Aims: The CoRoT and Kepler satellites have provided thousands of red-giant oscillation spectra. The analysis of these spectra requires efficient methods of identifying all eigenmode parameters. ...Methods: The assumption of new scaling laws allowed us to construct a theoretical oscillation pattern. We then obtained a highly precise determination of the large separation by correlating the observed patterns with this reference. Results: We demonstrate that this pattern is universal and are able to unambiguously assign the eigenmode radial orders and angular degrees. This solves one of the remaining problems of asteroseismology, hence allowing precise theoretical investigation of red-giant interiors.
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.
Towards the end of their lives, stars like the Sun greatly expand to become red giant stars. Such evolved stars could provide stringent tests of stellar theory, as many uncertainties of the internal ...stellar structure accumulate with age. Important examples are convective overshooting and rotational mixing during the central hydrogen-burning phase, which determine the mass of the helium core, but which are not well understood. In principle, analysis of radial and non-radial stellar oscillations can be used to constrain the mass of the helium core. Although all giants are expected to oscillate, it has hitherto been unclear whether non-radial modes are observable at all in red giants, or whether the oscillation modes have a short or a long mode lifetime, which determines the observational precision of the frequencies. Here we report the presence of radial and non-radial oscillations in more than 300 giant stars. For at least some of the giants, the mode lifetimes are of the order of a month. We observe giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex. No satisfactory theoretical explanation currently exists for our observations.
We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute ...sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Δν) and the frequency of maximum power (νmax). We focus on a sample of 50 low-luminosity stars (νmax > 100 μHz, L <~ 30 L sun) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a single échelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of δν02 versus Δν. The small separation δν01 of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.
Context: Measuring amplitudes of solar-like oscillations and the granulation power spectral density constitute two promising sources of information to improve our understanding and description of the ...convection in outer layers of stars. However, different instruments, using different techniques and different bandpasses, give measurements that cannot be directly compared to each other or to theoretical values. Aims: In this work, we define simple response functions to derive intrinsic oscillation amplitudes and granulation power densities, from photometry measurements obtained with a specific instrument on a specific star. Methods: We test this method on different photometry data sets obtained on the Sun with two different instruments in three different bandpasses. Results: We show that the results are in good agreement and we establish reference intrinsic values for the Sun with photometry. We also compute the response functions of the CoRoT instrument for a range of parameters representative of the Main Sequence solar-like pulsators to be observed with CoRoT. We show that these response functions can be conveniently described by simple analytic functions of the effective temperature of the target star.
Context. The star HD 49385 is the first G-type solar-like pulsator observed in the seismology field of the space telescope CoRoT. The satellite collected 137 days of high-precision photometric data ...on this star, confirming that it presents solar-like oscillations. HD 49385 was also observed in spectroscopy with the NARVAL spectrograph in January 2009. Aims. Our goal is to characterize HD 49385 using both spectroscopic and seismic data. Methods. The fundamental stellar parameters of HD 49385 are derived with the semi-automatic software VWA, and the projected rotational velocity is estimated by fitting synthetic profiles to isolated lines in the observed spectrum. A maximum likelihood estimation is used to determine the parameters of the observed p modes. We perform a global fit, in which modes are fitted simultaneously over nine radial orders, with degrees ranging from $\ell$ = 0 to $\ell$ = 3 (36 individual modes). Results. Precise estimates of the atmospheric parameters (Teff, M/H, log g) and of the ν sin i of HD 49385 are obtained. The seismic analysis of the star leads to a clear identification of the modes for degrees $\ell$ = 0,1,2. Around the maximum of the signal (ν $\simeq$ 1013 μHz), some peaks are found significant and compatible with the expected characteristics of $\ell$ = 3 modes. Our fit yields robust estimates of the frequencies, linewidths and amplitudes of the modes. We find amplitudes of ~5.6 ± 0.8 ppm for radial modes at the maximum of the signal. The lifetimes of the modes range from one day (at high frequency) to a bit more than two days (at low frequency). Significant peaks are found outside the identified ridges and are fitted. They are attributed to mixed modes.
Context. The advent of space-borne missions such as CoRoT or Kepler providing photometric data has brought new possibilities for asteroseismology across the H-R diagram. Solar-like oscillations are ...now observed in many stars, including red giants and main-sequence stars. Aims: Based on several hundred identified pulsating red giants, we aim to characterize their oscillation amplitudes and widths. These observables are compared with those of main-sequence stars in order to test trends and scaling laws for these parameters for main-sequence stars and red giants. Methods: An automated fitting procedure is used to analyze several hundred Fourier spectra. For each star, a modeled spectrum is fitted to the observed oscillation spectrum, and mode parameters are derived. Results: Amplitudes and widths of red-giant solar-like oscillations are estimated for several hundred modes of oscillation. Amplitudes are relatively high (several hundred ppm) and widths relatively small (very few tenths of a muHz). Conclusions: Widths measured in main-sequence stars show a different variation with the effective temperature from red giants. A single scaling law is derived for mode amplitudes of red giants and main-sequence stars versus their luminosity to mass ratio. However, our results suggest that two regimes may also be compatible with the observations.
Context. Solar-like oscillations have now been observed in several stars, thanks to ground-based spectroscopic observations and space-borne photometry. CoRoT, which has been in orbit since December ...2006, has observed the star HD49933 twice. The oscillation spectrum of this star has proven difficult to interpret. Aims. Thanks to a new timeseries provided by CoRoT, we aim to provide a robust description of the oscillations in HD49933, i.e., to identify the degrees of the observed modes, and to measure mode frequencies, widths, amplitudes and the average rotational splitting. Methods. Several methods were used to model the Fourier spectrum: Maximum Likelihood Estimators and Bayesian analysis using Markov Chain Monte-Carlo techniques. Results. The different methods yield consistent result, and allow us to make a robust identification of the modes and to extract precise mode parameters. Only the rotational splitting remains difficult to estimate precisely, but is clearly relatively large (several μHz in size).
Context. The estimate of solar-like oscillation properties, such as their frequencies, amplitudes and lifetimes, is challenging because of their low amplitudes and will benefit from long and ...uninterrupted observing runs. The space telescope CoRoT allows us to obtain high-performance photometric data over a long and quasi continuous period. Among its main targets are stars for which we expect solar-like oscillations. Aims. HD 181420, an F2 main sequence star, has been observed by CoRoT during its first long run covering about 156 days. With this unprecedently high-quality set of data, our aim is to derive the p-mode parameters that can be used to probe the stellar interior. Methods. The CoRoT data obtained on HD 181420 is analysed using a classical Fourier approach for the search for the p mode signature. The p-mode parameters are then derived using global fitting of the power spectrum by a Lorentzian model, as used widely in the solar case. Results. From the p-mode frequencies, the mean value of the large spacing is estimated to be 75 ${\rm \mu Hz}$. The p-mode amplitudes are slightly less than 4 ppm with a line width of about 8 ${\rm \mu Hz}$ at the maximum of the p modes. The inclination angle is estimated to be around $45 \degr$. The large mode line-width combined with the observed mode spacing make it difficult to identify the $\ell=2$ modes and to estimate the rotational splitting. We explore two scenarios for the identification of the modes.
Context. The F8 star HD 181906 (effective temperature ${\sim}6300$ K) was observed for 156 days by the CoRoT satellite during the first long run in the direction of the galactic centre. Analysis of ...the data reveals a spectrum of solar-like acoustic oscillations. However, the faintness of the target (mv = 7.65) means the signal-to-noise (S/N) in the acoustic modes is quite low, and this low S/N leads to complications in the analysis. Aims. We extract global variables of the star, as well as key parameters of the p modes observed in the power spectrum of the lightcurve. Methods. The power spectrum of the lightcurve, a wavelet transform and spot fitting were used to obtain the average rotation rate of the star and its inclination angle. Then, the autocorrelation of the power spectrum and the power spectrum of the power spectrum were used to properly determine the large separation. Finally, estimations of the mode parameters were done by maximizing the likelihood of a global fit, where several modes were fit simultaneously. Results. We have been able to infer the mean surface rotation rate of the star (~4 μHz) with indications of the presence of surface differential rotation, the large separation of the p modes (~87 μHz), hence also the “ridges” corresponding to overtones of the acoustic modes.
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