Determining global parameters of the oscillations of solar-like stars

Helioseismology has enabled us to better understand the solar interior, while also allowing us to better constrain solar models. But now is a tremendous epoch for asteroseismology as space missions dedicated to studying stellar oscillations have been launched within the last years (MOST and CoRoT). CoRoT has already proved valuable results for many types of stars, while Kepler, which was launched in March 2009, will provide us with a huge number of seismic data very soon. This is an opportunity to better constrain stellar models and to finally understand stellar structure and evolution. The goal of this research work is to estimate the global parameters of any solar-like oscillating target in an automatic manner. We want to determine the global parameters of the acoustic modes (large separation, range of excited pressure modes, maximum amplitude, and its corresponding frequency), retrieve the surface rotation period of the star and use these results to estimate the global parameters of the star (radius and mass).To prepare the analysis of hundreds of solar-like oscillating stars, we have developed a robust and automatic pipeline. The pipeline consists of data analysis techniques, such as Fast Fourier Transform, wavelets, autocorrelation, as well as the application of minimisation algorithms for stellar-modelling. We apply our pipeline to some simulated lightcurves from the asteroFLAG team and the Aarhus-asteroFLAG simulator, and obtain results that are consistent with the input data to the simulations. Our strategy gives correct results for stars with magnitudes below 11 with only a few 10% of bad determinations among the reliable results. We then apply the pipeline to the Sun and three CoRoT targets.In particular we determine the parameters of the Sun, HD49933, HD181906, and HD181420.Comment: 15 pages, 17 figures, accepted for publication in A&

Constraints on the structure of the core of subgiants via mixed modes: the case of HD49385

The solar-like pulsator HD49385 was observed with the CoRoT satellite during 137 days. The analysis of its oscillation spectrum yielded precise estimates of the mode frequencies over nine radial orders and pointed out some unusual characteristics: there exist some modes outside the identified ridges in the echelle diagram and the curvature of the l=1 ridge significantly differs from that of the l=0 ridge. We here propose a toy-model to describe avoided crossings with a strong coupling between the p-mode and g-mode cavities and we show that the peculiar features of the oscillation spectrum of HD49385 can be accounted for only by a low-frequency l=1 avoided crossing. This leads us to establish the post-main-sequence status of HD49385. We then investigate the information which is brought by the mixed modes about the structure of the core. Having shown that traditional optimization techniques are ill-suited for stars with mixed modes in avoided crossing, we propose a new approach to the computation of grids of models, which we apply to HD49385. As a result, we show that the amount of core overshooting in HD49385 is either very low (0<alpha_ov<0.05) or moderate (0.18<alpha_ov<0.20). The mixing length parameter is found to be significantly lower than the solar one (alpha_CGM=0.55\pm0.04 compared to the solar calibrated value alpha_sun=0.64). We also find that the revised solar abundances of Asplund give a better agreement than the classical ones of Grevesse & Noels. Finally, we show that the strength of the coupling between the cavities, which can be estimated through the properties of the observed avoided crossing, plays a dominant role in constraining the parameters of the models. At each step, we investigate the origin and meaning of our seismic diagnostics in terms of the physical structure of the star.Comment: accepted in A&A, 20 pages, 23 figure

Prime Focus Spectrograph (PFS) for the Subaru Telescope: Overview, recent progress, and future perspectives

PFS (Prime Focus Spectrograph), a next generation facility instrument on the 8.2-meter Subaru Telescope, is a very wide-field, massively multiplexed, optical and near-infrared spectrograph. Exploiting the Subaru prime focus, 2394 reconfigurable fibers will be distributed over the 1.3 deg field of view. The spectrograph has been designed with 3 arms of blue, red, and near-infrared cameras to simultaneously observe spectra from 380nm to 1260nm in one exposure at a resolution of ~1.6-2.7A. An international collaboration is developing this instrument under the initiative of Kavli IPMU. The project is now going into the construction phase aiming at undertaking system integration in 2017-2018 and subsequently carrying out engineering operations in 2018-2019. This article gives an overview of the instrument, current project status and future paths forward.Comment: 17 pages, 10 figures. Proceeding of SPIE Astronomical Telescopes and Instrumentation 201

Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants

Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Very distinct results are found depending on the evolutionary status of the star.Comment: Source tex file : 1713.tex Bibliography file : 1713.bbl 24 figures : 1713f1.eps, ..., 1713f24.ep

Theoretical damping rates and phase-lags for solar-like oscillations

International audienc

Theoretical damping rates and phase-lags for solar-like oscillations

International audienc

Theoretical damping rates and phase-lags for solar-like oscillations

International audienc