Hybrid Pulsator Research Articles

KIC 6761539, a fast rotating γ Dor – δ Sct hybrid star

AbstractKIC 6761539 is one of many fast rotating γ Doradus – δ Scuti hybrid pulsators. A search for possible regularities in the frequency spectrum is performed and a first stellar model is presented.

Detection of a large sample ofγDoradus stars fromKeplerspace photometry and high-resolution ground-based spectroscopy

The space-missions MOST, CoRoT, and Kepler deliver a huge amount of high-quality photometric data suitable to study numerous pulsating stars. Our ultimate goal is a detection and analysis of an extended sample of Gamma Dor-type pulsating stars with the aim to search for observational evidence of non-uniform period spacings and rotational splittings of gravity modes in main-sequence stars typically twice as massive as the Sun. We applied an automated supervised photometric classification method to select a sample of 69 Gamma Doradus candidate stars. We used an advanced method to extract the Kepler light curves from the pixel data information using custom masks. For 36 of the stars, we obtained high-resolution spectroscopy with the HERMES spectrograph installed at the Mercator telescope. We find that all stars for which spectroscopic estimates of Teff and logg are available fall into the region of the HR diagram where the Gamma Dor and Delta Sct instability strips overlap. The stars cluster in a 700 K window in effective temperature, logg measurements suggest luminosity class IV-V. From the Kepler photometry, we identify 45 Gamma Dor-type pulsators, 14 Gamma Dor/Delta Sct hybrids, and 10 stars which are classified as "possibly Gamma Dor/Delta Sct hybrid pulsators". The results of photometric and spectroscopic classifications according to the type of variability are in perfect agreement. We find a clear correlation between the spectroscopically derived vsini and the frequencies of independent pulsation modes and show that it has nothing to do with rotational modulation of the stars but is related to their stellar pulsations. Our sample and frequency determinations offer a good starting point for seismic modelling of slow to moderately rotating Gamma Dor stars.

Spectrum analysis of bright Kepler late B- to early F-stars★

The Kepler satellite mission delivers single band-pass light curves of a huge number of stars observed in the Cygnus-Lyra region opening a new window for asteroseismology. In order to accomplish one of the preconditions for the asteroseismic modelling of the stars, we aim to derive fundamental parameters and individual abundances for a sample of 18 Gamma Dor (GD)/Delta Sct (DSct) and 8 SPB/beta Cep candidate stars in the Kepler satellite field of view. We use the spectral synthesis method to model newly obtained, high-resolution spectra of 26 stars in order to derive their fundamental parameters and individual abundances. The stars are then placed into the log(Teff)-log(g) diagram and the obtained spectroscopic classification is compared to the existing photometric one. For hot stars, the KIC temperatures appear to be systematically underestimated, in agreement with previous findings. We also find that the temperatures derived from our spectra agree reasonably well with those derived from the SED fitting. According to their position in the log(Teff)-log(g) diagram, two stars are expected GD stars, four stars are expected DSct stars, and four stars are possibly DSct stars at the blue edge of the instability strip. Two stars are confirmed SPB variables, and one star falls into the SPB instability region but its parameters might be biased by binarity. Two of the four stars that fall into the DSct instability region show GD-type oscillation in their light curves implying that GD-like oscillations are much more common among the DSct stars than is theoretically expected. Moreover, one of the stars located at the hot border of the DSct instability strip is classified as DSct-GD hybrid pulsator from its light curve analysis. Given that these findings are fully consistent with recent investigations, we conclude that a revision of the GD and DSct instability strips is essential.

Why the peculiar δ Scuti star HD 187547 is a superstar

The δ Scuti pulsators occupy a region in the Hertzsprung-Russell diagram where several physical processes occur: the subsurface convection layers change from being deep and vigorous to being shallow and ineffective to transport energy. This transition has a large impact not only on pulsational stability but also on stellar evolution, activity, transport of angular momentum, mixing processes, etc.. It is therefore of great interest to understand how exactly the stellar structure changes with increasing temperature and mass. Theoretical models (Houdek et al. 1999; Samadi et al. 2002) predicted that the convection in the outer layers of δ Scuti stars is still efficient enough to excite solar-like oscillations. The Kepler target, HD 187547 (a.k.a. Superstar), was the first δ Scuti star to suggest that solar-like oscillations are indeed present in this type of stars (Antoci et al. 2011). There were several reasons to conclude that HD 187547 is a δ Scuti/solar-like hybrid pulsator. (1) The peaks at high frequencies are modes of pulsations approximately equidistantly spaced, as expected for high radial order pressure modes; these peaks are not combination frequencies as it is sometimes observed in δ Scuti stars. (2) The opacity mechanism cannot excite a continuous frequency range as observed in HD 187547 (Pamyatnykh 2000). (3) The identification as an Am star consistent with the low v sini, makes it very unlikely to be a δ Scuti/roAp hybrid, because strong large-scale magnetic fields, a necessity for roAp pulsators, have never been detected in Am stars (Auriere et al. 2010). (4) Although a large number of Am stars are found in binary systems, we find no evidence in the observed spectra for a companion, i.e. no significant RV shift over 170 days can be detected and the absorption lines can perfectly be reproduced by assuming a slowly-rotating chemically peculiar Am star. This means that the peaks at high frequencies are unlikely to be from a companion, because such a star would be an A or F-type star and would be visible in the spectrum and this is not the case. (5) The large separation Δν, which is the frequency difference between consecutive radial orders of the same degree l, predicts the frequency of maximum power (νmax) to be exactly where the mode with the highest amplitude in the supposed stochastic region is observed. Note that the Δν and νmax scaling relation is valid only for stochastic oscillations (e.g. Huber et al. 2011). (6) The statistical properties (Chang & Gough 1998) of the modes interpreted to be solar-like were very similar to what is expected for stochastic signals in general.

RELATIONSHIP BETWEEN LOW AND HIGH FREQUENCIES IN δ SCUTI STARS: PHOTOMETRICKEPLERAND SPECTROSCOPIC ANALYSES OF THE RAPID ROTATOR KIC 8054146

Two years of Kepler data of KIC 8054146 (delta Sct/gamma Dor hybrid) revealed 349 statistically significant frequencies between 0.54 and 191.36 c/d (6.3 microHz to 2.21 mHz). The 117 low frequencies cluster in specific frequency bands, but do not show the equidistant period spacings predicted for gravity modes of successive radial order, n, and reported for at least one other hybrid pulsator. The four dominant low frequencies in the 2.8 to 3.0 c/d (32 to 35 microHz) range show strong amplitude variability with timescales of months and years. These four low frequencies also determine the spacing of the higher frequencies in and beyond the delta Sct pressure-mode frequency domain. In fact, most of the higher frequencies belong to one of three families with spacings linked to a specific dominant low frequency. In the Fourier spectrum, these family regularities show up as triplets, high-frequency sequences with absolutely equidistant frequency spacings, side lobes (amplitude modulations) and other regularities in frequency spacings. Furthermore, within two families the amplitude variations between the low and high frequencies are related. We conclude that the low frequencies (gravity modes, rotation) and observed high frequencies (mostly pressure modes) are physically connected. This unusual behavior may be related to the very rapid rotation of the star: from a combination of high and low-resolution spectroscopy we determined that KIC 8054146 is a very fast rotator (v sin i = 300 +/- 20 km/s) with an effective temperature of 7600 +/- 200 K and a surface gravity log g of 3.9 +/- 0.3. Several astrophysical ideas explaining the origin of the relationship between the low and high frequencies are explored.

The CoRoT B-type binary HD 50230: a prototypical hybrid pulsator with g-mode period and p-mode frequency spacings

B-type stars are promising targets for asteroseismic modelling, since their frequency spectrum is relatively simple. We deduce and summarise observational constraints for the hybrid pulsator, HD50230, earlier reported to have deviations from a uniform period spacing of its gravity modes. The combination of spectra and a high-quality light curve measured by the CoRoT satellite allow a combined approach to fix the position of HD50230 in the HR diagram. To describe the observed pulsations, classical Fourier analysis was combined with short-time Fourier transformations and frequency spacing analysis techniques. Visual spectra were used to constrain the projected rotation rate of the star and the fundamental parameters of the target. In a first approximation, the combined information was used to interpret multiplets and spacings to infer the true surface rotation rate and a rough estimate of the inclination angle. We identify HD50230 as a spectroscopic binary and characterise the two components. We detect the simultaneous presence of high-order g modes and low-order p and g-modes in the CoRoT light curve, but were unable to link them to line profile variations in the spectroscopic time series. We extract the relevant information from the frequency spectrum, which can be used for seismic modelling, and explore possible interpretations of the pressure mode spectrum.

Strong interactions betweeng- andp-modes in the hybridγ Doradus-δ Scuti CoRoT star ID 105733033

CoRoT ID 105733033 is an excellent example of hybrid pulsators as it shows g- and p-modes with almost similar amplitudes in two clearly distinct frequency domains. Classical Fourier analysis allows the dectection of frequencies with an amplitude as small as 0.1 mmag up to 50c/d. The frequency spectrum of CoRoT ID 105733033 clearly consists of two distinct ranges, which are typical of gamma Doradus and delta Scuti pulsation. Focus was placed on the identification of linear combinations and frequencies due to the coupling between gamma Doradus and delta Scuti modes. We detect 198 gamma Doradus type frequencies in the range [0.25;4]c/d, of which 180 are not combination frequencies, and 24 of them are separated by a constant period-interval Delta P=0.03074d. According to the asymptotic theory, these 24 frequencies correspond to a series of g-modes of the same ell-degree and different radial orders n. We also detect 246 delta Scuti type frequencies in the range [10.1;63.4]c/d. The dominant frequency F=12.6759c/d was identified as the fundamental radial mode. Our most noteworthy result is that all the main gamma Doradus frequencies f_i are also detected in the delta Scuti domain as F +- f_i with four times smaller amplitudes. Once these frequencies were removed, only 59 can be considered as individual delta Scuti frequencies. A coupling between g- and p-modes is proposed to be a tool for detecting g-modes in the Sun, but this coupling has never yet been observed. Our present study may be valuable input to theoretical studies, addressing the mutual influence of g- and p-mode cavities and the deviation from classical theory. Furthermore, we identify a sequence of g-modes belonging to the same ell but with consecutive orders n.

THE CLASSIFICATION OFKEPLERB-STAR VARIABLES

The light curves of 252 B-star candidates in the Kepler data base are analyzed in a similar fashion to that done by Balona et al. (2011) to further characterize B star variability, increase the sample of variable B stars for future study, and to identify stars whose power spectra include particularly interesting features such as frequency groupings. Stars are classified as either constant light emitters, $\beta$ Cep stars, slowly pulsating B stars, hybrid pulsators, binaries or stars whose light curves are dominated by rotation (Bin/Rot), hot subdwarfs, or white dwarfs. One-hundred stars in our sample were found to be either light contants or to be variable at a level of less than 0.02 mmag. We increase the number of candidate B-star variables found in the Kepler data base by Balona et al. (2011) in the following fashion: $\beta$ Cep stars from 0 to 10, slowly pulsating B stars from 8 to 54, hybrid pulsators from 7 to 21, and Bin/Rot stars from 23 to 82. For comparison purposes, approximately 51 SPBs and 6 hybrids had been known prior to 2007. The number of $\beta$ Cep stars known prior to 2004 was 93. A secondary result of this study is the identification of an additional 11 pulsating white dwarf candidates, four of which possess frequency groupings.

Angular momentum transfer between oscillations and rotation in subdwarf B hybrid pulsators

Context. Subdwarf B pulsators exhibit pressure (p) and/or gravity (g) modes. Their frequency spectra range from very simple, with few frequencies, to very rich, with more than fifty peaks in some cases. Balloon09 is a hybrid pulsating subdwarf B, showing a large number of p- and g-modes including a triplet and a quintuplet-like structures which are interpreted as p-mode frequency splittings due to stellar rotation. Photometric observations undertaken in two subsequent years revealed a change in these rotational splittings of 0.24 - 0.58 {\mu}Hz/yr. Aims. We analyse the possibility of angular momentum interchange between stellar rotation and internal gravity waves as a mechanism for the observed rotational splitting variations. Methods. The expected change in the rotational splitting of eigenmodes resulting from this mechanism are computed in the nonadiabatic linear approximation for a stellar structure model that is representative of the target star. The fact that g-modes are also observed in Balloon09 makes it a particularly suitable candidate for our study, because the change in the rotational splittings are proportional to the amplitude squared of the g-modes which, in this case, can be estimated from the observations. Results. We find that this mechanism is able to predict changes in the splittings that can be of the same order of magnitude as the observed variations.

TheKeplercharacterization of the variability among A- and F-type stars

The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars. We propose three main groups to describe the observed variety in pulsating A-F type stars: gamma Dor, delta Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the gamma Dor and delta Sct range, also between 5 and 10 c/d, which is a challenge for the current models. We find indications for the existence of delta Sct and gamma Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the delta Sct and gamma Dor instability strips, and beyond. Non-variable stars seem to exist within the instability strips. The location of gamma Dor and delta Sct classes in the (Teff,logg)-diagram has been extended. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between gamma Dor and delta Sct stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of other pulsation mechanisms to supplement the kappa mechanism and convective blocking effect to drive hybrid pulsations.

Spectral analysis of Kepler SPB andβ Cephei candidate stars

We determine the fundamental parameters of SPB and Beta Cep candidate stars observed by the Kepler satellite mission and estimate the expected types of non-radial pulsators by comparing newly obtained high-resolution spectra with synthetic spectra computed on a grid of stellar parameters assuming LTE and check for NLTE effects for the hottest stars. For comparison, we determine Teff independently from fitting the spectral energy distribution of the stars obtained from the available photometry. We determine Teff, log(g), micro-turbulent velocity, vsin(i), metallicity, and elemental abundance for 14 of the 16 candidate stars, two of the stars are spectroscopic binaries. No significant influence of NLTE effects on the results could be found. For hot stars, we find systematic deviations of the determined effective temperatures from those given in the Kepler Input Catalogue. The deviations are confirmed by the results obtained from ground-based photometry. Five stars show reduced metallicity, two stars are He-strong, one is He-weak, and one is Si-strong. Two of the stars could be Beta Cep/SPB hybrid pulsators, four SPB pulsators, and five more stars are located close to the borders of the SPB instability region.

V391 Peg: Identification of the two main pulsation modes from ULTRACAM u′g′r′ amplitudes

AbstractV391 Peg (HS 2201+2610) is an extreme horizontal branch subdwarf B (sdB) star, it is an hybrid pulsator showing p‐ and g‐mode oscillations, and hosts a 3.2/sin i MJup planet at an orbital distance of about 1.7 AU. In order to improve the characterization of the star, we measured the pulsation amplitudes in the u′g′r′ Sloan photometric bands using ULTRACAM at the William Herschel 4.2 m telescope and we compared them with theoretical values. The preliminary results presented in this article conclusively show that the two main pulsation periods at 349.5 and 354.1 s are a radial and a dipole mode respectively. This is the first time that the degree index of multiple modes has been uniquely identified for an sdB star as faint as V391 Peg (B = 14.4), proving that multicolor photometry is definitely an efficient technique to constrain mode identification, provided that the data have a high enough quality (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First Kepler results on compact pulsators - III. Subdwarf B stars with V1093 Her and hybrid (DW Lyn) type pulsations

We present the discovery of nonradial pulsations in five hot subdwarf B (sdB) stars based on 27 days of nearly continuous time-series photometry using the Kepler spacecraft. We find that every sdB star cooler than $\approx 27\,500\,$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 one to 4.5~h and are associated with $g-$mode pulsations. Three stars also exhibit short periods indicative of $p-$modes with periods of 2 to 5~m and in addition, these stars exhibit periodicities between both classes from 15 to 45~m. We detect the coolest and longest-period V1093~Her-type pulsator to date, KIC010670103 ($T_eff\approx 20\,900\,$K, $P_max\approx 4.5$~h) as well as a suspected hybrid pulsator, KIC002697388 which is extremely cool ($T_{\rm eff}\approx 23\,900\,$K) and for the first time hybrid pulsators which have larger $g-$mode amplitudes than $p-$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.

First Kepler results on compact pulsators - I. Survey target selection and the first pulsators

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 for a V361 Hya type sdB variable (sdBV), seven show long-period pulsations characteristic 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.

RAT J0455+1305: a rare hybrid pulsating subdwarf B star

We present results on the second faintest pulsating subdwarf B (sdB) star, RAT J0455+1305, derived from photometric data obtained in 2009. It shows both short- and long-period oscillations, theoretically assigned as pressure and gravity modes. We identify six short-period frequencies (with one being a combination) and six long-period frequencies. This star is the fourth hybrid sdB star discovered so far which makes it of special interest as each type of mode probes a different part of the star. This star is similar to the sdB hybrid pulsator Balloon 090100001 in that it exhibits short-period mode groupings, which can be used to identify pulsation parameters and constrain theoretical models.

A major observational campaign on the hybrid pulsator HS0702+6043

HS0702+6043 (DW Lyn) is one of the very few known sdB pulsators which exhibit both pressure- and gravity-mode oscillations. Given its high potential for asteroseismology, we carried out an extensive campaign in integrated light photometry at the Steward Observatory 1.6 m Kuiper Telescope using the new CCD camera Mont4K, a joint venture between the Universite de Montreal and the University of Arizona. Over a period starting November 1st 2007 up to March 14th 2008, we were able to gather some 424.5 hours of useful data. To our knowledge, never before a pulsating sdB star has been observed at such high frequency resolution and sensitivity. We present the results of a preliminary analysis of this unique data set.

Complex asteroseismology of theβCep/slowly pulsating B-type pulsatorνEridani: constraints on opacities

We undertake another attempt towards seismic modelling of the most intensive studied main sequence pulsators of the early B spectral type, $\nu$ Eridani. Our analysis is extended by a requirement of fitting not only pulsational frequencies but also the complex amplitude of the bolometric flux variation, $f$, related to each mode frequency. This approach, called {\it complex asteroseismology}, provides a unique test of stellar parameters, atmospheres and opacities. In particular, the concordance of the empirical and theoretical values of $f$ we obtained for the high-order g mode opens a new gate in seismic studies of the main-sequence hybrid pulsators. The most intriguing and challenging result is that whereas an agreement of the theoretical and empirical values of $f$ for the radial mode can be achieved only with the OPAL data, a preference for the OP tables is derived from the analysis of the high-order gravity mode.

ASTEROSEISMOLOGY OF HYBRID PULSATORS MADE POSSIBLE: SIMULTANEOUS MOST SPACE PHOTOMETRY AND GROUND-BASED SPECTROSCOPY OF γ PEG

We have acquired simultaneous high-precision space photometry and radial velocities of the bright hybrid Beta Cep/SPB pulsator Gamma Peg. Frequency analyses reveal the presence of six g modes of high radial order together with eight low-order Beta Cep oscillations in both data sets. Mode identification shows that all pulsations have spherical degrees l = 0 - 2. An 8.5 solar mass model reproduces the observed pulsation frequencies; all theoretically predicted modes are detected. We suggest, contrary to previous authors, that Gamma Peg is a single star; the claimed orbital variations are due to g-mode pulsation. Gamma Peg is the first hybrid pulsator for which a sufficiently large number of high-order g modes and low order p and mixed modes have been detected and identified to be usable for in-depth seismic modeling.

The planet-hosting subdwarf B star V 391 Pegasi is a hybrid pulsator

Context. A noticeable fraction of subdwarf B stars shows either short-period ( p -mode) or long-period ( g -mode) luminosity variations, with two objects so far known to exhibit hybrid behaviour, i.e. showing both types of modes at the same time. The pulsating subdwarf B star V 391 Pegasi (or HS 2201+2610), which is close to the two known hybrid pulsators in the log g – T eff plane, has recently been discovered to host a planetary companion. Aims. In order to learn more about the planetary companion and its possible influence on the evolution of its host star (subdwarf B star formation is still not well understood), an accurate characterisation of the host star is required. As part of an ongoing effort to significantly improve the asteroseismic characterisation of the host star, we investigate the low-frequency behaviour of HS 2201+2610. Methods. We obtained rapid high signal-to-noise photometric CCD ( B -filter) and PMT (clear-filter) data at 2 m-class telescopes and carried out a careful frequency analysis of the light curves. Results. In addition to the previously known short-period luminosity variations in the range 342 s–367 s, we find a long-period variation with a period of 54 mn and an amplitude of 0.15 per cent. This can most plausibly be identified with a g -mode pulsation, so that HS 2201+2610 is a new addition to the short list of hybrid sdB pulsators. Conclusions. Along with the previously known pulsating subdwarf B stars HS 0702+6043 and Balloon 090100001 showing hybrid behaviour, the new hybrid HS 2201+2610 is the third member of this class. This important property of HS 2201+2610 can lead to a better characterisation of this planet-hosting star, helping the characterisation of its planetary companion as well. Current pulsation models cannot yet reproduce hybrid sdBV stars particularly well and improved pulsation models for this object have to include the hybrid behaviour.

Long-term EXOTIME photometry and follow-up spectroscopy of the sdB pulsator HS 0702+6043

Die Serie Communications in Asteroseismology (CoAst) stellt eine international gefragte Plattform für wissenschaftliche Publikationen der neuesten Forschungsergebnisse auf dem Gebiet der Asteroseismologie dar. Forschungsgegenstand der Asteroseismologie sind veränderliche und pulsierende Sterne. Neben den halbjährlich erscheinenden regulären Ausgaben mit Beiträgen aus der laufenden Forschung werden auch Sonderausgaben für Konferenzberichte bzw. Programmhandbücher herausgegeben. JENAM (Joint European and National Astronomy Meeting) ist die größte jährliche Zusammenkunft der europäischen Berufsastronomen, die diesmal im September 2008 in Wien stattfand. Die Konferenz beherbergte neun Minisymposien, wobei eines der Asteroseismologie gewidmet war. Die neuesten Entwicklungen auf diesem Gebiet sind im vorliegenden Band im Rahmen von Übersichts- und Detailartikeln zusammengefasst.