p-mode frequencies in solar-like stars: II ζ Her A (original) (raw)
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p -mode frequencies in solar-like stars
Astronomy and Astrophysics, 2004
... Di Mauro, MP, Christensen-Dalsgaard, J., Kjeldsen, H., et al. 2003, A&A, 404, 341 [EDP Sciences] [NASA ADS] [CrossRef]; Foster, G. 1995, AJ, 109, 1889 [NASA ADS] [CrossRef] (In the text); Frandsen, S., Carrier, F., Aerts, C., et al. ...
The detection of the rich p-mode spectrum and asteroseismology of Przybylski's star
Astronomy & Astrophysics, 2008
Aims. We investigate the oscillation spectrum of the most chemically-peculiar star in the sky, the rapidly-oscillating magnetic (roAp) star HD 101065. Methods. High-precision radial velocity (RV) measurements, spanning four consecutive nights on 3-6 March, 2004, were obtained with the HARPS echelle-spectrometer at the ESO 3.6-m telescope, and photometric data were acquired during 6 nights of observations at the 24-inch telescope of Siding Spring Observatory. The RVs were measured using all spectral lines in the wavelength range 4400-5100 Å. Results. We detected a rich spectrum of oscillation modes with semi-amplitudes ranging between 217 m s −1 and 1.6 m s −1. These belong to the complete spectrum of high-order, low = 0-2 modes, having a large spacing of Δν = 64.07 ± 0.9 μHz. We calculated the nonadiabatic frequencies of axisymmetric high-order p-modes for main-sequence models with dipole magnetic fields of two chemical compositions (X, Z) = (0.7, 0.02) and (X, Z) = (0.695, 0.025). The former composition provided the best fit model and yielded stellar parameters of mass, M = 1.525 ± 0.025 M , age = (1.5 ± 0.1) × 10 9 yr, effective temperature log T e = 3.821 ± 0.006, luminosity log L/L = 0.797 ± 0.026, surface gravity log g = 4.06 ± 0.04, and polar magnetic field strength, Bp = 8.7 ± 0.3 kG. We are able to identify the modes of all detected oscillation frequencies. The photometric U and B light curves have maxima that precede the RV maximum by 0.16-0.19 in phase.
Secular variations in the spectrum of solar p-modes
Solar Physics, 1994
The solar p-mode spectrum of very low I is measured with high accuracy for a long enough period of time so as to allow the search for solar cycle variations, in this paper solar cycle variations of the frequency and energy of the modes are confirmed. Moreover, a slight variation,within errors, of its rotational splitting with the solar cycle, is suggested.
CoRoT sounds the stars: p-mode parameters of Sun-like oscillations on HD 49933
Astronomy and Astrophysics, 2008
Context. The first asteroseismology results from CoRoT are presented, on a star showing Sun-like oscillations. We have analyzed a 60 day lightcurve of high-quality photometric data collected by CoRoT on the F5 V star HD 49933. The data reveal a rich spectrum of overtones of low-degree p modes. Aims. Our aim was to extract robust estimates of the key parameters of the p modes observed in the power spectrum of the lightcurve. Methods. Estimation of the mode parameters was performed using maximum likelihood estimation of the power spectrum. A global fitting strategy was adopted whereby 15 mode orders of the mode spectrum (45 modes) were fitted simultaneously. Results. The parameter estimates that we list include mode frequencies, peak linewidths, mode amplitudes, and a mean rotational frequency splitting. We find that the average large frequency (overtone) spacing derived from the fitted mode frequencies is 85.9 ± 0.15 μHz. The frequency of maximum amplitude of the radial modes is at 1760 μHz, where the observed rms mode amplitude is 3.75 ± 0.23 ppm. The mean rotational splitting of the non-radial modes appears to be in the range ≈2.7 μHz to ≈3.4 μHz. The angle of inclination offered by the star, as determined by fits to the amplitude ratios of the modes, appears to be in the range ≈50 degrees to ≈62 degrees.
Oscillation mode frequencies of 61 main-sequence and subgiant stars observed by Kepler
Astronomy & Astrophysics, 2012
Context. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology Aims. We provide the mode frequencies of the oscillations of various stars required to perform a comparison with those obtained from stellar modelling. Methods. We used a time series of nine months of data for each star. The 61 stars observed were categorised in three groups: simple, F-like and mixed-mode. The simple group includes stars for which the identification of the mode degree is obvious. The F-like group includes stars for which the identification of the degree is ambiguous. The mixed-mode group includes evolved stars for which the modes do not follow the asymptotic relation of low-degree frequencies. Following this categorisation, the power spectra of the 61 main sequence and subgiant stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We developed and describe a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individual scientists. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities. Results. We provide the mode frequencies of 61 stars (with their 1-σ error bars), as well as their associatedéchelle diagrams.
Excitation of radial P-modes in the Sun and stars
Solar Physics, 2004
P-mode oscillations in the Sun and stars are excited stochastically by Reynolds stress and entropy fluctuations produced by convection in their outer envelopes. The excitation rate of radial oscillations of stars near the main sequence from K to F and a subgiant K IV star have been calculated from numerical simulations of their surface convection zones. P-mode excitation increases with increasing effective temperature (until envelope convection ceases in the F stars) and also increases with decreasing gravity. The frequency of the maximum excitation decreases with decreasing surface gravity.
Amplitude variability or close frequencies in pulsating stars -- the Scuti star FG Vir
Monthly Notices of the Royal Astronomical Society, 2006
The nature of the observed amplitude variability of several modes in the δ Scuti star FG Vir is examined. This is made possible by the extensive photometry obtained during 2002, 2003 and 2004, as well as the long photometric time base starting in 1992. In this star, three frequencies show strong amplitude and phase variations. In the power spectrum, these frequencies also show up as frequency doublets. However, since true amplitude variability of a single frequency can also lead to (false) frequency doublets in the power spectrum, a specific test examining in detail the observed amplitude and phase variations of an assumed single frequency is applied. For the frequencies at 12.15 and 23.40 cycle d −1 it is shown that amplitude variability of a single mode can be ruled out. In particular, an important property of beating between two modes is fulfilled: the amplitude and phase vary synchronously with a phase shift close to 90 • . The origin of the amplitude variability of a third mode, viz. near 19.86 cycle d −1 , is not clear due to the long beat period of 20+ years, for which the amplitude/phase test suffers from gaps in the coverage. However, even for this frequency the amplitude variations can be expressed well by a mathematical two-mode model.
A new method for the spectroscopic identification of stellar non-radial pulsation modes
Astronomy and Astrophysics, 2006
Aims. We present the Fourier parameter fit method, a new method for spectroscopically identifying stellar radial and non-radial pulsation modes based on the high-resolution time-series spectroscopy of absorption-line profiles. In contrast to previous methods this one permits a quantification of the statistical significance of the computed solutions. The application of genetic algorithms in seeking solutions makes it possible to search through a large parameter space. Methods. The mode identification is carried out by minimizing χ 2 , using the observed amplitude and phase across the line profile and their modeled counterparts. Computations of the theoretical line profiles are based on a stellar displacement field, which is described as superposition of spherical harmonics and that includes the first order effects of the Coriolis force. Results. We made numerical tests of the method on a grid of different mono-and multi-mode models for 0 ≤ ≤ 4 in order to explore its capabilities and limitations. Our results show that whereas the azimuthal order m can be unambiguously identified for low-order modes, the error of is in the range of ±1. The value of m can be determined with higher precision than with other spectroscopic mode identification methods. Improved values for the inclination can be obtained from the analysis of non-axisymmetric pulsation modes. The new method is ideally suited to intermediatley rotating δ Scuti and β Cephei stars.
Monthly Notices of the Royal Astronomical Society, 2006
In a high-resolution spectroscopic survey of rapidly oscillating Ap (roAp) stars with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope of the European Southern Observatory, we find that almost all stars show significant variation of the radial velocity amplitudes-on a timescale of a few pulsation cycles-for lines of the rare earth ion Pr III and in the core of the Hα line. These variations in the radial velocity amplitudes are described by new frequencies in the amplitude spectra that are not seen in broad-band photometric studies of the same stars. The Pr III lines form high in the atmosphere of these stars at continuum optical depths of log τ 5000 −5 and tend to be concentrated towards the magnetic poles in many stars, and the core of the Hα line forms at continuum optical depths −5 log τ 5000 −2, whereas the photometry samples the atmosphere on average at continuum optical depths closer to log τ 5000 = 0 and averages over the visible hemisphere of the star. Therefore, there are three possible explanations for the newly discovered frequencies: (1) there are modes with nodes near to the level where the photometry samples that can be easily detected at the higher level of formation of the Pr III lines; or (2) there are higher degree, , non-radial oblique pulsation modes that are detectable in the spectroscopy because the Pr III is concentrated towards the magnetic poles where such modes have their highest amplitudes, but average out over the visible hemisphere in the photometry which samples the star's surface more uniformly; or (3) there is significant growth and decay of the principal mode amplitudes on a timescale of just a few pulsation cycles at the high level of formation of the Pr III lines and core of the Hα line. The third hypothesis implies that this level is within the magneto-acoustic boundary layer where energy is being dissipated by both outward acoustic running waves and inward magnetic slow waves. We suggest observations that can distinguish among these three possibilities. We propose that strong changes in pulsation phase seen with atmospheric height in roAp stars, in some cases more than π rad from the top to the bottom of a single spectral line, strongly affect the pulsation phases seen in photometry in various bandpasses which explains why phase differences between bandpasses for roAp stars have never been explicable with standard theories that assume single spherical harmonics within the observable atmosphere. We also discuss the photometric amplitude variations as a function of bandpass, and suggest that these are primarily caused by continuum variations, rather than by variability in the rare earth element lines. We propose further tests of this suggestion.