Enrico Corsaro - Academia.edu (original) (raw)

Papers by Enrico Corsaro

Astronomy & Astrophysics, 2015

Context. High-precision frequencies of acoustic modes in red giant stars are now available thanks... more Context. High-precision frequencies of acoustic modes in red giant stars are now available thanks to the long observing length and high-quality of the light curves provided by the NASA Kepler mission, thus allowing to probe the interior of evolved cool low-mass stars with unprecedented level of detail. Aims. We characterize the acoustic signature of the helium second ionization zone in a sample of 18 low-mass low-luminosity red giants by exploiting new mode frequency measurements derived from more than four years of Kepler observations. Methods. We analyze the second frequency differences of radial acoustic modes in all the stars of the sample by using the Bayesian code Diamonds. Results. We find clear acoustic glitches due to the signature of helium second ionization in all the stars of the sample. We measure the acoustic depth and the characteristic width of the acoustic glitches with a precision level on average around ∼2 % and ∼8 %, respectively. We find good agreement with theoretical predictions and existing measurements from the literature. Lastly, we derive the amplitude of the glitch signal at ν max for the second differences and for the frequencies with an average precision of ∼6 %, obtaining values in the range 0.14-0.24 µHz, and 0.08-0.33 µHz, respectively, which can be used to investigate the helium abundance in the stars.

EPJ Web of Conferences, 2015

In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we develop... more In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we developed a new code, termed Diamonds (high-DImensional And multi-MOdal NesteD Sampling), for fast Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional problems (like the peak bagging analysis of solar-like oscillations) and multi-modal problems (i.e. problems that show multiple solutions). We applied the code to the peak bagging analysis of solar-like oscillations observed in a challenging F-type star. By means of Diamonds one is able to detect the different backgrounds in the power spectrum of the star (e.g. stellar granulation and faculae activity) and to understand whether one or two oscillation peaks can be identified or not. In addition, we demonstrate a novel approach to peak bagging based on multimodality, which is able to reduce significantly the number of free parameters involved in the peak bagging model. This novel approach is therefore of great interest for possible future automatization of the entire analysis technique.

The Astrophysical Journal, 2016

EPJ Web of Conferences, 2015

The peak bagging analysis, namely the fitting and identification of single oscillation modes in s... more The peak bagging analysis, namely the fitting and identification of single oscillation modes in stars' power spectra, coupled to the very high-quality light curves of red giant stars observed by Kepler, can play a crucial role for studying stellar oscillations of different flavor with an unprecedented level of detail. A thorough study of stellar oscillations would thus allow for deeper testing of stellar structure models and new insights in stellar evolution theory. However, peak bagging inferences are in general very challenging problems due to the large number of observed oscillation modes, hence of free parameters that can be involved in the fitting models. Efficiency and robustness in performing the analysis is what may be needed to proceed further. For this purpose, we developed a new code implementing the Nested Sampling Monte Carlo (NSMC) algorithm, a powerful statistical method well suited for Bayesian analyses of complex problems. In this talk we show the peak bagging of a sample of high signal-to-noise red giant stars by exploiting recent Kepler datasets and a new criterion for the detection of an oscillation mode based on the computation of the Bayesian evidence. Preliminary results for frequencies and lifetimes for single oscillation modes, together with acoustic glitches, are therefore presented.

Astronomy and Astrophysics

The currently available Kepler light curves contain an outstanding amount of information but a de... more The currently available Kepler light curves contain an outstanding amount of information but a detailed analysis of the individual oscillation modes in the observed power spectra, also known as peak bagging, is computationally demanding and challenging to perform on a large number of targets. Our intent is to perform for the first time a peak bagging analysis on a sample of 19 low-mass low-luminosity red giants observed by Kepler for more than four years. This allows us to provide high-quality asteroseismic measurements that can be exploited for an intensive testing of the physics used in stellar structure models, stellar evolution and pulsation codes, as well as for refining existing asteroseismic scaling relations in the red giant branch regime. For this purpose, powerful and sophisticated analysis tools are needed. We exploit the Bayesian code Diamonds, using an efficient nested sampling Monte Carlo algorithm, to perform both a fast fitting of the individual oscillation modes and...

Astronomy & Astrophysics, 2015

Context. Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude... more Context. Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red-giant stars were targets of spectroscopic asteroseismic observing projects. Aims. We search for solar-like oscillations in the two bright red-giant stars γ Psc and θ 1 Tau from time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power ν max and the mean large frequency separation ∆ν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. Methods. The radial velocities of γ Psc and θ 1 Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach the accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired VLTI observations of γ Psc for an independent estimate of the radius. Also 22 days of observations of θ 1 Tau with the MOST-satellite were analysed. Results. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 µHz and a large frequency separation of 4.1±0.1 µHz. θ 1 Tau exhibits oscillation power around 90 µHz, with a large frequency separation of 6.9±0.2 µHz. Scaling relations indicate that γ Psc is a star of about 1 M ⊙ and 10 R ⊙ . θ 1 Tau appears to be a massive star of about 2.7 M ⊙ and 10 R ⊙ . The radial velocities of both stars were found to be modulated on time scales much longer than the oscillation periods.

Astronomy and Astrophysics

In asteroseismology an important diagnostic of the evolutionary status of a star is the small fre... more In asteroseismology an important diagnostic of the evolutionary status of a star is the small frequency separation which is sensitive to the gradient of the mean molecular weight in the stellar interior. It is thus interesting to discuss the classical age-activity relations in terms of this quantity. Moreover, as the photospheric magnetic field tends to suppress the amplitudes of acoustic oscillations, it is important to quantify the importance of this effect by considering various activity indicators. We propose a new class of age-activity relations that connects the Mt. Wilson SSS index and the average scatter in the light curve with the small frequency separation and the amplitude of the p-mode oscillations. We used a Bayesian inference to compute the posterior probability of various empirical laws for a sample of 19 solar-like active stars observed by the Kepler telescope. We demonstrate the presence of a clear correlation between the Mt. Wilson SSS index and the relative age of...

Astronomy & Astrophysics, 2014

We present photometric spot modeling of the nearly four-year long light-curve of the Kepler targe... more We present photometric spot modeling of the nearly four-year long light-curve of the Kepler target KIC 5955122 in terms of persisting dark circular surface features. With a Bayesian technique, we produced a plausible surface map that shows dozens of small spots. After some artifacts are removed, the residuals are at ±0.16 mmag. The shortest rotational period found is P = 16.4 ± 0.2 days. The equator-to-pole extrapolated differential rotation is 0.25 ± 0.02 rad/d. The spots are roughly half as bright as the unperturbed stellar photosphere. Spot latitudes are restricted to the zone ±60 • latitude. There is no indication for any near-pole spots. In addition, the p-mode pulsations enabled us to determine the evolutionary status of the star, the extension of the convective zone, and its radius and mass. We discuss the possibility that the clear signature of active regions in the light curve of the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the base of the convection zone. In particular, we argue that this star has evolved from an active to a quiet status during the Q0-Q16 period of observation, and we predict, according to our dynamo model, that the characteristic activity cycle is of the order of the solar one.

In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we develop... more In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we developed a new code, termed Diamonds (high-DImensional And multi-MOdal NesteD Sampling), for fast Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional problems (like the peak bagging analysis of solar-like oscillations) and multi-modal problems (i.e. problems that show multiple solutions). We applied the code to the peak bagging analysis of solar-like oscillations observed in a challenging F-type star. By means of Diamonds one is able to detect the different backgrounds in the power spectrum of the star (e.g. stellar granulation and faculae activity) and to understand whether one or two oscillation peaks can be identified or not. In addition, we demonstrate a novel approach to peak bagging based on multimodality, which is able to reduce significantly the number of free parameters involved in the peak bagging model. This novel approach is therefore of great interest for possible future automatization of the entire analysis technique.

Context. Thanks to the advent of the space-based missions CoRoT and NASA's Kepler, the asteroseis... more Context. Thanks to the advent of the space-based missions CoRoT and NASA's Kepler, the asteroseismology of solar-like oscillations is nowadays at the base of our understanding about stellar physics. The Kepler spacecraft especially, is releasing excellent photometric observations of more than three years length in very high duty cycle, which contain a large amount of information that has not yet been investigated. Aims. In order to exploit the full potential of Kepler light curves, sophisticated and robust analysis tools are now required more than ever. Characterizing single stars with an unprecedented level of accuracy and subsequently analyze stellar populations in detail will be fundamental to further constrain stellar structure and evolutionary models. Methods. We developed a new code, termed Diamonds, for Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional and multimodal problems. A detailed description of the features implemented in the code is given, with a focus on the novelties and differences with respect to other existing methods based on NSMC. Diamonds is then tested on the bright F8 V star KIC 9139163, a challenging target for peak bagging analysis due to its large number of oscillation peaks observed, coupled to the blending occurring between = 2, 0 peaks, and the strong stellar background signal. We further strain the performance of the approach by adopting a 1147.5 days-long Kepler light curve, accounting for more than 840,000 data bins in the power spectrum of the star. Results. The Diamonds code is able to provide robust results for the peak bagging analysis of KIC 9139163, while at the same time preserving a considerable computational efficiency for identifying the solution. We test the detection of different astrophysical backgrounds in the star and provide a criterion based on the Bayesian evidence for assessing in detail the peak significance of the detected oscillations. We present results for 59 individual oscillation frequencies, amplitudes and linewidths and provide a detailed comparison to the existing values in the literature, from which significant deviations are found when a different background is used. Lastly, we successfully demonstrate an innovative approach to peak bagging that exploits the capability of Diamonds to sample multimodal distributions, of great potential for possible future automatization of the analysis technique.

Astronomy & Astrophysics, 2014

Context. In asteroseismology an important diagnostic of the evolutionary status of a star is the ... more Context. In asteroseismology an important diagnostic of the evolutionary status of a star is the small frequency separation which is sensitive to the gradient of the mean molecular weight in the stellar interior. It is thus interesting to discuss the classical ageactivity relations in terms of this quantity. Moreover, as the photospheric magnetic field tends to suppress the amplitudes of acoustic oscillations, it is important to quantify the importance of this effect by considering various activity indicators. Aims. We propose a new class of age-activity relations that connects the Mt. Wilson S index and the average scatter in the light curve with the small frequency separation and the amplitude of the p-mode oscillations. Methods. We used a Bayesian inference to compute the posterior probability of various empirical laws for a sample of 19 solar-like active stars observed by the Kepler telescope. Results. We demonstrate the presence of a clear correlation between the Mt. Wilson S index and the relative age of the stars as indicated by the small frequency separation, as well as an anti-correlation between the S index and the oscillation amplitudes. We argue that the average activity level of the stars shows a stronger correlation with the small frequency separation than with the absolute age that is often considered in the literature. Conclusions. The phenomenological laws discovered in this paper have the potential to become new important diagnostics to link stellar evolution theory with the dynamics of global magnetic fields. In particular we argue that the relation between the Mt. Wilson S index and the oscillation amplitudes is in good agreement with the findings of direct numerical simulations of magneto-convection.

Astrophysical Journal, 2011

We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct ... more We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct a statistical study of the impact of stellar surface activity on the detectability of solar-like oscillations. We find that the number of stars with detected oscillations falls significantly with increasing levels of activity. The results present strong evidence for the impact of magnetic activity on the properties of near-surface convection in the stars, which appears to inhibit the amplitudes of the stochastically excited, intrinsically damped solar-like oscillations.

The Astrophysical Journal, 2011

We analyze the photometric short-cadence data obtained with the Kepler mission during the first 8... more We analyze the photometric short-cadence data obtained with the Kepler mission during the first 8 months of observations of two solar-type stars of spectral types G and F: KIC 11395018 and KIC 11234888, respectively, the latter having a lower signal-to-noise ratio (S/N) compared with the former. We estimate global parameters of the acoustic (p) modes such as the average large and small frequency separations, the frequency of the maximum of the p-mode envelope, and the average line width of the acoustic modes. We were able to identify and to measure 22 p-mode frequencies for the first star and 16 for the second one even though the S/N of these stars are rather low. We also derive some information about the stellar rotation periods from the analyses of the low-frequency parts of the power spectral densities. A model-independent estimation of the mean density, mass, and radius is obtained using the scaling laws. We emphasize the importance of continued observations for the stars with low S/N for an improved characterization of the oscillation modes. Our results offer a preview of what will be possible for many stars with the long data sets obtained during the remainder of the mission.

The Astrophysical Journal, 2012

We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 68... more We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters δν 02 , δν 01 , and , which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars and possibly a difference in structure of the red clump stars, from our measurements of the small separations δν 02 and δν 01 . Ensembleéchelle diagrams and upper limits to the linewidths of = 0 modes as a function of Δν of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the = 0 ridge width and the T eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.

The Astrophysical Journal, 2014

We present a novel method for estimating lower-limit surface gravities (log g) of Kepler targets ... more We present a novel method for estimating lower-limit surface gravities (log g) of Kepler targets whose data do not allow the detection of solar-like oscillations. The method is tested using an ensemble of solar-type stars observed in the context of the Kepler Asteroseismic Science Consortium. We then proceed to estimate lower-limit log g for a cohort of Kepler solar-type planet-candidate host stars with no detected

The Astrophysical Journal, 2011

We have analyzed solar-like oscillations in ∼1700 stars observed by the Kepler Mission, spanning ... more We have analyzed solar-like oscillations in ∼1700 stars observed by the Kepler Mission, spanning from the main-sequence to the red clump. Using evolutionary models, we test asteroseismic scaling relations for the frequency of maximum power (ν max ), the large frequency separation (∆ν) and oscillation amplitudes. We show that the difference of the ∆ν-ν max relation for unevolved and evolved stars can be explained by different distributions in effective temperature and stellar mass, in agreement with what is expected from scaling relations. For oscillation amplitudes, we show that neither (L/M ) s scaling nor the revised scaling relation by Kjeldsen & Bedding is accurate for red-giant stars, and demonstrate that a revised scaling relation with a separate luminosity-mass dependence can be used to calculate amplitudes from the main-sequence to red-giants to a precision of ∼25%. The residuals show an offset particularly for unevolved stars, suggesting that an additional physical dependency is necessary to fully reproduce the observed amplitudes. We investigate correlations between amplitudes and stellar activity, and find evidence that the effect of amplitude suppression is most pronounced for subgiant stars. Finally, we test the location of the cool edge of the instability strip in the Hertzsprung-Russell diagram using solar-like oscillations and find the detections in the hottest stars compatible with a domain of hybrid stochastically excited and opacity driven pulsation.

Science, 2011

In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data... more In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy.

Astronomy & Astrophysics, 2015

Context. High-precision frequencies of acoustic modes in red giant stars are now available thanks... more Context. High-precision frequencies of acoustic modes in red giant stars are now available thanks to the long observing length and high-quality of the light curves provided by the NASA Kepler mission, thus allowing to probe the interior of evolved cool low-mass stars with unprecedented level of detail. Aims. We characterize the acoustic signature of the helium second ionization zone in a sample of 18 low-mass low-luminosity red giants by exploiting new mode frequency measurements derived from more than four years of Kepler observations. Methods. We analyze the second frequency differences of radial acoustic modes in all the stars of the sample by using the Bayesian code Diamonds. Results. We find clear acoustic glitches due to the signature of helium second ionization in all the stars of the sample. We measure the acoustic depth and the characteristic width of the acoustic glitches with a precision level on average around ∼2 % and ∼8 %, respectively. We find good agreement with theoretical predictions and existing measurements from the literature. Lastly, we derive the amplitude of the glitch signal at ν max for the second differences and for the frequencies with an average precision of ∼6 %, obtaining values in the range 0.14-0.24 µHz, and 0.08-0.33 µHz, respectively, which can be used to investigate the helium abundance in the stars.

EPJ Web of Conferences, 2015

In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we develop... more In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we developed a new code, termed Diamonds (high-DImensional And multi-MOdal NesteD Sampling), for fast Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional problems (like the peak bagging analysis of solar-like oscillations) and multi-modal problems (i.e. problems that show multiple solutions). We applied the code to the peak bagging analysis of solar-like oscillations observed in a challenging F-type star. By means of Diamonds one is able to detect the different backgrounds in the power spectrum of the star (e.g. stellar granulation and faculae activity) and to understand whether one or two oscillation peaks can be identified or not. In addition, we demonstrate a novel approach to peak bagging based on multimodality, which is able to reduce significantly the number of free parameters involved in the peak bagging model. This novel approach is therefore of great interest for possible future automatization of the entire analysis technique.

The Astrophysical Journal, 2016

EPJ Web of Conferences, 2015

The peak bagging analysis, namely the fitting and identification of single oscillation modes in s... more The peak bagging analysis, namely the fitting and identification of single oscillation modes in stars' power spectra, coupled to the very high-quality light curves of red giant stars observed by Kepler, can play a crucial role for studying stellar oscillations of different flavor with an unprecedented level of detail. A thorough study of stellar oscillations would thus allow for deeper testing of stellar structure models and new insights in stellar evolution theory. However, peak bagging inferences are in general very challenging problems due to the large number of observed oscillation modes, hence of free parameters that can be involved in the fitting models. Efficiency and robustness in performing the analysis is what may be needed to proceed further. For this purpose, we developed a new code implementing the Nested Sampling Monte Carlo (NSMC) algorithm, a powerful statistical method well suited for Bayesian analyses of complex problems. In this talk we show the peak bagging of a sample of high signal-to-noise red giant stars by exploiting recent Kepler datasets and a new criterion for the detection of an oscillation mode based on the computation of the Bayesian evidence. Preliminary results for frequencies and lifetimes for single oscillation modes, together with acoustic glitches, are therefore presented.

Astronomy and Astrophysics

The currently available Kepler light curves contain an outstanding amount of information but a de... more The currently available Kepler light curves contain an outstanding amount of information but a detailed analysis of the individual oscillation modes in the observed power spectra, also known as peak bagging, is computationally demanding and challenging to perform on a large number of targets. Our intent is to perform for the first time a peak bagging analysis on a sample of 19 low-mass low-luminosity red giants observed by Kepler for more than four years. This allows us to provide high-quality asteroseismic measurements that can be exploited for an intensive testing of the physics used in stellar structure models, stellar evolution and pulsation codes, as well as for refining existing asteroseismic scaling relations in the red giant branch regime. For this purpose, powerful and sophisticated analysis tools are needed. We exploit the Bayesian code Diamonds, using an efficient nested sampling Monte Carlo algorithm, to perform both a fast fitting of the individual oscillation modes and...

Astronomy & Astrophysics, 2015

Context. Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude... more Context. Red giants are evolved stars which exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red-giant stars were targets of spectroscopic asteroseismic observing projects. Aims. We search for solar-like oscillations in the two bright red-giant stars γ Psc and θ 1 Tau from time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power ν max and the mean large frequency separation ∆ν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. Methods. The radial velocities of γ Psc and θ 1 Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach the accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired VLTI observations of γ Psc for an independent estimate of the radius. Also 22 days of observations of θ 1 Tau with the MOST-satellite were analysed. Results. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 µHz and a large frequency separation of 4.1±0.1 µHz. θ 1 Tau exhibits oscillation power around 90 µHz, with a large frequency separation of 6.9±0.2 µHz. Scaling relations indicate that γ Psc is a star of about 1 M ⊙ and 10 R ⊙ . θ 1 Tau appears to be a massive star of about 2.7 M ⊙ and 10 R ⊙ . The radial velocities of both stars were found to be modulated on time scales much longer than the oscillation periods.

Astronomy and Astrophysics

In asteroseismology an important diagnostic of the evolutionary status of a star is the small fre... more In asteroseismology an important diagnostic of the evolutionary status of a star is the small frequency separation which is sensitive to the gradient of the mean molecular weight in the stellar interior. It is thus interesting to discuss the classical age-activity relations in terms of this quantity. Moreover, as the photospheric magnetic field tends to suppress the amplitudes of acoustic oscillations, it is important to quantify the importance of this effect by considering various activity indicators. We propose a new class of age-activity relations that connects the Mt. Wilson SSS index and the average scatter in the light curve with the small frequency separation and the amplitude of the p-mode oscillations. We used a Bayesian inference to compute the posterior probability of various empirical laws for a sample of 19 solar-like active stars observed by the Kepler telescope. We demonstrate the presence of a clear correlation between the Mt. Wilson SSS index and the relative age of...

Astronomy & Astrophysics, 2014

We present photometric spot modeling of the nearly four-year long light-curve of the Kepler targe... more We present photometric spot modeling of the nearly four-year long light-curve of the Kepler target KIC 5955122 in terms of persisting dark circular surface features. With a Bayesian technique, we produced a plausible surface map that shows dozens of small spots. After some artifacts are removed, the residuals are at ±0.16 mmag. The shortest rotational period found is P = 16.4 ± 0.2 days. The equator-to-pole extrapolated differential rotation is 0.25 ± 0.02 rad/d. The spots are roughly half as bright as the unperturbed stellar photosphere. Spot latitudes are restricted to the zone ±60 • latitude. There is no indication for any near-pole spots. In addition, the p-mode pulsations enabled us to determine the evolutionary status of the star, the extension of the convective zone, and its radius and mass. We discuss the possibility that the clear signature of active regions in the light curve of the F9IV star KIC 5955122 is produced by a flux-transport dynamo action at the base of the convection zone. In particular, we argue that this star has evolved from an active to a quiet status during the Q0-Q16 period of observation, and we predict, according to our dynamo model, that the characteristic activity cycle is of the order of the solar one.

In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we develop... more In the context of high-quality asteroseismic data provided by the NASA Kepler Mission, we developed a new code, termed Diamonds (high-DImensional And multi-MOdal NesteD Sampling), for fast Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional problems (like the peak bagging analysis of solar-like oscillations) and multi-modal problems (i.e. problems that show multiple solutions). We applied the code to the peak bagging analysis of solar-like oscillations observed in a challenging F-type star. By means of Diamonds one is able to detect the different backgrounds in the power spectrum of the star (e.g. stellar granulation and faculae activity) and to understand whether one or two oscillation peaks can be identified or not. In addition, we demonstrate a novel approach to peak bagging based on multimodality, which is able to reduce significantly the number of free parameters involved in the peak bagging model. This novel approach is therefore of great interest for possible future automatization of the entire analysis technique.

Context. Thanks to the advent of the space-based missions CoRoT and NASA's Kepler, the asteroseis... more Context. Thanks to the advent of the space-based missions CoRoT and NASA's Kepler, the asteroseismology of solar-like oscillations is nowadays at the base of our understanding about stellar physics. The Kepler spacecraft especially, is releasing excellent photometric observations of more than three years length in very high duty cycle, which contain a large amount of information that has not yet been investigated. Aims. In order to exploit the full potential of Kepler light curves, sophisticated and robust analysis tools are now required more than ever. Characterizing single stars with an unprecedented level of accuracy and subsequently analyze stellar populations in detail will be fundamental to further constrain stellar structure and evolutionary models. Methods. We developed a new code, termed Diamonds, for Bayesian parameter estimation and model comparison by means of the Nested Sampling Monte Carlo (NSMC) algorithm, an efficient and powerful method very suitable for high-dimensional and multimodal problems. A detailed description of the features implemented in the code is given, with a focus on the novelties and differences with respect to other existing methods based on NSMC. Diamonds is then tested on the bright F8 V star KIC 9139163, a challenging target for peak bagging analysis due to its large number of oscillation peaks observed, coupled to the blending occurring between = 2, 0 peaks, and the strong stellar background signal. We further strain the performance of the approach by adopting a 1147.5 days-long Kepler light curve, accounting for more than 840,000 data bins in the power spectrum of the star. Results. The Diamonds code is able to provide robust results for the peak bagging analysis of KIC 9139163, while at the same time preserving a considerable computational efficiency for identifying the solution. We test the detection of different astrophysical backgrounds in the star and provide a criterion based on the Bayesian evidence for assessing in detail the peak significance of the detected oscillations. We present results for 59 individual oscillation frequencies, amplitudes and linewidths and provide a detailed comparison to the existing values in the literature, from which significant deviations are found when a different background is used. Lastly, we successfully demonstrate an innovative approach to peak bagging that exploits the capability of Diamonds to sample multimodal distributions, of great potential for possible future automatization of the analysis technique.

Astronomy & Astrophysics, 2014

Context. In asteroseismology an important diagnostic of the evolutionary status of a star is the ... more Context. In asteroseismology an important diagnostic of the evolutionary status of a star is the small frequency separation which is sensitive to the gradient of the mean molecular weight in the stellar interior. It is thus interesting to discuss the classical ageactivity relations in terms of this quantity. Moreover, as the photospheric magnetic field tends to suppress the amplitudes of acoustic oscillations, it is important to quantify the importance of this effect by considering various activity indicators. Aims. We propose a new class of age-activity relations that connects the Mt. Wilson S index and the average scatter in the light curve with the small frequency separation and the amplitude of the p-mode oscillations. Methods. We used a Bayesian inference to compute the posterior probability of various empirical laws for a sample of 19 solar-like active stars observed by the Kepler telescope. Results. We demonstrate the presence of a clear correlation between the Mt. Wilson S index and the relative age of the stars as indicated by the small frequency separation, as well as an anti-correlation between the S index and the oscillation amplitudes. We argue that the average activity level of the stars shows a stronger correlation with the small frequency separation than with the absolute age that is often considered in the literature. Conclusions. The phenomenological laws discovered in this paper have the potential to become new important diagnostics to link stellar evolution theory with the dynamics of global magnetic fields. In particular we argue that the relation between the Mt. Wilson S index and the oscillation amplitudes is in good agreement with the findings of direct numerical simulations of magneto-convection.

Astrophysical Journal, 2011

We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct ... more We use photometric observations of solar-type stars, made by the NASA Kepler Mission, to conduct a statistical study of the impact of stellar surface activity on the detectability of solar-like oscillations. We find that the number of stars with detected oscillations falls significantly with increasing levels of activity. The results present strong evidence for the impact of magnetic activity on the properties of near-surface convection in the stars, which appears to inhibit the amplitudes of the stochastically excited, intrinsically damped solar-like oscillations.

The Astrophysical Journal, 2011

We analyze the photometric short-cadence data obtained with the Kepler mission during the first 8... more We analyze the photometric short-cadence data obtained with the Kepler mission during the first 8 months of observations of two solar-type stars of spectral types G and F: KIC 11395018 and KIC 11234888, respectively, the latter having a lower signal-to-noise ratio (S/N) compared with the former. We estimate global parameters of the acoustic (p) modes such as the average large and small frequency separations, the frequency of the maximum of the p-mode envelope, and the average line width of the acoustic modes. We were able to identify and to measure 22 p-mode frequencies for the first star and 16 for the second one even though the S/N of these stars are rather low. We also derive some information about the stellar rotation periods from the analyses of the low-frequency parts of the power spectral densities. A model-independent estimation of the mean density, mass, and radius is obtained using the scaling laws. We emphasize the importance of continued observations for the stars with low S/N for an improved characterization of the oscillation modes. Our results offer a preview of what will be possible for many stars with the long data sets obtained during the remainder of the mission.

The Astrophysical Journal, 2012

We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 68... more We studied solar-like oscillations in 115 red giants in the three open clusters, NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters δν 02 , δν 01 , and , which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars and possibly a difference in structure of the red clump stars, from our measurements of the small separations δν 02 and δν 01 . Ensembleéchelle diagrams and upper limits to the linewidths of = 0 modes as a function of Δν of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the = 0 ridge width and the T eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.

The Astrophysical Journal, 2014

We present a novel method for estimating lower-limit surface gravities (log g) of Kepler targets ... more We present a novel method for estimating lower-limit surface gravities (log g) of Kepler targets whose data do not allow the detection of solar-like oscillations. The method is tested using an ensemble of solar-type stars observed in the context of the Kepler Asteroseismic Science Consortium. We then proceed to estimate lower-limit log g for a cohort of Kepler solar-type planet-candidate host stars with no detected

The Astrophysical Journal, 2011

We have analyzed solar-like oscillations in ∼1700 stars observed by the Kepler Mission, spanning ... more We have analyzed solar-like oscillations in ∼1700 stars observed by the Kepler Mission, spanning from the main-sequence to the red clump. Using evolutionary models, we test asteroseismic scaling relations for the frequency of maximum power (ν max ), the large frequency separation (∆ν) and oscillation amplitudes. We show that the difference of the ∆ν-ν max relation for unevolved and evolved stars can be explained by different distributions in effective temperature and stellar mass, in agreement with what is expected from scaling relations. For oscillation amplitudes, we show that neither (L/M ) s scaling nor the revised scaling relation by Kjeldsen & Bedding is accurate for red-giant stars, and demonstrate that a revised scaling relation with a separate luminosity-mass dependence can be used to calculate amplitudes from the main-sequence to red-giants to a precision of ∼25%. The residuals show an offset particularly for unevolved stars, suggesting that an additional physical dependency is necessary to fully reproduce the observed amplitudes. We investigate correlations between amplitudes and stellar activity, and find evidence that the effect of amplitude suppression is most pronounced for subgiant stars. Finally, we test the location of the cool edge of the instability strip in the Hertzsprung-Russell diagram using solar-like oscillations and find the detections in the hottest stars compatible with a domain of hybrid stochastically excited and opacity driven pulsation.

Science, 2011

In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data... more In addition to its search for extrasolar planets, the NASA Kepler mission provides exquisite data on stellar oscillations. We report the detections of oscillations in 500 solar-type stars in the Kepler field of view, an ensemble that is large enough to allow statistical studies of intrinsic stellar properties (such as mass, radius, and age) and to test theories of stellar evolution. We find that the distribution of observed masses of these stars shows intriguing differences to predictions from models of synthetic stellar populations in the Galaxy.