations (coherent or otherwise) in the amplitude spectra of these stars. Results. The rotation periods of 38 chemically peculiar stars are determined, 16 of which are the first determination of the rotation period in the literature. We confirm the discovery of high-overtone roAp pulsation modes in HD 177765 and find an additional 3 Ap and Bp stars that show evidence of high-overtone pressure modes found in roAp stars in the form of possible Nyquist alias frequencies in their amplitude spectra. Furthermore, we find 6 chemically peculiar stars that show evidence of intrinsic variability caused by gravity or pressure pulsation modes. Conclusions. The discovery of pulsations in a non-negligible fraction of chemically peculiar stars make these stars high-priority targets for spectropolarimetric campaigns to confirm the presence of their expected large-scale magnetic field. The ultimate goal is to perform magneto-asteroseismology and probe the interior physics of magnetic pulsating stars.

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, $\Delta\nu$, of 78.9 $\mu$Hz, twice that reported in the literature. Other frequency separations we detect may represent the small frequency separation, $\delta\nu$, 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 $\kappa$-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.

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 $\mu$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.

A theoretical tool for the study of radial velocities in the atmospheres of roAp stars

Over the last decade significant amounts of high-spectral and time-resolution spectroscopic data have been acquired for a number of rapidly oscillating Ap stars. Progress in the understanding of the information held by these data requires the development of theoretical models that can be directly compared with them. In this work we present a theoretical model for the radial velocities of roAp stars that takes full account of the coupling between the pulsations and the magnetic field. We explore the impact on the radial velocities of changing the position of the observer, the mode frequency and angular degree, as well as of changing the region of the disk where the elements are concentrated. We find that for integrations over the full disc, in the outermost layers the radial velocity is generally dominated by the acoustic waves, showing a rapid increase in amplitude. The most significant depth-variations in the radial velocity phase are seen for observers directed towards the equator and for even degree modes with frequencies close to, or above the acoustic cutoff. Comparison between the radial velocities obtained for spots of elements located around the magnetic poles and around the magnetic equator, shows that these present distinct amplitude-phase relations, resembling some of the differences seen in the observations. Finally, we discuss the conditions under which one may expect to find false nodes in the pulsation radial velocity of roAp stars.

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$^{-1}$ ($1758.45 \mu$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.

Poster Presentations

49 posters were displayed at IAU S339. The display space was adequate to allow all to be on show throughout the conference.Poster presenters were given the option of condensing the message(s) of their posters into written summaries and/or depositing a pdf of the original poster for inclusion in the on-line version of the Proceedings. Half of the poster authors responded; 12 chose to send written summaries, 9 submitted just their original pdfs, and 4 elected to send both. Workshop 1 also converted some catalogue material into an on-line poster. This Section contains the contributions that were submitted as written summaries, arranged in alphabetical order of lead author.Near-Infrared Observations of OGLE Classical and Type II Cepheid Variables in the LMC ………. 283A. Bhardwaj, L. M. Macri, S. M. Kanbur, C-C. Ngeow and H. P. SinghMultiwavelength Light-Curve Analysis of Cepheid Variables ………. 287A. Bhardwaj, S. M. Kanbur, M. Marconi, H. P. Singh, M. Rejkuba and C-C. NgeowSymbiotic Stars in the Local Group of Galaxies: POINT-AGAPE Catalogue Revisited ………. 291K. Drozd, J. Mikołajewska, M. Darnley, K. Iłkiewicz, N. Caldwell and M. SharaDetection of Sectoral Modes in the Eclipsing Binary KIC 4851217………. 295M. Fedurco, Š. Parimucha & P. GajdošKepler-410Ab and Transit Timing Variations ………. 299P. Gajdoš, Š. Parimucha and M. FedurcoSparse Spatio-Temporal Imaging of Radio Transients ………. 303J. Girard, M. Jiang, J-L. Starck and S. CorbelTesting the SALT High-Resolution Spectrograph for Pulsation Studies of roAp Stars ………. 307D. Holdsworth and D. KurtzProbing Convective Mixing in Stellar Interiors with α Centauri A and B………. 308M. Joyce and B. ChaboyerHighly Luminous SNe Associated with GRBs………. 309D. KannSearching for Pulsating Stars Using Clustering Algorithms………. 310R. Kgoadi, I. B. Whittingham and C. A. EngelbrechtPhase-resolved Spectroscopy and Photometry of the Eclipsing Polar UZ Fornacis 314Z. N. Khangale, S. B. Potter and P. A. WoudtMeteor Sky in Time-Domain Astronomy………. 318S. V. KolomiyetsFirst Results from Project SUNBIRD: Supernovæ UNmasked By Infra-Red Detection………. 322E. C. Kool, S. D. Ryder, E. Kankare, T. Reynolds, S. Mattila, M. Pérez-Torres and R. McDermidThe X-Ray Spectrum of the X-Ray Binary 4U 1728-34, observed with Suzaku ………. 323Y. Lei, H. Zhang, H. Yuan and Y. ZhangStatistical Research into Correlation between Solar Filaments and Flare Activities………. 327G. LinA CoRoT view of the ζ Aur binary HR 6902………. 329C. Maceroni, J. Montalbán, R. Da Silva, T. Semaan, B. Mosser, M. Rainer, E. Poretti and E. GriffinSearching for Long-Period Binary Central Stars of Planetary Nebulæ with SALT HRS………. 330B. Miszalski, R. Manick, J. Mikołajewska, K. Iłkiewicz, D. Kamath and H. Van WinckelCompanion(s) of the Eclipsing Binary KIC 3832716………. 331Š. Parimucha, M. Fedurco and P. GajdošEvaluating the Fraction of Obscured Supernovæ in Luminous Infrared Galaxies with Adaptive Optics Surveys………. 335T. Reynolds, S. Mattila, E. Kool, E. Kankare, S. Ryder and M. A. Pérez-TorresDetecting AGB stars in LG Dwarf Galaxies for Understanding Galaxy Formation and Evolution………. 336E. Saremi, A. Javadi, J. van Loon, H. Khosroshahi and M. TorkiAccurate Photometry with Digitized Photographic Plates of the Moscow Collection………. 340K. V. Sokolovsky D. M. Kolesnikova, N. N. Samus, S. V. Antipin and A.A. BelinskiThe All-Sky Automated Search for Supernovæ Going Global………. 344M. Stritzinger and the ASAS-SN TeamStellar Parameterisation using KPCA and SVM………. 345H. Yuan, Y. Zhang, Y., Yiqiao Dong, Z. Bai, G. Li, W. Zhang, H. Zhang and Y. ZhaoΔa Photometric Survey of the Small Magellanic Cloud………. 349M. Zejda, E. Paunzen and Z. MikulášekDesign, Build and Test of the VOEvent Network for the SVOM Chinese Ground Segment………. 353M. Zhang, M. Huang and C. Wu

Testing the SALT High-Resolution Spectrograph for Pulsation Studies of roAp Stars

AbstractWe present the first time-resolved spectroscopic observations, made with the SALT HRS instrument, of a rapidly oscillating Ap star. We used the instrument in the High Stability mode, with the fastest readout settings – a setup never previously used. Over a 2.5-hr track length, we obtained 280 spectra at 8-second integration times and a cadence of 30 seconds. The target, α Circini, is the brightest of the roAp stars, and thus provides an excellent opportunity to test the instrument. Previous time-resolved spectroscopic studies of this star have been conducted by Kurtz, Elkin & Mathys† with the VLT/UVES instrument, and by Mkrtichian & Hatzes‡ with the HARPS instrument on the ESO 3.6-m telescope. Those two studies provide us with benchmarks to compare the performance of SALT/HRS for this type of project. With the upcoming TESS mission, the ability to perform high-precision, time-resolved spectroscopy of pulsating stars will be key for the scientific output of SALT.

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 $\mu$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.

The first K2 roAp star: HD 24355 pulsating in a distorted quadruploe mode

We present an analysis of the first K2 observations of a rapidly oscillating Ap (roAp) star, HD 24355. The star was discovered to be a roAp star by [1], with a frequency of 224.31 d-1 (2596.18 μ Hz; P = 6.4 min) and an amplitude of 1.51 mmag in SuperWASP broadband photometry. Spectroscopic analysis of low-resolution spectra show the star 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. This is the largest number of sidelobes seen in a roAp star to date. We also see an unusual pulsational phase variation as the star rotates, showing this star to be the most distorted quadrupole roAp pulsator yet observed. Modelling of this star confirms its quadrupole nature, and allows us to constrain the magnetic field strength, angle of inclination and the angle of obliquity.

Detection of new pulsations in the roAp star HD 177765

We report the discovery of 2 previously undetected pulsation frequencies in the known roAp star HD 177765. Photometric observations by the Kepler space telescope during K2 Campaign 7 show low-amplitude pulsations (4-11 micro mag) previously unseen in photometry. We show the pulsations to be stable over the observation period, and demonstrate that the separation of the frequencies is not representative of the large frequency separation quantity needed to perform asteroseismic analysis.

HD 24355 observed by theKepler K2mission: 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 $\mu$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 $\mu$mag spaced-based discoveries. No further pulsation modes are identified in the K2 data, showing this star to be a single-mode pulsator.