Search

Context. The BRIght Target Explorer (BRITE) mission is a pioneering space project aimed at the long-term photometric monitoring of the brightest stars in the sky by means of a constellation of nanosatellites. Its main advantage is high photometric accuracy and time coverage which are inaccessible from the ground. Its main drawback is the lack of cooling of the CCD detectors and the absence of good shielding that would protect them from energetic particles. Aims. The main aim of this paper is the presentation of procedures used to obtain high-precision photometry from a series of images acquired by the BRITE satellites in two modes of observing, stare and chopping. The other aim is a comparison of the photometry obtained with two different pipelines and a comparison of the real scatter with expectations. Methods. We developed two pipelines corresponding to the two modes of observing. They are based on aperture photometry with a constant aperture, circular for stare mode of observing and thresholded for chopping mode. Impulsive noise is a serious problem for observations made in the stare mode of observing and therefore in the pipeline developed for observations made in this mode, hot pixels are replaced using the information from shifted images in a series obtained during a single orbit of a satellite. In the other pipeline, the hot pixel replacement is not required because the photometry is made in difference images. Results. The assessment of the performance of both pipelines is presented. It is based on two comparisons, which use data from six runs of the UniBRITE satellite: (i) comparison of photometry obtained by both pipelines on the same data, which were partly affected by charge transfer inefficiency (CTI), (ii) comparison of real scatter with theoretical expectations. It is shown that for CTI-affected observations, the chopping pipeline provides much better photometry than the other pipeline. For other observations, the results are comparable only for data obtained shortly after switching to chopping mode. Starting from about 2.5 years in orbit, the chopping mode of observing provides significantly better photometry for UniBRITE data than the stare mode. Conclusions. This paper shows that high-precision space photometry with low-cost nanosatellites is achievable. The proposed methods, used to obtain photometry from images affected by high impulsive noise, can be applied to data from other space missions or even to data acquired from ground-based observations. [ABSTRACT FROM AUTHOR]

A multisite photometric campaign for the β Cephei and eclipsing variable 16 Lacertae is reported. 749 h of high-quality differential photoelectric Stromgren, Johnson and Geneva time series photometry were obtained with 10 telescopes during 185 nights. After removing the pulsation contribution, an attempt was made to solve the resulting eclipse light curve by means of the computer program EBOP. Although a unique solution was not obtained, the range of solutions could be constrained by comparing computed positions of the secondary component in the Hertzsprung-Russell diagram with evolutionary tracks. For three highamplitude pulsation modes, the uvy and the Geneva UBG amplitude ratios are derived and compared with the theoretical ones for spherical-harmonic degrees l ≤ 4. The highest degree, l = 4, is shown to be incompatible with the observations. One mode is found to be radial, one is l = 1, while in the remaining case l = 2 or 3. The present multisite observations are combined with the archival photometry in order to investigate the long-term variation of the amplitudes and phases of the three high-amplitude pulsation modes. The radial mode shows a non-sinusoidal variation on a time-scale of 73 yr. The l = 1 mode is a triplet with unequal frequency spacing, giving rise to two beat-periods, 720.7 d and 29.1 yr. The amplitude and phase of the l = 2 or 3 mode vary on time-scales of 380.5 d and 43 yr. The light variation of 2 And, one of the comparison stars, is discussed in the appendix. [ABSTRACT FROM AUTHOR]

Context. The first two of a total of six nano-satellites that will constitute the BRITE-Constellation space photometry mission have recently been launched successfully. Aims. In preparation for this project, we carried out time-resolved colour photometry in a field that is an excellent candidate for BRITE measurements from space. Methods. We acquired 117 h of Strömgren uvy data during 19 nights. Our targets comprised the β Cephei stars κ and λ Sco, the eclipsing binary μ1 Sco, and the variable super/hypergiant ζ 1 Sco. Results. For κ Sco, a photometric mode identification in combination with results from the spectroscopic literature suggests a dominant (l,m) = (1, -1) β Cephei-type pulsation mode of the primary star. The longer period of the star may be a rotational variation or a g-mode pulsation. For λ Sco, we recover the known dominant β Cephei pulsation, a longer-period variation, and observed part of an eclipse. Lack of ultraviolet data precludes mode identification for this star. We noticed that the spectroscopic orbital ephemeris of the closer pair in this triple system is inconsistent with eclipse timings and propose a refined value for the orbital period of the closer pair of 5.95189 ± 0.00003 d.We also argue that the components of the λ Sco system are some 30% more massive than previously thought. The binary light curve solution of μ 1 Sco requires inclusion of the irradiation effect to explain the u light curve, and the system could show additional low amplitude variations on top of the orbital light changes. ζ 1 Sco shows long-term variability on a time scale of at least two weeks that we prefer to interpret in terms of a variable wind or strange mode pulsations. [ABSTRACT FROM AUTHOR]

We present three years of long-cadence and over one year of short-cadence photometry of the central star of the planetary nebula NGC 6826 obtained with the Kepler spacecraft, and temporally coinciding optical spectroscopy. The light curves are dominated by incoherent variability on time-scales of several hours, but contain a lower amplitude periodicity of 1.237 99 d. The temporal amplitude and shape changes of this signal are best explicable with a rotational modulation, and are not consistent with a binary interpretation. We argue that we do not observe stellar pulsations within the limitations of our data, and show that a binary central star with an orbital period less than seven days could only have escaped our detection in the case of low orbital inclination. Combining the photometric and spectroscopic evidence, we reason that the hourly variations are due to a variable stellar wind, and are global in nature. The physical cause of the wind variability of NGC 6826 and other ZZ Leporis stars is likely related to the mechanism responsible for wind variations in massive hot stars. [ABSTRACT FROM AUTHOR]

ABSTRACT We used extensive ground-based multisite and archival spectroscopy to derive observational constraints for a seismic modelling of the magnetic β Cep star V2052 Ophiuchi. The line-profile variability is dominated by a radial mode ( f1 = 7.148 46 d−1) and by rotational modulation ( Prot = 3.638 833 d). Two non-radial low-amplitude modes ( f2 = 7.756 03 d−1 and f3 = 6.823 08 d−1) are also detected. The four periodicities that we found are the same as the ones discovered from a companion multisite photometric campaign and known in the literature. Using the photometric constraints on the degrees ℓ of the pulsation modes, we show that both f2 and f3 are prograde modes with (ℓ, m) = (4, 2) or (4, 3). These results allowed us to deduce ranges for the mass ( M ∈ [8.2, 9.6] M⊙) and central hydrogen abundance ( X c ∈ [0.25, 0.32]) of V2052 Oph, to identify the radial orders n1 = 1, n2 = −3 and n3 = −2, and to derive an equatorial rotation velocity veq ∈ [71, 75] km s−1. The model parameters are in full agreement with the effective temperature and surface gravity deduced from spectroscopy. Only models with no or mild core overshooting (αov ∈ [0, 0.15] local pressure scale heights) can account for the observed properties. Such a low overshooting is opposite to our previous modelling results for the non-magnetic β Cep star θ Oph having very similar parameters, except for a slower surface rotation rate. We discuss whether this result can be explained by the presence of a magnetic field in V2052 Oph that inhibits mixing in its interior. [ABSTRACT FROM AUTHOR]

Context. The rapidly oscillating Ap (roAp) stars exhibit short time scale photometric and/or radial velocity variations, which are most important to test current pulsation models as well as our assumptions of the 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, only a limited number of about 45 roAp stars are known. Aims. In order to increase the sample, we obtained photometric time series of 20 good roAp candidates to search for pulsations. Methods. We present the time series analysis of about 60 h of CCD photometry taken at the 1 m Austrian-Croatian Telescope (Hvar Observatory) and derive effective temperatures for the programme objects. Results. The upper amplitude limits of the Fourier spectra are typically below 2 mmag in Bessell B with one good candidate for follow-up observations to find possible pulsation. In addition, we present a list of further roAp candidates, worth to be (re)investigated. [ABSTRACT FROM AUTHOR]

ABSTRACT During two weeks of nearly continuous optical photometry of the young open cluster NGC 2244 obtained by the Microvariability and Oscillations of STars ( MOST) satellite, we discovered two new slowly pulsating B (SPB) stars, GSC 00154−00785 and GSC 00154−01871. We present frequency analyses of the MOST light curves of these stars, which reveal two oscillation frequencies (0.61 and 0.71 cycle d−1) in GSC 00154−00785 and two (0.40 and 0.51 cycle d−1) in GSC 00154−01871. These frequency ranges are consistent with g modes of ℓ≤ 2 excited in models of main-sequence or pre-main-sequence (PMS) stars of masses 4.5-5 M⊙ and solar composition ( X, Z) = (0.7, 0.02). Published proper motion measurements and radial velocities are insufficient to establish unambiguously cluster membership for these two stars. However, the PMS models which fit best their eigenspectra have ages consistent with NGC 2244. If cluster membership can be confirmed, these would be the first known PMS SPB stars, and would open a new window on testing asteroseismically the interior structures of PMS stars. [ABSTRACT FROM AUTHOR]

BSTRACT PG 2303+243 is a cool DA variable (also called ZZ Ceti) star with a rich pulsation spectrum and variable amplitudes. A mini-campaign involving six observatories yielded time-resolved photometric measurements of PG 2303+243 during the period 2004 September 5-20. A duty cycle of 35 per cent was achieved. We detected 24 possible independent frequencies, their amplitudes and phases for future mode identification. We confirm the occurrence of short-term amplitude and frequency variations. Our analysis suggests an l= 1 rotational splitting around 8.4 μHz, implying a rotation period of 16.5 h. [ABSTRACT FROM AUTHOR]

ABSTRACT The Kepler Mission has acquired 33.5 d of continuous 1-min photometry of KPD 1946+4340, a short-period binary system that consists of a subdwarf B star (sdB) and a white dwarf. In the light curve, eclipses are clearly seen, with the deepest occurring when the compact white dwarf crosses the disc of the sdB (0.4 per cent) and the more shallow ones (0.1 per cent) when the sdB eclipses the white dwarf. As expected, the sdB is deformed by the gravitational field of the white dwarf, which produces an ellipsoidal modulation of the light curve. Spectacularly, a very strong Doppler beaming (also known as Doppler boosting) effect is also clearly evident at the 0.1 per cent level. This originates from the sdB's orbital velocity, which we measure to be 164.0 ± 1.9 km s−1 from supporting spectroscopy. We present light-curve models that account for all these effects, as well as gravitational lensing, which decreases the apparent radius of the white dwarf by about 6 per cent, when it eclipses the sdB. We derive system parameters and uncertainties from the light curve using Markov chain Monte Carlo simulations. Adopting a theoretical white dwarf mass-radius relation, the mass of the subdwarf is found to be 0.47 ± 0.03 M⊙ and the mass of the white dwarf 0.59 ± 0.02 M⊙. The effective temperature of the white dwarf is 15 900 ± 300 K. With a spectroscopic effective temperature of Teff= 34 730 ± 250 K and a surface gravity of log g= 5.43 ± 0.04, the subdwarf has most likely exhausted its core helium, and is in a shell He burning stage. The detection of Doppler beaming in Kepler light curves potentially allows one to measure radial velocities without the need of spectroscopic data. For the first time, a photometrically observed Doppler beaming amplitude is compared to a spectroscopically established value. The sdB's radial velocity amplitude derived from the photometry ( 168 ± 4 km s−1) is in perfect agreement with the spectroscopic value. After subtracting our best model for the orbital effects, we searched the residuals for stellar oscillations but did not find any significant pulsation frequencies. [ABSTRACT FROM AUTHOR]

We present the discovery of non-radial pulsations in five hot subdwarf B (sdB) stars based on 27 d of nearly continuous time series photometry using the Kepler spacecraft. We find that every sdB star cooler than K that Kepler has observed (seven so far) is a long-period pulsator of the V1093 Her (PG 1716) class or a hybrid star with both short and long periods. The apparently non-binary long-period and hybrid pulsators are described here. The V1093 Her periods range from 1 to 4.5 h and are associated with -mode pulsations. Three stars also exhibit short periods indicative of -modes with periods of 2-5 min and in addition, these stars exhibit periodicities between both classes from 15 to 45 min. We detect the coolest and longest-period V1093 Her-type pulsator to date, KIC010670103 ( K, h) as well as a suspected hybrid pulsator, KIC002697388, which is extremely cool ( K) and for the first time hybrid pulsators which have larger -mode amplitudes than -mode ones. All of these pulsators are quite rich with many frequencies and we are able to apply asymptotic relationships to associate periodicities with modes for KIC010670103. Kepler data are particularly well suited for these studies as they are long duration, extremely high duty cycle observations with well-behaved noise properties. [ABSTRACT FROM AUTHOR]

We present the discovery of non-radial pulsations in a hot subdwarf B star based on 30.5 d of nearly continuous time series photometry using the Kepler spacecraft. KIC 010139564 is found to be a short-period pulsator of the V361 Hya (EC 14026) class with more than 10 independent pulsation modes whose periods range from 130 to 190 s. It also shows one periodicity at a period of 3165 s. If this periodicity is a high-order -mode, then this star may be the hottest member of the hybrid DW Lyn stars. In addition to the resolved pulsation frequencies, additional periodic variations in the light curve suggest that a significant number of additional pulsation frequencies may be present. The long duration of the run, the extremely high duty cycle and the well-behaved noise properties allow us to explore the stability of the periodic variations, and to place strong constraints on how many of them are independent stellar oscillation modes. We find that most of the identified periodicities are indeed stable in phase and amplitude, suggesting a rotation period of 2-3 weeks for this star, but further observations are needed to confirm this suspicion. [ABSTRACT FROM AUTHOR]

We present results from the first two quarters of a survey to search for pulsations in compact stellar objects with the Kepler spacecraft. The survey sample and the various methods applied in its compilation are described, and spectroscopic observations are presented to separate the objects into accurate classes. From the Kepler photometry we clearly identify nine compact pulsators and a number of interesting binary stars. Of the pulsators, one shows the strong, rapid pulsations typical of a V361 Hya-type sdB variable (sdBV); seven show long-period pulsation characteristics of V1093 Her-type sdBVs; and one shows low-amplitude pulsations with both short and long periods. We derive effective temperatures and surface gravities for all the subdwarf B stars in the sample and demonstrate that below the boundary region where hybrid sdB pulsators are found, all our targets are pulsating. For the stars hotter than this boundary temperature a low fraction of strong pulsators (10 per cent) is confirmed. Interestingly, the short-period pulsator also shows a low-amplitude mode in the long-period region, and several of the V1093 Her pulsators show low-amplitude modes in the short-period region, indicating that hybrid behaviour may be common in these stars, also outside the boundary temperature region where hybrid pulsators have hitherto been found. [ABSTRACT FROM AUTHOR]

The survey phase of the Kepler Mission includes a number of hot subdwarf B (sdB) stars to search for non-radial pulsations. We present our analysis of two sdB stars that are found to be -mode pulsators of the V1093 Her class. These two stars also display the distinct irradiation effect typical of sdB stars with a close M-dwarf companion with orbital periods of less than half a day. Because the orbital period is so short, the stars should be in synchronous rotation, and if so, the rotation period should imprint itself on the multiplet structure of the pulsations. However, we do not find clear evidence for such rotational splitting. Though the stars do show some frequency spacings that are consistent with synchronous rotation, they also display multiplets with splittings that are much smaller. Longer-duration time series photometry will be needed to determine if those small splittings are in fact rotational splitting, or caused by slow amplitude or phase modulation. Further data should also improve the signal-to-noise ratio, perhaps revealing lower-amplitude periodicities that could confirm the expectation of synchronous rotation. The pulsation periods seen in these stars show period spacings that are suggestive of high-overtone -mode pulsations. [ABSTRACT FROM AUTHOR]

We present a detailed study of the pulsation of α Circini, the brightest of the rapidly oscillating Ap stars. We have obtained 84 d of high-precision photometry from four runs with the star tracker on the WIRE satellite. Simultaneously, we collected ground-based Johnson B observations on 16 nights at the South African Astronomical Observatory. In addition to the dominant oscillation mode at 2442 μHz, we detect two new modes that lie symmetrically around the principal mode to form a triplet. The average separation between these modes is and they are nearly equidistant with the separations differing by only 3.9 nHz. We compare the observed frequencies with theoretical pulsation models based on constraints from the recently determined interferometric radius and effective temperature, and the recently updated Hipparcos parallax. We show that the theoretical large separations for models of α Cir with global parameters within the 1σ observational uncertainties vary between 59 and 65 μHz. This is consistent with the large separation being twice the observed value of , indicating that the three main modes are of alternating even and odd degrees. The frequency differences in the triplet are significantly smaller than those predicted from our models, for all possible combinations of mode degrees, and may indicate that the effects of magnetic perturbations need to be taken into account. The WIRE light curves are modulated by a double wave with a period of 4.479 d, and a peak-to-peak amplitude of 4 mmag. This variation is due to the rotation of the star and is a new discovery, made possible by the high precision of the WIRE photometry. The rotational modulation confirms an earlier indirect determination of the rotation period. The main pulsation mode at 2442 μHz has two sidelobes split by exactly the rotation frequency. From the amplitude ratio of the sidelobes to the central peak, we show that the principal mode is consistent with an oblique axisymmetric dipole mode or with a magnetically distorted mode of higher degree with a dominant dipolar component. [ABSTRACT FROM AUTHOR]

We have carried out single and multisite photometry of the three β Cephei stars β and 15 CMa as well as KZ Mus. For the two stars in CMa, we obtained 270 h of measurement in the Strömgren uvy and Johnson V filters, while 150 h of time-resolved Strömgren uvy photometry was acquired for KZ Mus. All three stars are multiperiodic variables, with three (β CMa) and four (15 CMa, KZ Mus) independent pulsation modes. Two of the mode frequencies of 15 CMa are new discoveries and one of the known modes showed amplitude variations over the last 33 yr. Taken together, this fully explains the diverse behaviour of the star reported in the literature. Mode identification by means of the amplitude ratios in the different passbands suggests one radial mode for each star. In addition, β CMa has a dominant mode while its third mode is non-radial with unknown ℓ. The non-radial modes of 15 CMa, which are , form an almost equally split triplet that, if physical, would imply that we see the star under an inclination angle larger than 55°. The strongest non-radial mode of KZ Mus is , followed by the radial mode and a dipole mode. Its weakest known mode is non-radial with unknown ℓ, confirming previous mode identifications for the pulsations of the star. The phased light curve for the strongest mode of 15 CMa has a descending branch steeper than the rising branch. A stillstand phenomenon during the rise to maximum light is indicated. Given the low photometric amplitude of this non-radial mode this is at first sight surprising, but it can be explained by the aspect angle of the mode. [ABSTRACT FROM AUTHOR]

We report a multisite photometric campaign for the β Cephei star 12 Lacertae. 750 h of high-quality differential photoelectric Strömgren, Johnson and Geneva time-series photometry were obtained with nine telescopes during 190 nights. Our frequency analysis results in the detection of 23 sinusoidal signals in the light curves. Ten of those correspond to independent pulsation modes, and the remainder are combination frequencies. We find some slow aperiodic variability such as that seemingly present in several β Cephei stars. We perform mode identification from our colour photometry, derive the spherical degree ℓ for the five strongest modes unambiguously and provide constraints on ℓ for the weaker modes. We find a mixture of modes of . In particular, we prove that the previously suspected rotationally split triplet within the modes of 12 Lac consists of modes of different ℓ; their equal frequency splitting must thus be accidental. One of the periodic signals we detected in the light curves is argued to be a linearly stable mode excited to visible amplitude by non-linear mode coupling via a 2:1 resonance. We also find a low-frequency signal in the light variations whose physical nature is unclear; it could be a parent or daughter mode resonantly coupled. The remaining combination frequencies are consistent with simple light-curve distortions. The range of excited pulsation frequencies of 12 Lac may be sufficiently large that it cannot be reproduced by standard models. We suspect that the star has a larger metal abundance in the pulsational driving zone, a hypothesis also capable of explaining the presence of β Cephei stars in the Large Magellanic Cloud. [ABSTRACT FROM AUTHOR]

We have carried out a three-site photometric campaign for the β Cephei star θ Oph from 2003 April to August. 245 h of differential photoelectric u v y photometry were obtained during 77 clear nights. The frequency analysis of our measurements has resulted in the detection of seven pulsation modes within a narrow frequency interval between 7.116 and 7.973 c d−1. No combination or harmonic frequencies have been found. We have performed a mode identification of the individual pulsations from our colour photometry that shows the presence of one radial mode, one rotationally split triplet and possibly three components of a rotationally split quintuplet. We discuss the implications of our findings and point out the similarity of the pulsation spectrum of θ Oph to that of another β Cephei star, V836 Cen. [ABSTRACT FROM AUTHOR]

We report on the second multisite photometric campaign devoted toν Eridani (ν Eri). The campaign, carried out from 2003 September 11 to 2004 February 16, was very nearly a replica of the first campaign, 2002–2003: the five telescopes and photometers we used were the same as those in the first campaign, the comparison stars and observing procedure were identical, and the numbers and time baselines of the data were comparable.Forν Eri, analysis of the new data adds four independent frequencies to the nine derived previously from the 2002–2003 data: three in the range 7.20–7.93 d−1 and a low one, equal to 0.614 d−1. Combining the new and the old data results in two further independent frequencies, equal to 6.7322 and 6.2236 d−1. Altogether, the oscillation spectrum is shown to consist of 12 high and two low frequencies. The latter haveuamplitudes about twice as large as thevandyamplitudes, a signature of high radial-order g modes. Thus, we confirm the suggestion, put forward on the basis of the data of the first campaign, thatν Eri is both aβ Cephei and a slowly pulsating B (SPB) star.Nine of the 12 high frequencies form three triplets, of which two are new. The triplets represent rotationally splitmodes, although in case of the smallest-amplitude one this may be questioned. Mean separations and asymmetries of the triplets are derived with accuracy sufficient for meaningful comparison with models.The first comparison star,μ Eri, is shown to be an SPB variable with an oscillation spectrum consisting of six frequencies, three of which are equidistant in period. The star is also found to be an eclipsing variable. The eclipse is a transit, probably total, the secondary is fainter than the primary by several magnitudes, and the system is widely detached.The second comparison star,ξ Eri, is confirmed to be aδ Scuti variable. To the frequency of 10.8742 d−1 seen already in the data of the first campaign, another, equal to 17.2524 d−1, is added. [ABSTRACT FROM AUTHOR]

Using the large photometric and spectroscopic data sets of the ν Eridani multisite campaign given in our two recent papers (Aerts et al. and Handler et al.), we present an extended frequency analysis and a photometric mode identification. For the extended frequency analysis, we used an improved radial velocity time series, the second-moment time series and the line profiles themselves. In the radial velocity time series, we can now detect an additional pulsation frequency that was previously only found in photometric time series. We also report several new candidate pulsation frequencies. For seven frequencies, the photometric mode identification indicates that they belong to a radial mode and six dipole modes, and for three frequencies the degree ℓ could not be unambiguously determined. We also placed ν Eri in the Hertzsprung–Russell diagram by determining using Geneva plus Strömgren photometric calibrations, spectral energy distribution fitting, by non-local thermodynamic equilibrium hydrogen, helium and silicon line profile fitting, and by determining using the Hipparcos parallax and an Hβ calibration. [ABSTRACT FROM AUTHOR]

We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 h of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights. The frequency analysis of our measurements shows that the variability of ν Eri can be decomposed into 23 sinusoidal components, eight of which correspond to independent pulsation frequencies between 5 and 8 cd−1. Some of these are arranged in multiplets, which suggests rotational m-mode splitting of non-radial pulsation modes as the cause. If so, the rotation period of the star must be between 30 and 60 d. One of the signals in the light curves of ν Eri has a very low frequency of 0.432 cd−1. It can be a high-order combination frequency or, more likely, an independent pulsation mode. In the latter case, ν Eri would be both a β Cephei star and a slowly pulsating B (SPB) star. The photometric amplitudes of the individual pulsation modes of ν Eri appear to have increased by about 20 per cent over the last 40 years. So have the amplitudes of the dominant combination frequencies of the star. Among the latter, we could only identify sum frequencies with certainty, not difference frequencies, which suggests that neither light-curve distortion in its simplest form nor resonant mode coupling is their single cause. One of our comparison stars, μ Eridani, turned out to be variable with a dominant time-scale of 1.62 d. We believe either that it is an SPB star just leaving its instability strip or that its variations are of rotational origin. [ABSTRACT FROM AUTHOR]

We undertook a multisite spectroscopic campaign for the β Cephei star ν Eridani. A total of 2294 high-resolution spectra were obtained from telescopes at 11 different observatories around the world. The time base of dedicated multisite observations is 88 d. To this data set we have added 148 older, previously unpublished spectra, such that the overall time-span of the 2442 spectra is 430 d. The analysis of the radial velocity variations derived from the Si iii triplet centred on 4560 Å leads to 19 significant frequencies, of which seven correspond to independent pulsation frequencies. Five of these are members of multiplets with an average spacing of . Our spectroscopic results agree well with those derived from a simultaneous multisite photometric campaign of the star, albeit that we do not recover their low frequency at 0.43218 cd−1. We find three different candidate frequencies below 1 cd−1 instead. We also find that the radial velocity amplitude of the main mode has increased by some 30 per cent over the last 15 years, which is consistent with the photometry data. We derive a relative equivalent width variation of 6.5 per cent, which is completely dominated by the main radial mode. The phase difference between the radial velocity and light variations for the main frequency is , which is clearly deviant from the adiabatic value and confirms the radial nature of the dominant mode. The spectral line broadening leads to an upper limit of 20 km s−1 for v sin i, which is consistent with the long rotation period derived from the frequency splittings. [ABSTRACT FROM AUTHOR]

We have acquired new time series photometry of the two pulsating DB white dwarf stars KUV 05134+2605 and PG 1654+160 with the Whole Earth Telescope. Additional single-site photometry is also presented. We use all these data plus all available archival measurements to study the temporal behaviour of the pulsational amplitudes and frequencies of these stars for the first time. We demonstrate that both KUV 05134+2605 and PG 1654+160 pulsate in many modes, the amplitudes of which are variable in time; some frequency variability of PG 1654+160 is also indicated. Beating of multiple pulsation modes cannot explain our observations; the amplitude variability must therefore be intrinsic. We cannot find stable modes to be used for determinations of the evolutionary period changes of the stars. Some of the modes of PG 1654+160 appear at the same periods whenever detected. The mean spacing of these periods (≈40 s) suggests that they are probably caused by non-radial gravity-mode pulsations of spherical degree l = 1. If so, PG 1654+160 has a mass around 0.6 M[sub ⊙]. The time-scales of the amplitude variability of both stars (down to two weeks) are consistent with theoretical predictions of resonant mode coupling, a conclusion which might however be affected by the temporal distribution of our data. [ABSTRACT FROM AUTHOR]

We undertook two time-series photometric multisite campaigns for the rapidly oscillating Ap star HD 122970. The first one, conducted in 1998, resulted in 119 h of data and in the detection of three pulsation frequencies. The presence of possible further modes which held the promise of deriving a mode identification motivated a second worldwide campaign in the year 2001. This second campaign resulted in 203 h of measurement, but did not reveal further modes. Rather, one of the previously detected signals disappeared. The two modes common to both data sets have different spherical degree. They also showed slight frequency modulation, and one of them varied in amplitude as well. Possible causes of the latter behaviour include intrinsic instability of the pulsation spectrum or precession of the pulsational axis and orbital motion in a binary system. Frequency analysis of the Hipparcos observations of the star did not allow us to determine the stellar rotation period. The amplitude and phase behaviour of the two modes of HD 122970 in the Strömgren uvby bands is quite similar to that observed for other roAp stars. [ABSTRACT FROM AUTHOR]

Time-series photometry of the Hipparcos variable stars HD 199434 and 21190 is reported. Both stars are pulsators of the δ Scuti type. Reclassifications of the MK types of the stars, based on new spectrograms, are given. HD 21190 is found to be F2III SrEuSi:, making it the most evolved Ap star known. Its Strömgren photometric indices support the peculiar spectral type. It is also one of the most evolved δ Scuti stars known. Its combined Ap–δ Scuti nature makes it an important test of models of pulsation in peculiar stars recently developed by Turcotte et al., although it is more extreme than any model they examined. Physical parameters of both stars are estimated from Strömgren and Hβ photometry, and Hipparcos absolute magnitudes. We attempt mode identifications based on amplitude ratios and phase differences from our photometry. The dominant pulsation of HD 21190 may be an overtone radial mode. The model fits for HD 199434 are even less satisfactory, but favour an ℓ=2 mode. Given the good quality and wavelength coverage of our data, the poor results from the application of the photometric theory of mode identification may call into question the use of that technique. [ABSTRACT FROM AUTHOR]

We report 36.6h of time-resolved CCD photometry of the DB white dwarf star PG 2246 + 121 and the discovery that it is a new pulsating variable. Analysis of our compact single-site data set allowed the detection of three mode multiplets, two triplets at 256 and 329 s, respectively, and one doublet at 286 s. The frequency splitting within those structures is exactly the same within the length and accuracy of our data set. We argue that these multiplets are the result of non-radial g-mode pulsations, most probably of spherical degree ... = 1, which then yields a formal stellar rotation period of 2.00 ± 0.12 d. We suggest that the excited modes are three consecutive radial overtones of order 3-7, most likely k = 4, 5, 6. This discovery's impact on the understanding of pulsating DB white dwarfs is discussed. [ABSTRACT FROM AUTHOR]

We report multisite observations devoted to the main-sequence δ Scuti star XX Pyx, conducted as the 17th run of the Delta Scuti Network. Over 125 nights a total of 550 h of usable time-series photometric B- and V-filter data were acquired involving both photoelectric and CCD measurements at eight observatories spread around the world, which represents the most extensive single time-series for any pulsating star other than the Sun obtained so far. We describe our observations and reduction methods, and present the frequency analysis of our new data. First, we detect six new pulsation and five new combination frequencies in the star's light curves. We also discover evidence for amplitude and/or frequency variations of some of the modes during the observations. These can occur on time-scales as short as 20 d and show quite diverse behaviour. To take them into account in the frequency analysis, a so-called non-linear frequency analysis method was developed, allowing us to quantify the temporal variability of the modes and to compensate for it. Following that we continue the frequency search and we also incorporate published multisite observations. In this way, we reveal three more pulsation and two more combination frequencies. In the end, we report a total of 30 significant frequencies – 22 of which correspond to independent pulsation modes. This is the largest number of independent modes ever detected in the light curves of a δ Scuti star. The frequencies of the modes show preferred separations as already suggested by previous work on this star; they are also arranged in clear patterns. These results lead to a refinement of the stellar mean density [equation] and to a new constraint on the rotation rate of XX Pyx (ν[sub rot]=1.1±0.3 d[sup -1]). However, our attempts to identify the modes by pattern recognition failed. Moreover, mode identification from multicolour photometry failed as well because the high pulsation frequencies make this method unfavourable. The diverse behaviour of the amplitude and frequency variations of some of the modes leaves resonances as the only presently known possibility for their explanation. [ABSTRACT FROM AUTHOR]

We have searched the Kepler light curves of ∼3900 M-star targets for evidence of periodicities that indicate, by means of the effects of starspots, rapid stellar rotation. Several analysis techniques, including Fourier transforms, inspection of folded light curves, “sonograms,” and phase tracking of individual modulation cycles, were applied in order to distinguish the periodicities due to rapid rotation from those due to stellar pulsations, eclipsing binaries, or transiting planets. We find 178 Kepler M-star targets with rotation periods, Prot, of <2 days, and 110 with Prot < 1 day. Some 30 of the 178 systems exhibit two or more independent short periods within the same Kepler photometric aperture, while several have 3 or more short periods. Adaptive optics imaging and modeling of the Kepler pixel response function for a subset of our sample support the conclusion that the targets with multiple periods are highly likely to be relatively young physical binary, triple, and even quadruple M star systems. We explore in detail the one object with four incommensurate periods all less than 1.2 days, and show that two of the periods arise from one of a close pair of stars, while the other two arise from the second star, which itself is probably a visual binary. If most of these M-star systems with multiple periods turn out to be bound M stars, this could prove a valuable way discovering young hierarchical M-star systems; the same approach may also be applicable to G and K stars. The ∼5% occurrence rate of rapid rotation among the ∼3900 M star targets is consistent with spin evolution models that include an initial contraction phase followed by magnetic braking, wherein a typical M star can spend several hundred Myr before spinning down to periods longer than 2 days. [ABSTRACT FROM AUTHOR]

ABSTRACT We report a multisite photometric campaign for the β Cep stars V2052 Oph and V986 Oph. 670 h of high-quality differential photoelectric Strömgren, Johnson and Geneva time-series photometry was obtained with eight telescopes on five continents during 182 nights. Frequency analyses of the V2052 Oph data enabled the detection of three pulsation frequencies, the first harmonic of the strongest signal, and the rotation frequency with its first harmonic. Pulsational mode identification from analysing the colour amplitude ratios confirms the dominant mode as being radial, whereas the other two oscillations are most likely l = 4. Combining seismic constraints on the inclination of the rotation axis with published magnetic field analyses we conclude that the radial mode must be the fundamental mode. The rotational light modulation is in phase with published spectroscopic variability, and consistent with an oblique rotator for which both magnetic poles pass through the line of sight. The inclination of the rotation axis is 54° < i < 58° and the magnetic obliquity 58° < β < 66°. The possibility that V2052 Oph has a magnetically confined wind is discussed. The photometric amplitudes of the single oscillation of V986 Oph are most consistent with an l = 3 mode, but this identification is uncertain. Additional intrinsic, apparently temporally incoherent light variations of V986 Oph are reported. Different interpretations thereof cannot be distinguished at this point, but this kind of variability appears to be present in many OB stars. The prospects of obtaining asteroseismic information for more rapidly rotating β Cep stars, which appear to prefer modes of higher l, are briefly discussed. [ABSTRACT FROM AUTHOR]