Many stars exhibit stellar pulsations, favoring them for asteroseismic analyses. Interpreting the oscillations requires some knowledge of the oscillation mode geometry (spherical degree, radial and azimuthal orders). The delta Scuti stars (1.5 - 2.5 M_sol) often show just one or few pulsation frequencies. Although this may promise a successful seismological analysis, we may not know enough about either the mode or the star to use the oscillation frequency to improve the determination of the stellar model, or probe the star's structure. For the observed frequencies to be used successfully as seismic probes of these objects, we need to concentrate on stars for which we can reduce the number of free parameters in the problem, such as binary systems or open clusters. We investigate how much our understanding of a delta Scuti star is improved when it is in a detached eclipsing binary system instead of being a single field star. We use singular value decomposition to explore the precision we expect in stellar parameters (mass, age and chemical composition) for both cases. We examine how the parameter uncertainties propagate to the luminosity - effective temperature diagram and determine when the effort of obtaining a new measurement is justified. We show that for the single star, a correct identification of the oscillation mode is necessary to produce strong constraints on the stellar model properties, while for the binary system the observations without the pulsation mode provide the same or better constraints on the stellar parameters. In the latter case, ...

The European Coordination Action on HELio- and ASteroseismology (HELAS) has completed its fourth and final year of initial funding by the European Commission. Set up as a network which combines solar and stellar physics communities in the important and vigorously evolving field of seismology, HELAS has been able to coordinate the efforts of European astronomers with remarkable success. Four large international conferences including the HELAS-IV conference on Lanzarote as well as many workshops were organized with a substantial contribution from HELAS. About a dozen workshops, addressing specialized questions in global and local helioseismology and asteroseismology were entirely organized by HELAS. Data analysis tools to prepare the European communities for the upcoming influx of data from new missions have been prepared, tested and demonstrated. Lecture notes and outreach material have been assembled and prepared for general access. As a result, HELAS has an important impact on the scientific output of the astrophysics seismology com-munities and significantly increased the visibility of European research in this field. This paper summarizes the activities and accomplishments of HELAS (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present an analysis of the variability of the solar oscillation spectrum during solar cycle 23 and its extended minimum. We use simultaneous observations of the low-degree solar p modes collected by the space-based, Sun-as-a-star GOLF (radial velocity) and VIRGO (intensity) instruments, and by the ground-based, multi-site network GONG. We investigate in particular the response of the p-mode eigenfrequencies to the observed peculiar deep solar minimum of surface activity of 2007-2009 as compared with the previous solar cycle 23. We study the different temporal variations of the p-mode frequencies with individual angular degrees.

We study the response of the low-degree solar p-mode frequencies to the unusual extension of the minimum of solar surface activity since 2007. Helioseismic observations collected by the space-based, Sun-as-a-star GOLF instrument and by the ground-based, multi-site network GONG (integrated signal) are analyzed. Temporal variations of the low-degree (l=0,1,2), p-mode frequencies are obtained. Although the known correlation of the frequency changes with the solar surface activity is recovered for the period 1996-2007, since the second half of 2007 and until July 2009 (latest period analyzed) we notice a peculiar behavior amongst modes of different angular degrees. In particular, a clear increase of the l=0 and l=2 p-mode frequencies is consistently obtained since late 2007, while the l=1 frequencies follow the general decreasing trend of surface activity. We interpret these differences in the frequency shifts of individual low-degree modes as indicative of variations at high latitudes in the magnetic flux beneath the surface of the Sun related to the onset of solar cycle 24.

We study the response of the low-degree, solar p-mode frequencies to the unusually extended minimum of solar surface activity since 2007. A total of 4768 days of observations collected by the space-based, Sun-as-a-star helioseismic GOLF instrument are analyzed. A multi-step iterative maximum-likelihood fitting method is applied to subseries of 365 days and 91.25 days to extract the p-mode parameters. Temporal variations in the l = 0, 1, and 2 p-mode frequencies are then obtained from April 1996 to May 2009. While the p-mode frequency shifts are closely correlated with solar surface activity proxies during the past solar cycles, the frequency shifts of the l = 0 and 1 = 2 modes increase from the second half of 2007, when no significant surface activity is observable. On the other hand, the l = 1 modes follow the general decreasing trend of solar surface activity. The different behaviors between the l = 0 and l = 2 modes and the l = 1 modes may be interpreted as different geometrical responses to the spatial distribution of the solar magnetic field beneath the surface of the Sun. The analysis of the low-degree, solar p-mode frequency shifts indicates that the solar activity cycle 24 started in late 2007, despite the absence of activity on the solar surface.

Solar magnetism is measured with different indexes: for instance the MPSI and the MWSI, number of sunspots, radio flux at 10.7 cm, Ca II K, Mg II K, EUV, He I or L_alpha. Bachmann & White (1994) had compared these indicators of the solar activity showing a hysteresis of the solar cycle variations and a time lag between these indices not related to instrumental effects. Later on, Ozguc & Atac (2001) extended this study of hysteresis phenomenon between Flare index and other solar indices (mean magnetic field, coronal index). In its original working configuration, GOLF/SoHO was able to measure during 26 days the solar mean magnetic field (Garcia et al. 1999). We check here if the velocity data could be used as another solar magnetism proxy with the advantage of having a duty cycle >95% during the last 12 years. We will compare the GOLF data with some of the above-mentioned solar activity indexes.

The primary challenge of GOLF-NG (Global Oscillations at Low Frequency New Generation) is the detection of the low-frequency solar gravity and acoustic modes, as well as the possibility to measure the high-frequency chromospheric modes. On June 8th 2008, the first sunlight observations with the multichannel resonant GOLF-NG prototype spectrometer were obtained at the Observatorio del Teide (Tenerife). The instrument performs integrated (Sun-as-a-star), Doppler velocity measurements, simultaneously at eight different heights in the D1 sodium line profile, corresponding to photospheric and chromospheric layers of the solar atmosphere. In order to study its performances, to validate the conceived strategy, and to estimate the necessary improvements, this prototype has been running on a daily basis over the whole summer of 2008 at the Observatorio del Teide. We present here the results of the first GOLF-NG observations, clearly showing the characteristics of the 5-minute oscillatory signal at different heights in the solar atmosphere. We compare these signals with simultaneous observations from GOLF/SOHO and from the Mark-I instrument -- a node of the BiSON network, operating at the same site.

Context. The F8 star HD 181906 (effective temperature similar to 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 (m(v) = 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 (similar to 4 mu Hz) with indications of the presence of surface differential rotation, the large separation of the p modes (similar to 87 mu Hz), hence also the "ridges" corresponding to overtones of the acoustic modes.

For distant stars, as observed by the NASA Kepler satellite, parallax information is currently of fairly low quality and is not complete. This limits the precision with which the absolute sizes of the stars and their potential transiting planets can be determined by traditional methods. Asteroseismology will be used to aid the radius determination of stars observed during NASA's Kepler mission. We report on the recent asteroFLAG hare-and-hounds Exercise#2, where a group of `hares' simulated data of F-K main-sequence stars that a group of `hounds' sought to analyze, aimed at determining the stellar radii. Based on the asteroseismic large frequency spacing, obtained from simulations of 4-year time series data from the Kepler mission, we demonstrate that the stellar radii can be correctly and precisely determined, when combined with traditional stellar parameters from the Kepler Input Catalogue. The radii found by the various methods used by each independent hound generally agree with the true values of the artificial stars to within 3%, when the large frequency spacing is used. This is 5--10 times better than the results where seismology is not applied. These results give strong confidence that radius estimation can be performed to better than 3% for solar-like stars using automatic pipeline reduction. Even when the stellar distance and luminosity are unknown we can obtain the same level of agreement.

The detection of the signature of dipole gravity modes has opened the path to study the solar inner radiative zone. Indeed, g modes should be the best probes to infer the properties of the solar nuclear core that represents more than half of the total mass of the Sun. Concerning the dynamics of the solar core, we can study how future observations of individual g modes could enhance our knowledge of the rotation profile of the deep radiative zone. Applying inversions on a set of real p-mode splittings coupled with either one or several g modes, we have checked the improvement of the inferred rotation profile when different error bars are considered for the g modes. Moreover, using a new methodology based on the analysis of the almost constant separation of the dipole gravity modes, we can introduce new constraints on solar models. For that purpose, we can compare g-mode predictions computed from several models including different physical inputs with the g-mode asymptotic signature detected in GOLF data and calculate the correlation. This work shows the great consistency between the signature of dipole gravity modes and our knowledge of p-modes: incompatibility of data with a present standard model including the Asplund composition

Two research paths are described to obtain better understanding of the origin of global solar activity. First, observations with a multichannel resonant spectrometer may reveal the dynamics of the solar core, the tachocline, and the temporal evolution of activity between the photosphere and chromosphere. Such new observations will deliver constraints for 3D simulations of solar activity. Second, we examine the ab-initio introduction of a non-force-free field expressed in spherical harmonics into the solar structure equations and estimate its impact on the inner and subsurface layers, its time evolution, and its role in angular momentum transport.

We stand on the threshold of a critical expansion of asteroseismology of Sun-like stars, the study of stellar interiors by observation and analysis of their global acoustic modes of oscillation. The Sun-like oscillations give a very rich spectrum allowing the internal structure and dynamics to be probed down into the stellar cores to very high precision. Asteroseismic observations of many stars will allow multiple-point tests of crucial aspects of stellar evolution and dynamo theory. The aims of the asteroFLAG collaboration are to help the community to refine existing, and to develop new, methods for analysis of the asteroseismic data on the Sun-like oscillators.

We report on initial results from the first phase of Exercise #1 of the asteroFLAG hare and hounds. The asteroFLAG group is helping to prepare for the asteroseismology component of NASA's Kepler mission, and the first phase of Exercise #1 is concerned with testing extraction of estimates of the large and small frequency spacings of the low-degree p modes from Kepler-like artificial data. These seismic frequency spacings will provide key input for complementing the exoplanet search data. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Frequencies, powers and damping rates of the solar p modes are all observed to vary over the 11-yr solar activity cycle. Here, we show that simultaneous variations in these parameters give rise to a subtle cross-talk effect, which we call the ‘devil in the detail’, that biases p-mode frequencies estimated from analysis of long power frequency spectra. We also show that the resonant peaks observed in the power frequency spectra show small distortions due to the effect. Most of our paper is devoted to a study of the effect for Sun-as-a-star observations of the low-l p modes. We show that for these data the significance of the effect is marginal. We also touch briefly on the likely l dependence of the effect, and discuss the implications of these results for solar structure inversions.

In this work we investigate p-mode power variation with solar atmosphere. To this aim, we use THEMIS observations of the Na D1 (lambda 5896 angstrom) and K (lambda 7699 angstrom) spectral lines. While the formation heights of the K spectral line are essentially located in the photospheric layer, the formation heights of the Na D I line span a much wider region: from photosphere up to chromosphere. Hence, we had the opportunity to infer p-mode power variation up to the chromospheric layer. By analyzing power spectra obtained by temporal series at different points of the Na D1 and K spectral lines, we confirm and quantify the increase in p-mode power towards higher atmospheric layers. Furthermore, the large span in formation heights of the Na D1 line induces a larger enhancement of p-mode power with solar atmosphere compared to the K spectral line. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The PICARD mission is a CNES micro-satellite to be launched in 2009. Its goal is to better understand the Sun and the potential impact of its activity on earth climate by measuring simultaneously the solar total and spectral irradiance, diameter, shape and oscillations. We present the scientific objectives, instrumental requirements and data products of the helioseismology program of PICARD which aims to observe the low to medium l p-mode oscillations in intensity and search for g-mode oscillation signatures at the limb. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Since the beginning of this century we have attended a blooming of the gravity-mode research thanks to the unprecedented quality of the data available, either from space with SoHO, or from the ground-based networks as BiSON or GONG. From the first upper limit of the gravity-mode amplitudes fixed at 10 mm/s at 200 mu Hz given by Appourchaux et al. (2000), on one hand, a peak was supposed to be a component of the l = 1, n = 1 mixed mode (Garcia et al. 2001a, 2001b; Gabriel et al. 2002) and, on the other hand, a couple of patterns - multiplets - were attributed to gravity modes (Turck-Chieze et al. 2004; Mathur et al. 2007). One of these patterns, found around 220 mu Hz, could be labeled as the l = 2, n = -3 g mode, which is expected to be the one with the highest surface amplitude (Cox & Guzik 2004). Finally, in 2007, Garcia et al. were able to measure the fingertips of the dipole gravity modes looking for their asymptotic properties. In the present paper we present an update of the recent developments on this subject with special attention to the 220 mu Hz region, the dipole asymptotic properties and the impact of the incoming g-mode observations on the knowledge of the solar structure and rotation profile. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Accurate determination of the rotation rate in the radiative zone of the Sun from helioseismic observations requires rotational frequency splittings of exceptional quality as well as reliable inversion techniques. Here we present inferences based on mode parameters calculated from 2088 day MDI, GONG, and GOLF time series that were fitted to estimate very low frequency rotational splittings (ν < 1.7 mHz). These low-frequency modes provide data of exceptional quality, since the width of the mode peaks is much smaller than the rotational splitting, and hence it is much easier to separate the rotational splittings from the effects caused by the finite lifetime and the stochastic excitation of the modes. We have also implemented a new inversion methodology that allows us to infer the rotation rate of the radiative interior from mode sets that span ℓ = 1 to 25. Our results are compatible with the Sun rotating like a rigid solid in most of the radiative zone, and slowing down in the core (r/R☉ < 0.2). A resolution analysis of the inversion was carried out for the solar rotation inverse problem. This analysis effectively establishes a direct relationship between the mode set included in the inversion and the sensitivity and information content of the resulting inferences. We show that such an approach allows us to determine the effect of adding low-frequency and low-degree p-modes, high-frequency and low-degree p-modes, and some g-modes on the derived rotation rate in the solar radiative zone, and in particular the solar core. We conclude that the level of uncertainty that is needed to infer the dynamical conditions in the core when only p-modes are included is unlikely to be reached in the near future, and hence sustained efforts are needed toward the detection and characterization of g-modes.

Context. Below 1 mHz, the power spectrum of helioseismic velocity measurements is dominated by the spectrum of convective motions (granulation and supergranulation) and it is difficult to detect the low-order acoustic modes and gravity modes.Aims. We attempt to understand more clearly the behavior of solar granulation as a function of observing height in the solar atmosphere and with magnetic activity during solar cycle 23.Methods. We analyze the Power Spectral Density (PSD) of eleven years of GOLF/SOHO velocity time series data using a Harvey-type model to characterize the properties of the convective motions in the solar oscillation power spectrum. We study the evolution of the granulation with both altitude in the solar atmosphere and solar activity.Results. We first demonstrate that the traditional use of a Lorentzian profile to describe the envelope of the p modes is unsuitable for GOLF data. To model properly the solar spectrum, we must instead adopt a second Lorentzian profile. Secondly, we show that the granulation clearly evolves with height in the photosphere but does not present any significant variation with the activity cycle.