Michel Breger

The Kepler characterization of the variability among A- and F-type stars. - I: General overview

Context. The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. Aims. 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, and observationally investigate the relation between γ Doradus (γ Dor), δ Scuti (δ Sct), and hybrid stars. Methods. We compile a database of physical parameters for the sample stars from the literature and new ground-based observations. We analyse the Kepler light curve of each star and extract the pulsational frequencies using different frequency analysis methods. We construct two new observables, “energy ”a nd “efficiency”, related to the driving energy of the pulsation mode and the convective efficiency of the outer convective zone, respectively. Results. We propose three main groups to describe the observed variety in pulsating A-F type stars: γ Dor, δ 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 higher 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 γ Dor and δ Sct range, also between 5 and 10 d −1 , which is a challenge for the current models. We find indications for the existence of δ Sct and γ Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the δ Sct and γ Dor instability strips and beyond. Non-variable stars seem to exist within the instability strips. The location of γ Dor and δ Sct classes in the (Teff ,l ogg)-diagram has been extended. We investigate two newly constructed variables, “efficiency ”a nd “energy”, as a means to explore the relation between γ Dor and δ Sct stars. Conclusions. Our results suggest a revision of the current observational instability strips of δ Sct and γ Dor stars and imply an investigation of pulsation mechanisms to supplement the κ mechanism and convective blocking effect to drive hybrid pulsations. Accurate physical parameters for all stars are needed to confirm these findings.

A survey for high optical polarization in quasars with core-dominant radio structure - Is there a beamed optical continuum?

129 new optical linear polarization measurements of 84 quasars (defined as starlike on Sky Survey prints) are presented. 23 new highly polarized objects (p > 3%) were discovered, of which 14 are known to have broad emission lines. This represents a significant increase in the numbers of these objects that were known. With the addition of data from the literature, our measurements constitute well-defined samples of flat-spectrum core-dominant radio quasars brighter than about 18 mag, one selected from the list published by Perley (1982, AJ, 87), and another from that by Kuhr et al. (1981)

Polarimetry and spectrophotometry of the QSO IRAS 13349 + 2438 and the unification of active galaxies

We have made new observations of the bright (V=14.7) infrared-selected QSO IRAS 13349+2438 (Beichman et al.), which provide important clues about the structure of the inner few parsecs of active galactic nuclei (AGNs). The principal observational facts are as follows: 1. There is high linear optical polarization, increasing with decreasing wavelength from 1.4% at 2.2 μm (K-band) to 8% at 0.36 μm (U-band). 2. The position angle of polarization is independent of wavelength in the optical region and is aligned with the direction of elongation of the r-band optical image, presumably the major axis of the host galaxy

Regularities in frequency spacings of δ Scuti stars: the Kepler star KIC 9700322★

In the faint star KIC 9700322 observed by the Kepler satellite, 76 frequencies with amplitudes from 14 to 29000 ppm were detected. The two dominant frequencies at 9.79 and 12.57 c/d (113.3 and 145.5 mu Hz), interpreted to be radial modes, are accompanied by a large number of combination frequencies. A small additional modulation with a 0.16 c/d frequency is also seen/ this is interpreted to be the rotation frequency of the star. The corresponding prediction of slow rotation is confirmed by a spectrum from which v sin i = 19 pm 1 km/s is obtained. The analysis of the spectrum shows that the star is one of the coolest {delta} Sct variables. We also determine Teff = 6700 pm 100 K and log g = 3.7 pm 0.1, compatible with the observed frequencies of the radial modes. Normal solar abundances are found. An ell = 2 frequency quintuplet is also detected with a frequency separation consistent with predictions from the measured rotation rate. A remarkable result is the absence of additional independent frequencies down to an amplitude limit near 14 ppm, suggesting that the star is stable against most forms of nonradial pulsation. The frequency spectrum of this star emphasizes the need for caution in interpreting low frequencies in {delta} Sct stars as independent gravity modes. A low frequency peak at 2.7763 c/d in KIC 9700322 is, in fact, the frequency difference between the two dominant modes and is repeated over and over in various frequency combinations involving the two dominant modes. The relative phases of the combination frequencies show a strong correlation with frequency, but the physical significance of this result is not clear.

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

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

Asteroseismology of KIC 11754974: a high-amplitude SX Phe pulsator in a 343-d binary system

The candidate SX Phe star KIC 11754974 shows a remarkably high number of combination frequencies in the Fourier amplitude spectrum: 123 of the 166 frequencies in our multifrequency fit are linear combinations of independent modes. Predictable patterns in frequency spacings are seen in the Fourier transform of the light curve. We present an analysis of 180 d of short-cadence Kepler photometry and of new spectroscopic data for this evolved, late A-type star. We infer from the 1150-d, long-cadence light curve, and in two different ways, that our target is the primary of a 343-d, non-eclipsing binary system. According to both methods, the mass function is similar, f (M) = 0.0207 ± 0.0003M⊙. The observed pulsations are modelled extensively, using separate, state-of-the-art, time-dependent convection (TDC) and rotating models. The models match the observed temperature and low metallicity, finding a mass of 1.50-1.56M⊙. The models suggest that the whole star is metal poor, and that the low metallicity is not just a surface abundance peculiarity. This is the best frequency analysis of an SX Phe star, and the only Kepler δ Sct star to be modelled with both TDC and rotating models.

Evidence of Resonant Mode Coupling and the Relationship between Low and High Frequencies in a Rapidly Rotating a Star

In the theory of resonant mode coupling, the parent and child modes are directly related in frequency and phase. The oscillations present in the fast rotating δ Sct star KIC 8054146 allow us to test the most general and generic aspects of such a theory. The only direct way to separate the parent and coupled (child) modes is to examine the correlations in amplitude variability between the different frequencies. For the dominant family of related frequencies, only a single mode and a triplet are the origins of nine dominant frequency peaks ranging from 2.93 to 66.30 cycles day–1 (as well as dozens of small-amplitude combination modes and a predicted and detected third high-frequency triplet). The mode-coupling model correctly predicts the large amplitude variations of the coupled modes as a product of the amplitudes of the parent modes, while the phase changes are also correctly modeled. This differs from the behavior of normal combination frequencies in that the amplitudes are three orders of magnitude larger and may exceed even the amplitudes of the parent modes. We show that two dominant low frequencies at 5.86 and 2.93 cycles day–1 in the gravity-mode region are not harmonics of each other, and their properties follow those of the almost equidistant high-frequency triplet. We note that the previously puzzling situation of finding two strong peaks in the low-frequency region related by nearly a factor of two in frequency has been seen in other δ Sct stars as well.

Pulsation analysis and its impact on primary transit modeling in WASP-33

Aims. To date, WASP-33 is the only δ Scuti star known to be orbited by a hot Jupiter. The pronounced stellar pulsations, showing periods comparable to the primary transit duration, interfere with the transit modeling. Therefore our main goal is to study the pulsation spectrum of the host star to redetermine the orbital parameters of the system by means of pulsation-cleaned primary transit light curves. Methods. Between August 2010 and October 2012 we obtained 457 h of photometry of WASP-33 using small and middle-class telescopes located mostly in Spain and in Germany. Our observations comprise the wavelength range between the blue and the red, and provide full phase coverage of the planetary orbit. After a careful detrend, we focus our pulsation studies in the high frequency regime, where the pulsations that mostly deform the primary transit exist. Results. The data allow us to identify, for the first time in the system, eight significant pulsation frequencies. The pulsations are likely associated with low-order p-modes. Furthermore, we find that pulsation phases evolve in time. We use our knowledge of the pulsations to clean the primary transit light curves and carry out an improved transit modeling. Surprisingly, taking into account the pulsations in the modeling has little influence on the derived orbital parameters. However, the uncertainties in the best-fit parameters decrease. Additionally, we find indications for a possible dependence between wavelength and transit depth, but only with marginal significance. A clear pulsation solution, in combination with an accurate orbital period, allows us to extend our studies and search for star-planet interactions (SPI). Although we find no conclusive evidence of SPI, we believe that the pulsation nature of the host star and the proximity between members make WASP-33 a promising system for further SPI studies.

Discovery of binarity, spectroscopic frequency analysis, and mode identification of the δ Scuti star 4 CVn

More than 40 years of ground-based photometric observations of the δu2009Sct star 4u2009CVn has revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree l ≤ 2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1u2009m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4u2009CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discover that 4u2009CVn is an eccentric binary system with an orbital period Porb = 124.44 ± 0.03 d and an eccentricity e = 0.311 ± 0.003. We detect 20 oscillation frequencies, 9 of which previously unseen in photometric data; attempt mode identification for the two dominant modes, f1 = 7.3764 d-1 and f2 = 5.8496 d-1; and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, veqsini ≃ 106.7 km s-1, translates to a rotation rate of veq/vcrit ≥ 33%. This relatively high rotation rate hampers unique mode identification, since higher order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4u2009CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.

The Pleiades cluster. IV - The visit of a molecular CO cloud

The location, size, and mass of the CO molecular cloud seen in the direction of the Pleiades cluster is determined from a study of the polarization and reddening of cluster members and nonmembers. Arguments are presented against both a foreground and background location of the molecular cloud, so that the cloud should be presently situated inside the cluster. Stellar reddening determinations with the appropriate value of R = 3.3 for the region, as well as star counts, lead to a determination of a total extinction of A(V) in the range of 1.0-1.6 mag for the central region of the CO cloud. The extinction determinations for cluster members and background stars indicate a mass of 20 solar masses for the CO cloud visiting the Pleiades cluster. 20 references.