arXiv subjects

W. W. Weiss

Publications and source records attributed to W. W. Weiss.

At least 91 records · Page 5Linked to original sources

arXiv2008

Pulsations in the atmosphere of the rapidly oscillating Ap star 10 Aquilae

The rapidly oscillating Ap (roAp) star 10 Aql shows one of the lowest photometric pulsation amplitudes and is characterized by an unusual spectroscopic pulsational behavior compared to other roAp stars. More than 1000 spectra were taken during 7 nights over a time span of 21 days with high-resolution spectrographs at the 8-m ESO VLT and 3.6-m TNG telescopes giving access to radial velocity variations of about 150 lines from different chemical species. A comparison of pulsation signatures in lines formed at different atmospheric heights allowed us to resolve the vertical structure of individual pulsation modes in 10 Aql which is the first time for a multiperiodic roAp star. The inferred propagation of pulsation waves in 10 Aql is qualitatively similar to other roAp stars: pulsation amplitudes become measurable in the layers where Y and Eu are concentrated, increase in layers where the Halpha core is formed, reach a maximum of 200-300 m/s in the layers probed by Ce, Sm, Dy lines and then decrease to 20-50 m/s in the layers where NdIII and PrIII lines are formed. A unique pulsation feature of 10 Aql is a second pulsation maximum indicated by TbIII lines which form in the uppermost atmospheric layers and oscillate with amplitudes of up to 350 m/s. The dramatic decline of pulsations in the atmospheric layers probed by the strong PrIII and NdIII lines accounts for the apparent peculiarity of 10 Aql when compared to other roAp stars. The phase-amplitude diagrams and bisector measurements of the NdIII 5102 A line reveal a rapid change of phase and amplitude with height for all three main pulsation modes, indicating the presence of a pulsation node in the stellar atmosphere. (abridged)

astro-ph↗

arXiv2008

MOST detects SPBe pulsations in HD 127756 & HD 217543: Asteroseismic rotation rates independent of vsini

The MOST (Microvariability and Oscillations of Stars) satellite has discovered SPBe (Slowly Pulsating Be) oscillations in the stars HD 127756 (B1/B2 Vne) and HD 217543 (B3 Vpe). For HD 127756, 30 significant frequencies are identified from 31 days of nearly continuous photometry; for HD 217543, up to 40 significant frequencies from 26 days of data. In both cases, the oscillations fall into three distinct frequency ranges, consistent with models of the stars. The variations are caused by nonradial g-modes (and possibly r-modes) distorted by rapid rotation and excited by the opacity mechanism near the iron opacity bump. A comparison of pulsation models and observed frequency groups yields a rotation frequency for each star, independently of vsini. The rotation rates of these stars, as well as those of the SPBe stars previously discovered by MOST, HD 163868 and $β$ CMi, are all close to their critical values.

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arXiv2008

MOST photometry of the roAp star 10 Aql

Context: We present 31.2 days of nearly continuous MOST photometry of the roAp star 10Aql. Aims:The goal was to provide an unambiguous frequency identification for this little studied star, as well as to discuss the detected frequencies in the context of magnetic models and analyze the influence of the magnetic field on the pulsation. Methods: Using traditional Fourier analysis techniques on three independent data reductions, intrinsic frequencies for the star are identified. Theoretical non-adiabatic axisymmetric modes influenced by a magnetic field having polar field strengths Bp = 0-5kG were computed to compare the observations to theory. Results: The high-precision data allow us to identify three definite intrinsic pulsation frequencies and two other candidate frequencies with low S/N. Considering the observed spacings, only one (50.95microHz) is consistent with the main sequence nature of roAp stars. The comparison with theoretical models yields a best fit for a 1.95Msun model having solar metallicity, suppressed envelope convection, and homogenous helium abundance. Furthermore, our analysis confirms the suspected slow rotation of the star and sets new lower limits to the rotation period (Prot>1 month) and inclination (i>30\pm10deg.). Conclusions:The observed frequency spectrum is not rich enough to unambiguously identify a model. On the other hand, the models hardly represent roAp stars in detail due to the approximations needed to describe the interactions of the magnetic field with stellar structure and pulsation. Consequently, errors in the model frequencies needed for the fitting procedure can only be estimated. Nevertheless, it is encouraging that models which suppress convection and include solar metallicity, in agreement with current concepts of roAp stars, fit the observations best.

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arXiv2008

Cinderella - Comparison of INDEpendent RELative Least-squares Amplitudes

The identification of increasingly smaller signal from objects observed with a non-perfect instrument in a noisy environment poses a challenge for a statistically clean data analysis. We want to compute the probability of frequencies determined in various data sets to be related or not, which cannot be answered with a simple comparison of amplitudes. Our method provides a statistical estimator for a given signal with different strengths in a set of observations to be of instrumental origin or to be intrinsic. Based on the spectral significance as an unbiased statistical quantity in frequency analysis, Discrete Fourier Transforms (DFTs) of target and background light curves are comparatively examined. The individual False-Alarm Probabilities are used to deduce conditional probabilities for a peak in a target spectrum to be real in spite of a corresponding peak in the spectrum of a background or of comparison stars. Alternatively, we can compute joint probabilities of frequencies to occur in the DFT spectra of several data sets simultaneously but with different amplitude, which leads to composed spectral significances. These are useful to investigate a star observed in different filters or during several observing runs. The composed spectral significance is a measure for the probability that none of coinciding peaks in the DFT spectra under consideration are due to noise. Cinderella is a mathematical approach to a general statistical problem. Its potential reaches beyond photometry from ground or space: to all cases where a quantitative statistical comparison of periodicities in different data sets is desired. Examples for the composed and the conditional Cinderella mode for different observation setups are presented.

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arXiv2008

MOST photometry and modeling of the rapidly oscillating (roAp) star gamma Equ

Aims. Despite photometry and spectroscopy of its oscillations obtained over the past 25 years, the pulsation frequency spectrum of the rapidly oscillating Ap (roAp) star gamma Equ has remained poorly understood. Better time-series photometry, combined with recent advances to incorporate interior magnetic field geometry into pulsational models, enable us to perform improved asteroseismology of this roAp star. Methods. We obtained 19 days of continuous high-precision photometry of gamma Equ with the MOST (Microvariability & Oscillations of STars) satellite. The data were reduced with two different reduction techniques and significant frequencies were identified. Those frequencies were fitted by interpolating a grid of pulsation models that include dipole magnetic fields of various polar strengths. Results. We identify 7 frequencies in gamma Equ that we associate with 5 high-overtone p-modes and 1st and 2nd harmonics of the dominant p-mode. One of the modes and both harmonics are new discoveries for this star. Our best model solution (1.8 M_sun, log T_eff ~ 3.882; polar field strength ~8.1 kG) leads to unique mode identifications for these frequencies (ell = 0, 1, 2 and 4). This is the first purely asteroseismic fit to a grid of magnetic models. We measure amplitude and phase modulation of the primary frequency due to beating with a closely spaced frequency which had never been resolved. This casts doubts on theories that such modulation - unrelated to the rotation of the star - is due to a stochastic excitation mechanism.

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arXiv2008

MOST Spacebased Photometry of the Transiting Exoplanet System HD 189733: Precise Timing Measurements for Transits Across an Active Star

We have measured transit times for HD 189733b passing in front of its bright (V = 7.67) chromospherically active and spotted parent star. Nearly continuous broadband optical photometry of this system was obtained with the MOST (Microvariability & Oscillations of STars) space telescope during 21 days in August 2006, monitoring 10 consecutive transits. We have used these data to search for deviations from a constant orbital period which can indicate the presence of additional planets in the system that are as yet undetected by Doppler searches. There are no transit timing variations above the level of ${\pm}45$ s, ruling out super-Earths (of masses $1 - 4 M_{\earth}$) in the 1:2 and 2:3 inner resonances and planets of 20 $M_{\earth}$ in the 2:1 outer resonance of the known planet. We also discuss complications in measuring transit times for a planet that transits an active star with large star spots, and how the transits can help constrain and test spot models. This has implications for the large number of such systems expected to be discovered by the CoRoT and Kepler missions.

astro-ph↗

arXiv2008

MOST Spacebased Photometry of the Transiting Exoplanet System HD 209458: Transit Timing to Search for Additional Planets

We report on the measurement of transit times for the HD 209458 planetary system from photometry obtained with the MOST (Microvariability & Oscillations of STars) space telescope. Deviations from a constant orbital period can indicate the presence of additional planets in the system that are yet undetected, potentially with masses approaching an Earth mass. The MOST data sets of HD 209458 from 2004 and 2005 represent unprecedented time coverage with nearly continuous observations spanning 14 and 43 days and monitoring 3 transits and 12 consecutive transits, respectively. The transit times we obtain show no variations on three scales: (a) no long-term change in P since before 2004 at the 25 ms level, (b) no trend in transit timings during the 2005 run, and (c) no individual transit timing deviations above 80 sec level. Together with previously published transit times from Agol & Steffen (2007), this allows us to place limits on the presence of additional close-in planets in the system, in some cases down to below an Earth mass. This result, along with previous radial velocity work, now eliminates the possibility that a perturbing planet could be responsible for the additional heat source needed to explain HD 209458b's anomalous low density.

astro-ph↗

arXiv2008

A heuristic derivation of the uncertainty of the frequency determination in time series data

Context: Several approaches to estimate frequency, phase and amplitude errors in time series analyses were reported in the literature, but they are either time consuming to compute, grossly overestimating the error, or are based on empirically determined criteria. Aims: A simple, but realistic estimate of the frequency uncertainty in time series analyses. Methods: Synthetic data sets with mono- and multi-periodic harmonic signals and with randomly distributed amplitude, frequency and phase were generated and white noise added. We tried to recover the input parameters with classical Fourier techniques and investigated the error as a function of the relative level of noise, signal and frequency difference. Results: We present simple formulas for the upper limit of the amplitude, frequency and phase uncertainties in time-series analyses. We also demonstrate the possibility to detect frequencies which are separated by less than the classical frequency resolution and that the realistic frequency error is at least 4 times smaller than the classical frequency resolution.

astro-ph↗

arXiv2007

Spectroscopic study of pulsations in the atmosphere of roAp star 10 Aql

We present the analysis of spectroscopic time-series observations of the roAp star 10 Aql. Observations were carried out in July 2006 with the UVES and SARG spectrographs simultaneously with the MOST mini-satellite photometry. All these data were analysed for radial velocity (RV) variations. About 150 lines out of the 1000 measured reveal clear pulsation signal. Frequency analysis of the spectroscopic data gives four frequencies. Three highest amplitude frequencies in spectroscopy coincide with the photometric ones. Phase-amplitude diagrams created for the lines of different elements/ions show that atmospheric pulsations may be represented by a superposition of the standing and running wave components, similar to other roAp stars. The highest RV amplitudes, 300-400 m/s, were measured for Ce II, Dy III, Tb III, and two unidentified lines at lambda 5471, 5556 A. We discovered ~0.4 period phase jump in the RV measurements across the Nd III line profiles. It indicates the presence of the pulsation node in stellar atmosphere. The phase jump occurs at nearly the same atmospheric layers for the two main frequencies. There is no rotational modulation in the average spectra for the 6 different nights we analysed.

astro-ph↗

arXiv2007

Nonradial p-modes in the G9.5 giant epsilon Oph? Pulsation model fits to MOST photometry

The G9.5 giant eps Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10-20d. The best fit model itself, constrained only by the observed frequencies, lies within 1 sigma of eps Oph's position in the HR-diagram and the interferometrically determined radius.

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arXiv2007

HD 114839 - An Am star showing both delta Scuti and gamma Dor pulsations discovered through MOST photometry

Using MOST (Microvariability and Oscillations of STars) satellite guide star photometry, we have discovered a metallic A star showing hybrid p- and g-mode pulsations. HD 114839 was observed nearly continuously for 10 days in March, 2005. We identify frequencies in three groups: the first centered near 2 cycles/day, in the gamma Dor pulsation range, and two others near 8 and 20, both in the delta Scuti range. This is only the fourth known such hybrid pulsator, including another MOST discovery (Rowe et al. 2006, this issue).

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arXiv2007

Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data

The MOST (Microvariability & Oscillations of STars) satellite obtains ultraprecise photometry from space with high sampling rates and duty cycles. Astronomical photometry or imaging missions in low Earth orbits, like MOST, are especially sensitive to scattered light from Earthshine, and all these missions have a common need to extract target information from voluminous data cubes. They consist of upwards of hundreds of thousands of two-dimensional CCD frames (or sub-rasters) containing from hundreds to millions of pixels each, where the target information, superposed on background and instrumental effects, is contained only in a subset of pixels (Fabry Images, defocussed images, mini-spectra). We describe a novel reduction technique for such data cubes: resolving linear correlations of target and background pixel intensities. This stepwise multiple linear regression removes only those target variations which are also detected in the background. The advantage of regression analysis versus background subtraction is the appropriate scaling, taking into account that the amount of contamination may differ from pixel to pixel. The multivariate solution for all pairs of target/background pixels is minimally invasive of the raw photometry while being very effective in reducing contamination due to, e.g., stray light. The technique is tested and demonstrated with both simulated oscillation signals and real MOST photometry.

physics.data-an↗

arXiv2006

Line profile variations in rapidly oscillating Ap stars: resolution of the enigma

We have carried out the first survey of the pulsational line profile variability in rapidly oscillating Ap (roAp) stars. We analysed high signal-to-noise time-series observations of ten sharp-lined roAp stars obtained with the high-resolution spectrographs attached to the VLT and CFHT telescopes. We investigated in detail the variations of Pr III, Nd II, Nd III and Tb III lines and discovered a prominent change of the profile variability pattern with height in the atmospheres of all studied roAp stars. In every investigated star profile variability of at least one rare-earth ion is characterized by unusual blue-to-red moving features - a behaviour inexplicable in the framework of the standard oblique pulsator model of slowly rotating roAp stars. Using analysis of the line profile moments and spectrum synthesis calculations, we demonstrate that unusual oscillations in spectral lines of roAp stars arise from the pulsational modulation of line widths. This variation occurs approximately in quadrature with the radial velocity changes, and its amplitude rapidly increases with height in stellar atmosphere. We propose that the line width modulation is a consequence of the periodic expansion and compression of turbulent layers in the upper atmospheres of roAp stars. Thus, the line profile changes observed in slowly rotating magnetic pulsators should be interpreted as a superposition of two types of variability: the usual time-dependent velocity field due to an oblique low-order pulsation mode and an additional line width modulation, synchronized with the changes of stellar radius. Our explanation of the line profile variations of roAp stars solves the long-standing observational puzzle and opens new possibilities for constraining geometric and physical properties of the stellar magnetoacoustic pulsations.

astro-ph↗

arXiv2006

Pulsation in the atmosphere of the roAp star HD 24712. I. Spectroscopic observations and radial velocity measurements

We have investigated the structure of the pulsating atmosphere of one of the best studied rapidly oscillating Ap stars, HD 24712. For this purpose we analyzed spectra collected during 2001-2004. An extensive data set was obtained in 2004 simultaneously with the photometry of the Canadian MOST mini-satellite. This allows us to connect directly atmospheric dynamics observed as radial velocity variations with light variations seen in photometry. We directly derived for the first time and for different chemical elements, respectively ions, phase shifts between photometric and radial velocity pulsation maxima indicating, as we suggest, different line formation depths in the atmosphere. This allowed us to estimate for the first time the propagation velocity of a pulsation wave in the outer stellar atmosphere of a roAp star to be slightly lower than the sound speed. We confirm large pulsation amplitudes (150-400 m/s) for REE lines and the Halpha core, while spectral lines of the other elements (Mg, Si, Ca, and Fe-peak elements) have nearly constant velocities. We did not find different pulsation amplitudes and phases for the lines of rare-earth elements before and after the Balmer jump, which supports the hypothesis of REE concentration in the upper atmosphere above the hydrogen line-forming layers. We also discuss radial velocity amplitudes and phases measured for individual spectral lines as tools for a 3D tomography of the atmosphere of HD 24712.

astro-ph↗

arXiv2006

MOST Photometry of the roAp Star HD 134214

We present 10.27$ \rm{hrs}$ of photometry of the roAp star HD 134214 obtained by the MOST satellite. The star is shown to be monoperiodic and oscillating at a frequency of 2948.97$ \pm 0.55 μ\rm{Hz}$. This is consistent with earlier ground based photometric campaigns (e.g. Kreidl et al. 1994). We do not detect any of the additional frequencies identified in the recent spectroscopic study by Kurtz et al. (2006) down to an amplitude limit of 0.36$ \rm{mmag}$ ($2σ$ significance limit).

astro-ph↗

arXiv2006

Discovery of hybrid $γ$ Dor and $δ$ Sct pulsations in BD+18 4914 through MOST spacebased photometry

We present a total of 57 days of contiguous, high-cadence photometry (14 days in 2004 and 43 in 2005) of the star BD+18 4914 obtained with the MOST satellite. We detect 16 frequencies down to a signal-to-noise of 3.6 (amplitude \~ 0.5 mmag). Six of these are less than 3 cycles/day, and the other ten are between 7 and 16 cycles/day. We intrepret the low frequencies as g-mode $γ$ Doradus-type pulsations and the others as $δ$ Scuti-type p-modes, making BD+18 4914 one of the few known hybrid pulsators of its class. If the g-mode pulsations are high-overtone non-radial modes with identical low degree l, we can assign a unique mode classification of n={12, 20, 21, 22, 31, 38} based on the frequency ratio method.

astro-ph↗

arXiv2006

MOST Detects g-Modes in the Late-Type be Star beta CMi (B8Ve)

The Microvariability and Oscillations of stars (MOST) satellite has detected low-amplitude light variations ($Δm\sim$1 mmag) in the Be star $β$ CMi (B8Ve). The observations lasted 41 days and the variations have typical periods $\sim 0.3$ days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of $m=-1$ excited by the Fe-bump of opacity in an intermediate-mass ($\approx 3.5 M_\odot$) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars.

astro-ph↗

arXiv2006

Most Detects G- and P-Modes in the B Supergiant HD 163899 (B2Ib/II)

The {\it Microvariability and Oscillations of Stars (MOST)} satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies $\la$ 2.8 c d$^{-1}$ with amplitudes of a few milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between $20M_\odot$ at $\log T_{\rm eff} \approx 4.41$ and $15M_\odot$ at $\log T_{\rm eff} \approx 4.36$ are roughly consistent with the observed frequency range. Excitation by the Fe-bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The {\it MOST} light curve of HD 163899 shows that such a reflection of g-modes actually occurs, and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from $α$ Cyg variables. Such g-modes have great potential for asteroseismology.

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