The Experts below are selected from a list of 165 Experts worldwide ranked by ideXlab platform
H.-g. Ludwig - One of the best experts on this subject based on the ideXlab platform.
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Stellar granulation as seen in disk-Integrated Intensity
Astronomy and Astrophysics - A&A, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. Kallinger, M. N. Lund, B. Mosser, A. BaglinAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:Context. Solar granulation has been known for a long time to be a surface manifestation of convection. The space-borne missions CoRoT and Kepler enable us to observe the signature of this phenomena in disk-Integrated Intensity on a large number of stars. Aims. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated wit h granulation obeys characteristic scaling relations. We thus ai med at providing simple theoretical modeling to reproduce these scaling relations, and subsequently at inferring the physical prop erties of granulation across the HR diagram. Methods. We developed a simple 1D theoretical model. The input parameters were extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involve d in the model were calibrated with solar observations. Two different prescriptions for representing the Fourier transform of the ti me-correlation of the eddy velocity were compared: a Lorentzian and an exponential form. Finally, we compared our theoretical pre diction with 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (hereafter ab initio approach). Results. Provided that the free parameters are appropriately adjust ed, our theoretical model reproduces the observed solar gra nulation spectrum quite satisfactorily ; the best agreement is obtai ned for an exponential form. Furthermore, our model results in granulation spectra that agree well with the ab initio approach using two 3D RHD models that are representative of the surface layers of an F-dwarf and a red-giant star. Conclusions. We have developed a theoretical model that satisfactory reproduces the solar granulation spectrum and gives results consistent with the ab initio approach. The model is used in a companion paper as theoretical framework for interpretating the observed scaling relations.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:The solar granulation is known for a long time to be a surface manifestation of convection. Thanks to the current space-borne missions CoRoT and Kepler, it is now possible to observe in disk-Integrated Intensity the signature of this phenomena in a growing number of stars. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aim at providing a simple theoretical modeling to reproduce these scaling relations and subsequently at inferring the physical properties of granulation properties across the HR diagram. We develop a simple 1D theoretical model that enable us to test any prescription concerning the time-correlation between granules. The input parameters of the model are extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model are calibrated with solar observations. Two different prescriptions for representing the eddy time-correlation in the Fourier space are compared: a Lorentzian and an exponential form. Finally, we compare our theoretical prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (ab-initio approach). Provided that the free parameters are appropriately adjusted, our theoretical model satisfactorily reproduces the shape and the amplitude of the observed solar granulation spectrum. The best agreement is obtained with an exponential form. Furthermore, our theoretical model results in granulation spectra that consistently agree with the these calculated on the basis of the ab-initio approach with two 3D RHD models. Comparison between theoretical granulation spectra calculated with the present model and high precision photometry measurements of stellar granulation is undertaken in a companion paper.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness fluctuations (sigma) associated with the granulation scale as a function of the peak frequency (nu_max) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model. For each PDS we derived tau_eff and sigma and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the Kepler measurements. We derive theoretical scaling relations for tau_eff and sigma, which show the same dependence on nu_max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (Ma) estimated at the photosphere. The theoretical scaling relations for tau_eff and sigma match the observations well on a global scale. Our modelling provides additional theoretical support for the observed variations of sigma and tau_eff with nu_m max. It also highlights the important role of Ma in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on Ma. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with Ma.
Navid Qureshi - One of the best experts on this subject based on the ideXlab platform.
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Mag2Pol: a program for the analysis of spherical neutron polarimetry, flipping ratio and Integrated Intensity data
Journal of Applied Crystallography, 2019Co-Authors: Navid QureshiAbstract:Mag2Pol is a graphical user interface program which is devoted to the treatment of data from polarized neutron diffractometers with spherical polarization analysis. Nuclear and magnetic structure models can be introduced using space-group symbols and individual symmetry operators, respectively, and viewed in an OpenGL widget. The program calculates nuclear/magnetic structure factors, flipping ratios and polarization matrices for magnetic Bragg reflections, taking into account structural twins and magnetic domains. Spherical neutron polarimetry data can be analyzed by refining a magnetic structure model including magnetic domain populations in a least-squares fit and can also be correlated with an Integrated Intensity data set in a joint refinement. Further features are the simultaneous refinement of nuclear and magnetic structures with Integrated Intensity data and the analysis of flipping ratios either with tabulated magnetic form factors or using a multipole expansion of the magnetization density.
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Mag2Pol: A program for the analysis of spherical neutron polarimetry, flipping ratio and Integrated Intensity data
arXiv: Strongly Correlated Electrons, 2018Co-Authors: Navid QureshiAbstract:Mag2Pol is a graphical user interface program which is devoted to the analysis of data from polarized neutron diffractometers with spherical polarization analysis. Nuclear and magnetic structure models can be introduced using space group symbols and individual symmetry operators, respectively, and viewed in an OpenGL widget. The program calculates nuclear/magnetic structure factors, flipping ratios and polarization matrices for magnetic Bragg reflections taking into account structural twins and magnetic domains. Spherical neutron polarimetry data can be analyzed by refining a magnetic structure model including magnetic domain populations in a least-squares fit and can also be correlated to an Integrated Intensity data set in a joint refinement. Further features are the simultaneous refinement of nuclear and magnetic structures with Integrated Intensity data and the analysis of flipping ratios either with tabulated magnetic form factors or using a multipole expansion of the magnetization density.
R. Samadi - One of the best experts on this subject based on the ideXlab platform.
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Stellar granulation as seen in disk-Integrated Intensity
Astronomy and Astrophysics - A&A, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. Kallinger, M. N. Lund, B. Mosser, A. BaglinAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:Context. Solar granulation has been known for a long time to be a surface manifestation of convection. The space-borne missions CoRoT and Kepler enable us to observe the signature of this phenomena in disk-Integrated Intensity on a large number of stars. Aims. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated wit h granulation obeys characteristic scaling relations. We thus ai med at providing simple theoretical modeling to reproduce these scaling relations, and subsequently at inferring the physical prop erties of granulation across the HR diagram. Methods. We developed a simple 1D theoretical model. The input parameters were extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involve d in the model were calibrated with solar observations. Two different prescriptions for representing the Fourier transform of the ti me-correlation of the eddy velocity were compared: a Lorentzian and an exponential form. Finally, we compared our theoretical pre diction with 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (hereafter ab initio approach). Results. Provided that the free parameters are appropriately adjust ed, our theoretical model reproduces the observed solar gra nulation spectrum quite satisfactorily ; the best agreement is obtai ned for an exponential form. Furthermore, our model results in granulation spectra that agree well with the ab initio approach using two 3D RHD models that are representative of the surface layers of an F-dwarf and a red-giant star. Conclusions. We have developed a theoretical model that satisfactory reproduces the solar granulation spectrum and gives results consistent with the ab initio approach. The model is used in a companion paper as theoretical framework for interpretating the observed scaling relations.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:The solar granulation is known for a long time to be a surface manifestation of convection. Thanks to the current space-borne missions CoRoT and Kepler, it is now possible to observe in disk-Integrated Intensity the signature of this phenomena in a growing number of stars. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aim at providing a simple theoretical modeling to reproduce these scaling relations and subsequently at inferring the physical properties of granulation properties across the HR diagram. We develop a simple 1D theoretical model that enable us to test any prescription concerning the time-correlation between granules. The input parameters of the model are extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model are calibrated with solar observations. Two different prescriptions for representing the eddy time-correlation in the Fourier space are compared: a Lorentzian and an exponential form. Finally, we compare our theoretical prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (ab-initio approach). Provided that the free parameters are appropriately adjusted, our theoretical model satisfactorily reproduces the shape and the amplitude of the observed solar granulation spectrum. The best agreement is obtained with an exponential form. Furthermore, our theoretical model results in granulation spectra that consistently agree with the these calculated on the basis of the ab-initio approach with two 3D RHD models. Comparison between theoretical granulation spectra calculated with the present model and high precision photometry measurements of stellar granulation is undertaken in a companion paper.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness fluctuations (sigma) associated with the granulation scale as a function of the peak frequency (nu_max) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model. For each PDS we derived tau_eff and sigma and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the Kepler measurements. We derive theoretical scaling relations for tau_eff and sigma, which show the same dependence on nu_max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (Ma) estimated at the photosphere. The theoretical scaling relations for tau_eff and sigma match the observations well on a global scale. Our modelling provides additional theoretical support for the observed variations of sigma and tau_eff with nu_m max. It also highlights the important role of Ma in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on Ma. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with Ma.
T. Kallinger - One of the best experts on this subject based on the ideXlab platform.
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Stellar granulation as seen in disk-Integrated Intensity
Astronomy and Astrophysics - A&A, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. Kallinger, M. N. Lund, B. Mosser, A. BaglinAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness fluctuations (sigma) associated with the granulation scale as a function of the peak frequency (nu_max) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model. For each PDS we derived tau_eff and sigma and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the Kepler measurements. We derive theoretical scaling relations for tau_eff and sigma, which show the same dependence on nu_max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (Ma) estimated at the photosphere. The theoretical scaling relations for tau_eff and sigma match the observations well on a global scale. Our modelling provides additional theoretical support for the observed variations of sigma and tau_eff with nu_m max. It also highlights the important role of Ma in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on Ma. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with Ma.
K. Belkacem - One of the best experts on this subject based on the ideXlab platform.
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Stellar granulation as seen in disk-Integrated Intensity
Astronomy and Astrophysics - A&A, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. Kallinger, M. N. Lund, B. Mosser, A. BaglinAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:Context. Solar granulation has been known for a long time to be a surface manifestation of convection. The space-borne missions CoRoT and Kepler enable us to observe the signature of this phenomena in disk-Integrated Intensity on a large number of stars. Aims. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated wit h granulation obeys characteristic scaling relations. We thus ai med at providing simple theoretical modeling to reproduce these scaling relations, and subsequently at inferring the physical prop erties of granulation across the HR diagram. Methods. We developed a simple 1D theoretical model. The input parameters were extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involve d in the model were calibrated with solar observations. Two different prescriptions for representing the Fourier transform of the ti me-correlation of the eddy velocity were compared: a Lorentzian and an exponential form. Finally, we compared our theoretical pre diction with 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (hereafter ab initio approach). Results. Provided that the free parameters are appropriately adjust ed, our theoretical model reproduces the observed solar gra nulation spectrum quite satisfactorily ; the best agreement is obtai ned for an exponential form. Furthermore, our model results in granulation spectra that agree well with the ab initio approach using two 3D RHD models that are representative of the surface layers of an F-dwarf and a red-giant star. Conclusions. We have developed a theoretical model that satisfactory reproduces the solar granulation spectrum and gives results consistent with the ab initio approach. The model is used in a companion paper as theoretical framework for interpretating the observed scaling relations.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
Astronomy and Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations. Aims. We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I. Methods. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets. Results. We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants. Conclusions. Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.
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stellar granulation as seen in disk Integrated Intensity i simplified theoretical modeling
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. LudwigAbstract:The solar granulation is known for a long time to be a surface manifestation of convection. Thanks to the current space-borne missions CoRoT and Kepler, it is now possible to observe in disk-Integrated Intensity the signature of this phenomena in a growing number of stars. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aim at providing a simple theoretical modeling to reproduce these scaling relations and subsequently at inferring the physical properties of granulation properties across the HR diagram. We develop a simple 1D theoretical model that enable us to test any prescription concerning the time-correlation between granules. The input parameters of the model are extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model are calibrated with solar observations. Two different prescriptions for representing the eddy time-correlation in the Fourier space are compared: a Lorentzian and an exponential form. Finally, we compare our theoretical prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (ab-initio approach). Provided that the free parameters are appropriately adjusted, our theoretical model satisfactorily reproduces the shape and the amplitude of the observed solar granulation spectrum. The best agreement is obtained with an exponential form. Furthermore, our theoretical model results in granulation spectra that consistently agree with the these calculated on the basis of the ab-initio approach with two 3D RHD models. Comparison between theoretical granulation spectra calculated with the present model and high precision photometry measurements of stellar granulation is undertaken in a companion paper.
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stellar granulation as seen in disk Integrated Intensity ii theoretical scaling relations compared with observations
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: R. Samadi, K. Belkacem, H.-g. Ludwig, E. Caffau, T. L. Campante, G. R. Davies, T. KallingerAbstract:A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (tau_eff) and the root-mean-square (rms) brightness fluctuations (sigma) associated with the granulation scale as a function of the peak frequency (nu_max) of the solar-like oscillations. We aim at providing a theoretical background to the observed scaling relations based on a model developed in the companion paper. We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-Integrated Intensity on the basis of the theoretical model. For each PDS we derived tau_eff and sigma and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the Kepler measurements. We derive theoretical scaling relations for tau_eff and sigma, which show the same dependence on nu_max as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (Ma) estimated at the photosphere. The theoretical scaling relations for tau_eff and sigma match the observations well on a global scale. Our modelling provides additional theoretical support for the observed variations of sigma and tau_eff with nu_m max. It also highlights the important role of Ma in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on Ma. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with Ma.