Mode width fitting with a simple bayesian approach. Application to CoRoT targets HD 181420 and HD 49933

We investigate the asteroseismology of two solar-like targets as observed with the CoRoT satellite, with particular attention paid to the mode fitting. HD 181420 and HD 49933 are typical CoRoT solar-like targets (156 and 60-day runs). The low signal-to-noise ratio (SNR) of about 3-10 prevents us from unambiguously identifying the individual oscillation modes. In particular, convergence problems appear at the edges of the oscillation spectrum. HD 181420 and HD 49933 are typical CoRoT solar-like targets (156 and 60-day runs). The low signal-to-noise ratio (SNR) of about 3-10 prevents us from unambiguously identifying the individual oscillation modes. In particular, convergence problems appear at the edges of the oscillation spectrum. We apply a Bayesian approach to the analysis of these data. We compare the global fitting of the power spectra of this time series, obtained by the classical maximum likelihood (MLE) and the maximum a posteriori (MAP) estimators. We examine the impact of the choice of the priors upon the fitted parameters. We also propose to reduce the number of free parameters in the fitting, by replacing the individual estimate of mode height associated with each overtone by a continuous function of frequency (Gaussian profile). The MAP appears as a powerful tool to constrain the global fits, but it must be used carefully and only with reliable priors. The mode width of the stars increases with the frequency over all the oscillation spectrum.Comment: 10 pages, 9 figures, 2 table

Helioseismology with PICARD

PICARD is a CNES micro-satellite launched in June 2010 (Thuillier at al. 2006). Its main goal is to measure the solar shape, total and spectral irradiance during the ascending phase of the activity cycle. The SODISM telescope onboard PICARD also allows us to conduct a program for helioseismology in intensity at 535.7 nm (Corbard et al. 2008). One-minute cadence low-resolution full images are available for a so-called medium-$l$ program, and high-resolution images of the limb recorded every 2 minutes are used to study mode amplification near the limb in the perspective of g-mode search. First analyses and results from these two programs are presented here.Comment: 6 pages, 6 figures, Eclipse on the Coral Sea: Cycle 24 Ascending, GONG 2012 / LWS/SDO-5 / SOHO 27, November 12 - 16, 2012, Palm Cove, Queensland. Accepted for publication in Journal of Physics Conference Series on March 1st 201

Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933

Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions. We aim to model the photometric granulation background of HD49933 as well as the Sun, and compare the predictions to the measurements obtained by the COROT and SOHO satellite missions. We construct hydrodynamical model atmospheres representing HD49933 and the Sun, and use a previously developed scaling technique to obtain the observable disk-integrated brightness fluctuations. We further performed exploratory magneto-hydrodynamical simulations to gauge the impact of small scale magnetic fields on the synthetic light-curves. We find that the granulation-related brightness fluctuations depend on metallicity. We obtain a satisfactory correspondence between prediction and observation for the Sun, validating our approach. For HD49933, we arrive at a significant over-estimation by a factor of two to three in total power. Locally generated magnetic fields are unlikely to be responsible, otherwise existing fields would need to be rather strong to sufficiently suppress the granulation signal. Presently suggested updates on the fundamental stellar parameters do not improve the correspondence; however, an ad-hoc increase of the HD49933 surface gravity by about 0.2dex would eliminate most of the discrepancy. We diagnose a puzzling discrepancy between the predicted and observed granulation background in HD49933, with only rather ad-hoc ideas for remedies at hand.Comment: 7 pages, 5 figures, accepted for publication in A&

The CoRoT target HD175726: an active star with weak solar-like oscillations

Context. The CoRoT short runs give us the opportunity to observe a large variety of late-type stars through their solar-like oscillations. We report observations of the star HD175726 that lasted for 27 days during the first short run of the mission. The time series reveals a high-activity signal and the power spectrum presents an excess due to solar-like oscillations with a low signal-to-noise ratio. Aims. Our aim is to identify the most efficient tools to extract as much information as possible from the power density spectrum. Methods. The most productive method appears to be the autocorrelation of the time series, calculated as the spectrum of the filtered spectrum. This method is efficient, very rapid computationally, and will be useful for the analysis of other targets, observed with CoRoT or with forthcoming missions such as Kepler and Plato. Results. The mean large separation has been measured to be 97.2+-0.5 microHz, slightly below the expected value determined from solar scaling laws.We also show strong evidence for variation of the large separation with frequency. The bolometric mode amplitude is only 1.7+-0.25 ppm for radial modes, which is 1.7 times less than expected. Due to the low signal-to-noise ratio, mode identification is not possible for the available data set of HD175726. Conclusions. This study shows the possibility of extracting a seismic signal despite a signal-to-noise ratio of only 0.37. The observation of such a target shows the efficiency of the CoRoT data, and the potential benefit of longer observing runs.Comment: 8 pages. Accepted in A&

II.2 Description of processes and corrections from observation to delivery

This book is dedicated to all the people interested in the CoRoT mission and the beautiful data that were delivered during its six year duration. Either amateurs, professional, young or senior researchers, they will find treasures not only at the time of this publication but also in the future twenty or thirty years. It presents the data in their final version, explains how they have been obtained, how to handle them, describes the tools necessary to understand them, and where to find them. It also highlights the most striking first results obtained up to now. CoRoT has opened several unexpected directions of research and certainly new ones still to be discovered

The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO

Solar flares presumably have an impact on the deepest layers of the solar atmosphere and yet the observational evidence for such an impact is scarce. Using ten years of measurements of the Na D$_{1}$ and Na D$_2$ Fraunhofer lines, measured by GOLF onboard SOHO, we show that this photospheric line is indeed affected by flares. The effect of individual flares is hidden by solar oscillations, but a statistical analysis based on conditional averaging reveals a clear signature. Although GOLF can only probe one single wavelength at a time, we show that both wings of the Na line can nevertheless be compared. The varying line asymmetry can be interpreted as an upward plasma motion from the lower solar atmosphere during the peak of the flare, followed by a downward motion.Comment: 13 pages, 7 figure

Searching for p-modes in MOST Procyon data: Another view

Photometry of Procyon obtained by the MOST satellite in 2004 has been searched for p modes by several groups, with sometimes contradictory interpretations. We explore two possible factors that complicate the analysis and may lead to erroneous reports of p modes in these data. Two methods are used to illustrate the role of subtle instrumental effects in the photometry: time-frequency analysis, and a search for regularly spaced peaks in a Fourier spectrum based on the echelle diagramme approach. We find no convincing evidence of a p-mode signal in the MOST Procyon data. We can account for an apparent excess of power close to the p-mode frequency range and signs of structure in an echelle diagramme in terms of instrumental effects.Comment: Article accepted, to appear in A&

Seismic and spectroscopic characterization of the solar-like pulsating CoRoT target HD 49385

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. Our goal is to characterize HD 49385 using both spectroscopic and seismic data. 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 l=0 to l=3 (36 individual modes). Precise estimates of the atmospheric parameters (Teff, [M/H], log g) and of the vsini of HD 49385 are obtained. The seismic analysis of the star leads to a clear identification of the modes for degrees l=0,1,2. Around the maximum of the signal (nu=1013 microHz), some peaks are found significant and compatible with the expected characteristics of l=3 modes. Our fit yields robust estimates of the frequencies, linewidths and amplitudes of the modes. We find amplitudes of about 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.Comment: 13 pages, 14 figures, accepted in A&