arXiv subjects

S. Turck-Chieze

Publications and source records attributed to S. Turck-Chieze.

36 records · Page 2Linked to original sources

arXiv2006

Detection of periodic signatures in the solar power spectrum. On the track of l=1 gravity modes

In the present work we show robust indications of the existence of g modes in the Sun using 10 years of GOLF data. The present analysis is based on the exploitation of the collective properties of the predicted low-frequency (25 to 140 microHz) g modes: their asymptotic nature, which implies a quasi equidistant separation of their periods for a given angular degree (l). The Power Spectrum (PS) of the Power Spectrum Density (PSD), reveals a significant structure indicating the presence of features (peaks) in the PSD with near equidistant periods corresponding to l=1 modes in the range n=-4 to n=-26. The study of its statistical significance of this feature was fully undertaken and complemented with Monte Carlo simulations. This structure has a confidence level better than 99.86% not to be due to pure noise. Furthermore, a detailed study of this structure suggests that the gravity modes have a much more complex structure than the one initially expected (line-widths, magnetic splittings...). Compared to the latest solar models, the obtained results tend to favor a solar core rotating significantly faster than the rest of the radiative zone. In the framework of the Phoebus group, we have also applied the same methodology to other helioseismology instruments on board SoHO and ground based networks.

astro-ph↗

arXiv2006

Dynamical processes in the solar radiative interior

Recent seismic observations coming from acoustic and gravity modes clearly show that the solar standard model has reached its limits and can no longer be used to interpret satisfactorily seismic observations. In this paper, we present a review of the non-standard processes that may be added to the solar models in order to improve our understanding of the helioseismic data. We also present some results obtained when applying ``non-standard'' stellar evolution to the modelling of the Sun.

astro-ph↗

arXiv2006

Procyon-A and Eta-Bootis: Observational Frequencies Analyzed by the Local-Wave Formalism

In the present analysis of Procyon-A and Eta-Bootis, we use the local-wave formalism which, despite its lack of precision inherent to any semi-analytical method, uses directly the model profile without any modification when calculating the acoustic mode eigenfrequencies. These two solar-like stars present steep variations toward the center due to the convective core stratification, and toward the surface due to the very thin convective zone. Based on different boundary conditions, the frequencies obtained with this formalism are different from that of the classical numerical calculation. We point out that (1) the frequencies calculated with the local-wave formalism seem to agree better with observational ones. All the frequencies detected with a good confident level including those classified as 'noise' find an identification, (2) some frequencies can be clearly identified here as indications of the core limit.

astro-ph↗

arXiv2006

Curvelet analysis of asteroseismic data I: Method description and application to simulated sun-like stars

Context: The detection and identification of oscillation modes (in terms of their $\ell$, $m$ and successive $n$) is a great challenge for present and future asteroseismic space missions. The "peak tagging" is an important step in the analysis of these data to provide estimations of stellar oscillation mode parameters, i.e., frequencies, rotation rates, and further studies on the stellar structure. Aims: To increase the signal-to-noise ratio of the asteroseismic spectra computed from time series representative of MOST and CoRoT observations (30- and 150-day observations). Methods: We apply the curvelet transform -- a recent image processing technique which looks for curved patterns -- to echelle diagrams built using asteroseismic power spectra. In this diagram the eigenfrequencies appear as smooth continuous ridges. To test the method we use Monte Carlo simulations of several sun-like stars with different combinations of rotation rates, rotation-axis inclination and signal-to-noise ratios. Results: The filtered diagrams enhance the contrast between the ridges of the modes and the background allowing a better tagging of the modes and a better extraction of some stellar parameters. Monte Carlo simulations have also shown that the region where modes can be detected is enlarged at lower and higher frequencies compared to the raw spectra. Even more, the extraction of the mean rotational splitting from modes at low frequency can be done more easily than using the raw spectrum.

astro-ph↗

arXiv2005

The magnetism of the solar interior for a complete MHD solar vision

The solar magnetism is no more considered as a purely superficial phenomenon. The SoHO community has shown that the length of the solar cycle depends on the transition region between radiation and convection. Nevertheless, the internal solar (stellar) magnetism stays poorly known. Starting in 2008, the American instrument HMI/SDO and the European microsatellite PICARD will enrich our view of the Sun-Earth relationship. Thus obtaining a complete MHD solar picture is a clear objective for the next decades and it requires complementary observations of the dynamics of the radiative zone. For that ambitious goal, space prototypes are being developed to improve gravity mode detection. The Sun is unique to progress on the topology of deep internal magnetic fields and to understand the complex mechanisms which provoke photospheric and coronal magnetic changes and possible longer cycles important for human life. We propose the following roadmap in Europe to contribute to this "impressive" revolution in Astronomy and in our Sun-Earth relationship: SoHO (1995-2007), PICARD (2008-2010), DynaMICS (2009-2017) in parallel to SDO (2008-2017) then a world-class mission located at the L1 orbit or above the solar poles.

astro-ph↗

arXiv2005

Helioseismology, Neutrinos and Radiative Zones

The solar interior has been scrutinized by two different and independent probes during the last twenty years with important revisions of the solar model, including a recent heavy element abundance revision. Today, we get a quantitatively coherent picture (even incomplete) of the solar (stellar) radiative zones. In this review, we recall the clues for solar gravitational settling definitively established by the seismic determination of the photospheric helium content. We comment also on the need for mixing in the transition region between radiation and convection in the case of the Sun and of population II stars. We finally list the open questions and the importance to continue more precise investigations of the solar (stellar) radiative zone in detecting gravity modes with the project DynaMICS.

astro-ph↗

arXiv2005

Seismic constraints on open clusters

We derive knowledge on the global and structural parameters of low-mass stars using asteroseismology and taking advantage of the stellar collective behavior within open clusters. We build stellar models and compute the seismic signal expected from main sequence objects in the 0.8-1.6 Msun range. We first evaluate apparent magnitudes and oscillations-induced luminosity fluctuations expected in the Hyades, the Pleiades and the alpha Persei clusters. The closest cluster presents a feasible challenge to observational asteroseismology in the present and near future. We combine seismological and classical computations to address three questions: what can be inferred about 1) mass, 2) composition and 3) extension of outer convection zones of solar analogs in the Hyades. The first issue relies on the strong sensitivity of the large separation to mass. Then large separations and second differences are used to respectively constrain metal and helium fractions in the Hyades.When plotted for several masses, the relation of effective temperature vs large separation is found to be strongly dependent on the metal content. Besides this the second difference main modulation is related to the second ionization of helium.The second difference modulations are also partly due to the discontinuity in stellar stratification at the convective envelope / radiative core transition. They permit direct insight in the stellar structure. We compute acoustic radii of the convective bases for different values of the mixing length theoryparameter alpha_MLT in convection modelling, i.e. different convective efficiency in the superadiabatic layers. For a given effectivetemperature we show that the acoustic radius changes with convection efficiency.

astro-ph↗

arXiv2004

Surprising Sun

Important revisions of the solar model ingredients appear after 35 years of intense work which have led to an excellent agreement between solar models and solar neutrino detections. We first show that the updated CNO composition suppresses the anomalous position of the Sun in the known galactic enrichment. The following law: He/H= 0.075 + 44.6 O/H in fraction number is now compatible with all the indicators. We then examine the existing discrepancies between the standard model and solar - seismic and neutrino - observations and suggest some directions of investigation to solve them. We update our predicted neutrino fluxes using the recent composition, new nuclear reaction rates and seismic models as the most representative of the central plasma properties. We get 5.31 +- 0.6 10^6/cm^{2}/s for the total ^8B neutrinos, 66.5 SNU and 2.76 SNU for the gallium and chlorine detectors, all in remarquable agreement with the detected values including neutrino oscillations for the last two. We conclude that the acoustic modes and detected neutrinos see the same Sun, but that clear discrepancies in solar modelling encourage further observational and theoretical efforts.

astro-ph↗

arXiv2003

The Rotation Of The Deep Solar Layers

From the analysis of low-order GOLF+MDI sectoral modes and LOWL data (l > 3), we derive the solar radial rotation profile assuming no latitudinal dependance in the solar core. These low-order acoustic modes contain the most statistically significant information about rotation of the deepest solar layers and should be least influenced by internal variability associated with the solar dynamo. After correction of the sectoral splittings for their contamination by the rotation of the higher latitudes, we obtain a flat rotation profile down to 0.2 solar radius.

astro-ph↗

arXiv2002

New Solar Seismic Models and the Neutrino Puzzle

The SoHO spacecraft made astrophysicists achieve a major breakthrough in the knowledge of the Sun. In helioseismology, both GOLF and MDI experiments aboard SoHO greatly improve the accuracy of seismic data. More specifically, the detection of an enhanced number of low degree low order modes improves the accuracy on the sound speed and density profiles in the solar core. After a description of these profiles, we build solar seismic models. Different models are considered and enable us to derive precise emitted neutrino fluxes. These ones are validated by the seismic data and are in agreement with the recent detected neutrinos, assuming 3 neutrino flavors. The seismic models are also used to put limits on large scale magnetic fields in the solar interior. This analysis puts some upper bounds of about 3x10^7 G in the radiative zone. Such a field could slightly improve the emitted neutrino flux, which remains in agreement with the Sudbury Neutrino Observatory result of 2001. From the models we deduce gravity mode predictions, and the electron and neutron radial densities that are useful to calculate the neutrino oscillations. We also begin to discuss how the external magnetic field may influence such quantities.

astro-ph↗

arXiv2001

Models of Metal Poor Stars with Gravitational Settling and Radiative Accelerations: I. Evolution and Abundance Anomalies

Evolutionary models have been calculated for Pop II stars of 0.5 to 1.0$M_\odot$ from the pre-main-sequence to the lower part of the giant branch. Rosseland opacities and radiative accelerations were calculated taking into account the concentration variations of 28 chemical species, including all species contributing to Rosseland opacities in the OPAL tables. The effects of radiative accelerations, thermal diffusion and gravitational settling are included. While models were calculated both for Z=0.00017 and 0.0017, we concentrate on models with Z=0.00017 in this paper. These are the first Pop II models calculated taking radiative acceleration into account. It is shown that, at least in a 0.8$M_\odot$ star, it is a better approximation not to let Fe diffuse than to calculate its gravitational settling without including the effects of $g_{rad}(Fe)$. In the absence of any turbulence outside of convection zones, the effects of atomic diffusion are large mainly for stars more massive than 0.7$M_\odot$. Overabundances are expected in some stars with $\teff \ge 6000K$. Most chemical species heavier than CNO are affected. At 12 Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while others are limited to 3 (e.g. for Fe). The calculated surface abundances are compared to recent observations of abundances in globular clusters as well as to observations of Li in halo stars. It is shown that, as in the case of Pop I stars, additional turbulence appears to be present.

astro-ph↗

arXiv2001

Lithium Depletion in Pre-Mainsequence Solar-Like Stars

We examine the internal structure of solar-like stars in detail between 0.8 and 1.4M Sun and during pre-main sequence phase. Recent opacity computations of OPAL along with a new hydrodynamical mixing process have been considered. We also introduce up-to-date nuclear reaction rates and explore the impact of accretion, mixing-length parameter, non-solar distributions among metals and realistic rotation history. We compare models predictions of lithium depletion to the $^7Li$ content observations of the Sun and to 4 young clusters of different metallicities and ages. We show that we can distinguish two phases in lithium depletion: 1- a rapid nuclear destruction in the T-Tauri phase before 20 Myrs : this is independent of the mass used within our range but largely dependent on the extension and temperature of the convective zone, 2- a second phase where the destruction is slow and moderate and which is largely dependent on the (magneto)hydrodynamic instability located at the base of the convective zone. In terms of composition, we show the interest on considering helium and especially the mixture of heavy elements : carbon, oxygen, silicium and iron. We outline the importance of O/Fe ratio. We note a reasonable agreement on lithium depletion for the two best known cases, the Sun and the Hyades cluster for solar-like stars. Other clusters suggest that processes which may partly inhibit the predicted premainsequence depletion cannot be excluded, in particular for stars below ~ 0.9M Sun. Finally we suggest different research areas such as initial stellar models and more realistic atmospheres which could contribute to a better understanding of this early phase of evolution and which should become the object of subsequent research.

astro-ph↗

arXiv2000

Structure of the solar core: Effect of asymmetry of peak profiles

Recent studies have established that peaks in solar oscillation power spectra are not Lorentzian in shape, but have a distinct asymmetry. Fitting a symmetric Lorentzian profile to the peaks therefore produces a shift in frequency of the modes. Accurate determination of low-frequency modes is essential to infer the structure of the solar core by inversion of the mode frequencies. In this paper we investigate how the changes in frequencies of low-degree modes obtained by fitting symmetric and asymmetric peak profiles change the inferred properties of the solar core. We use data obtained by the Global Oscillations at Low Frequencies (GOLF) project on board the SoHO spacecraft. Two different solar models and inversion procedures are used to invert the data to determine the sound speed in the solar core. We find that for a given set of modes no significant difference in the inferred sound-speed results from taking asymmetry into account when fitting the low-degree modes.

astro-ph↗

arXiv1999

Standard Solar models in the Light of New Helioseismic Constraints II. Mixing Below the Convective Zone

In previous work, we have shown that recent updated standard solar models cannot reproduce the radial profile of the sound speed at the base of the convective zone (CZ) and fail to predict the Li7 depletion. In parallel, helioseismology has shown that the transition from differential rotation in the CZ to almost uniform rotation in the radiative solar interior occurs in a shallow layer called the tachocline. This layer is presumably the seat of large scale circulation and of turbulent motions. Here, we introduce a macroscopic transport term in the structure equations, which is based on a hydrodynamical description of the tachocline proposed by Spiegel and Zahn, and we calculate the mixing induced within this layer. We discuss the influence of different parameters that represent the tachocline thickness, the Brunt-Vaissala frequency at the base of the CZ, and the time dependence of this mixing process along the Sun's evolution. We show that the introduction of such a process inhibits the microscopic diffusion by about 25%. Starting from models including a pre-main sequence evolution, we obtain: a) a good agreement with the observed photospheric chemical abundance of light elements such as He3, He4, Li7 and Be9, b) a smooth composition gradient at the base of the CZ, and c) a significant improvement of the sound speed square difference between the seismic sun and the models in this transition region, when we allow the phostospheric heavy element abundance to adjust, within the observational incertitude, due to the action of this mixing process. The impact on neutrino predictions is also discussed.

astro-ph↗

arXiv1998

Macroscopic Processes in the Solar Interior

With the recent results of helioseismology aboard SOHO, solar models are more and more constrained (Brun, Turck-Chieze and Morel 1998 (astro-ph/9806272)). New physical processes, mainly connected to macroscopic motions, must be introduced to understand these new observations. In this poster, we present solar models including a turbulent pressure in the outer layers and mixing due to the tachocline (Spiegel and Zahn 1992). Our results lead us to conclude that: - Turbulent pressure improves the absolute value of the acoustic mode frequencies (~ 10 microHz at 4mHz), - Mixing in a tachocline thickness of 0.05 +/- 0.03 Ro (Corbard et al. 1997) looks promising.

astro-ph↗

arXiv1998

Standard Solar Models in the Light of New Helioseismic Constraints: I The Solar Core

In this paper, we examine a new updated solar model taking advantage of recent reexamination of the nuclear reactions rates and of the microscopic diffusion of helium and heavy elements. Our best model fits the helioseismic data reasonably well: the base of the convective zone is at r=0.715 Ro, the photospheric helium in mass fraction is 0.243 and the sound speed square difference between the Sun and the model is < 1%. This model leads to a reestimate of the neutrinos fluxes : 7.18 SNU for the chlorine experiment, 127.2 SNU for the gallium detector and 4.82 10^6/cm2/s for the B8 neutrino flux. Acoustic and gravity mode predictions are also estimated. We present 3 models which respect the helioseismic constraints and fit the seismic observations better by taking advantage of the known physical uncertainties: nuclear reaction rates, CNO abundances and microscopic diffusion. We conclude that the standard model in side its inherent uncertainties, is robust in the light of the present acoustic mode detection and that the neutrino fluxes prediction is not well constrained.

astro-ph↗

arXiv1998

Solar Fusion Cross Sections

We review and analyze the available information for nuclear fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

astro-ph↗

arXiv1997

Are Solar Acoustic Modes Correlated ?

We have studied the statistical properties of the energy of individual acoustic modes, extracted from 310 days of GOLF data near the solar minimum. The exponential distribution of the energy of each mode is clearly seen. The modes are found to be uncorrelated with a +-0.6 % accuracy, thus supporting the hypothesis of stochastic excitation by the solar convection. Nevertheless, the same analysis performed on the same modes just before the solar maximum, using IPHIR data, rejects the hypothesis of no correlation at a 99.3 % confidence level. A simple model suggests that 31.3 +-9.4 % of the energy of each mode is coherent among the modes studied in IPHIR data, correponding to a mean correlation of 10.7 +-5.9 %.

astro-ph↗