Line-profile variations of stochastically excited oscillations in four evolved stars
TL;DR: In this paper, line-profile variations in stochastically excited solar-like oscillations of four evolved stars were used to derive the azimuthal order of the observed mode and the surface rotational frequency.
Abstract: Context. Since solar-like oscillations were first detected in red-giant stars, the presence of non-radial oscillation modes has been debated. Spectroscopic line-profile analysis was used in the first attempt to perform mode identification, which revealed that non-radial modes are observable. Despite the fact that the presence of non-radial modes could be confirmed, the degree or azimuthal order could not be uniquely identified. Here we present an improvement to this first spectroscopic line-profile analysis. Aims. We aim to study line-profile variations in stochastically excited solar-like oscillations of four evolved stars to derive the azimuthal order of the observed mode and the surface rotational frequency. Methods. Spectroscopic line-profile analysis is applied to cross-correlation functions, using the Fourier parameter fit method on the amplitude and phase distributions across the profiles. Results. For four evolved stars, β Hydri (G2IV), � Ophiuchi (G9.5III), η Serpentis (K0III) and δ Eridani (K0IV) the line-profile variations reveal the azimuthal order of the oscillations with an accuracy of ±1. Furthermore, our analysis reveals the projected rotational velocity and the inclination angle. From these parameters we obtain the surface rotational frequency. Conclusions. We conclude that line-profile variations in cross-correlation functions behave differently for different frequencies and provide additional information about the surface rotational frequency and azimuthal order.
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TL;DR: In this article, a physically motivated model of the time-steady mass loss rates of cool main-sequence stars and evolved giants was developed to model the energy flux of magnetohydrodynamic turbulence from a subsurface convection zone to its eventual dissipation and escape through open magnetic flux tubes.
Abstract: The basic mechanisms responsible for producing winds from cool, late-type stars are still largely unknown. We take inspiration from recent progress in understanding solar wind acceleration to develop a physically motivated model of the time-steady mass loss rates of cool main-sequence stars and evolved giants. This model follows the energy flux of magnetohydrodynamic turbulence from a subsurface convection zone to its eventual dissipation and escape through open magnetic flux tubes. We show how Alfven waves and turbulence can produce winds in either a hot corona or a cool extended chromosphere, and we specify the conditions that determine whether or not coronal heating occurs. These models do not utilize arbitrary normalization factors, but instead predict the mass loss rate directly from a star's fundamental properties. We take account of stellar magnetic activity by extending standard age-activity-rotation indicators to include the evolution of the filling factor of strong photospheric magnetic fields. We compared the predicted mass loss rates with observed values for 47 stars and found significantly better agreement than was obtained from the popular scaling laws of Reimers, Schroder, and Cuntz. The algorithm used to compute cool-star mass loss rates is provided as a self-contained and efficient computer code. We anticipate that the results from this kind of model can be incorporated straightforwardly into stellar evolution calculations and population synthesis techniques.
323 citations
TL;DR: In this article, the internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales, and the authors anticipate that more information is still hidden in the data.
Abstract: The internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales. Long near-uninterrupted high-precision photometric timeseries observations from dedicated space missions such as CoRoT and Kepler have provided seismic inferences of the global and internal properties of a large number of evolved stars, including red giants. These inferences are confronted with predictions from theoretical models to improve our understanding of stellar structure and evolution. Our knowledge and understanding of red giants have indeed increased tremendously using these seismic inferences, and we anticipate that more information is still hidden in the data. Unraveling this will further improve our understanding of stellar evolution. This will also have significant impact on our knowledge of the Milky Way Galaxy as well as on exo-planet host stars. The latter is important for our understanding of the formation and structure of planetary systems.
142 citations
TL;DR: In this article, the authors used high-resolution spectra of 430 stars with spectral types in the range O4 - B9 (all luminosity classes) compiled in the framework of the IACOB project to provide new empirical clues about macroturbulent spectral line broadening in O- and B-type stars.
Abstract: Context. The term macroturbulent broadening is commonly used to refer to a certain type of non-rotational broadening affecting the spectral line profiles of O- and B-type stars. It has been proposed to be a spectroscopic signature of the presence of stellar oscillations;however, we still lack a definitive confirmation of this hypothesis. Aims. We aim to provide new empirical clues about macroturbulent spectral line broadening in O- and B-type stars to evaluate its physical origin. Methods. We used high-resolution spectra of 430 stars with spectral types in the range O4 - B9 (all luminosity classes) compiled in the framework of the IACOB project. We characterized the line broadening of adequate diagnostic metal lines using a combined Fourier transform and goodness-of-fit technique. We performed a quantitative spectroscopic analysis of the whole sample using automatic tools coupled with a huge grid of fast wind models to determine their effective temperatures and gravities. We also incorporated quantitative information about line asymmetries into our observational description of the characteristics of the line profiles, and performed a comparison of the shape and type of line-profile variability found in a small sample of O stars and B supergiants with still undefined pulsational properties and B main-sequence stars with variable line profiles owing to a well-identified type of stellar oscillations or to the presence of spots in the stellar surface. Results. We present a homogeneous and statistically significant overview of the (single snapshot) line-broadening properties of stars in the whole O and B star domain. We find empirical evidence of the existence of various types of non-rotational broadening agents acting in the realm of massive stars. Even though all these additional sources of line-broadening could be quoted and quantified as a macroturbulent broadening from a practical point of view, their physical origin can be different. Contrarily to the early-to late-B dwarfs and giants, which present a mixture of cases in terms of line-profile shape and variability, the whole O- type and B supergiant domain (or, roughly speaking, stars with M-ZAMS greater than or similar to 15 M-circle dot) is fully dominated by stars with a remarkable non-rotational broadening component and very similar profiles (including type of variability). We provide some examples illustrating how this observational dataset can be used to evaluate scenarios aimed at explaining the existence of sources of non-rotational broadening in massive stars.
105 citations
Cites background from "Line-profile variations of stochast..."
...In the same line, Briquet et al. (2004) demonstrated that the moments of a line-profile allow to distinguish line asymmetry due to surface spots versus oscillations and Hekker & Aerts (2010) showed the utility of using the moments even in the case of stochastically excited oscillations....
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TL;DR: In this article, the internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales, and the authors anticipate that more information is still hidden in the data.
Abstract: The internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales. Long near-uninterrupted high-precision photometric timeseries observations from dedicated space missions such as CoRoT and Kepler have provided seismic inferences of the global and internal properties of a large number of evolved stars, including red giants. These inferences are confronted with predictions from theoretical models to improve our understanding of stellar structure and evolution. Our knowledge and understanding of red giants have indeed increased tremendously using these seismic inferences, and we anticipate that more information is still hidden in the data. Unraveling this will further improve our understanding of stellar evolution. This will also have significant impact on our knowledge of the Milky Way Galaxy as well as on exo-planet host stars. The latter is important for our understanding of the formation and structure of planetary systems.
71 citations
Cites background or methods from "Line-profile variations of stochast..."
...A first mention of dense and/or irregular frequency patterns in the solar-like oscillations of red giants was made by Hekker et al. (2009). These authors already indicated that this could be explained by the fact that the observed oscillations are influenced by their behaviour in both the p-mode and g-mode cavity....
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...This technique has mainly been developed for stars with coherent oscillations (Zima et al, 2004; Zima, 2008), but also proved useful to reveal for the first time the presence of non-radial oscillations inred giants (Hekker et al, 2006; Hekker and Aerts, 2010)....
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...Nevertheless, Hekker et al. (2013) showed that there is good agreement between ν obtained from the power spectrum of the power spectrum (e....
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...2004; Zima 2008), but also proved useful for providing evidence for the presence of non-radial oscillations in red giants (Hekker et al. 2006; Hekker and Aerts 2010)....
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References
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TL;DR: In this paper, a fair agreement between estimated velocity amplitudes and the values quoted in the ESO press release was found using a stochastic excitation model, and the theoretical results were compared with the recent measurements by an international group of astronomers.
Abstract: Amplitudes of stochastically excited oscillations of models of ξ Hydrae (HR4450) are presented. The theoretical results are compared with the recent measurements by an international group of astronomers announced in the ESO press release 10/02. Using a stochastic excitation model we find fair agreement between estimated velocity amplitudes and the values quoted in the ESO press release.
80 citations
"Line-profile variations of stochast..." refers methods in this paper
...Based on theory of more luminous red giants (Dziembowski et al. 2001), the detected frequenciesof these stars were interpreted as radial modes and the stars were modelled based on this assumption (Houdek & Gough 2002: De Ridder et al. 2006)....
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77 citations
"Line-profile variations of stochast..." refers methods in this paper
...To identify the best fit to the data,χ2 values have been computed, as foreseen in FAMIAS....
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...With FAMIAS (Zima 2008), we compute frequencies from the first moment (Aerts et al. 1992) for the new CCFs, similar to what was done by Hekker et al. (2006) for the uncorrected CCFs....
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...SH wants to thank Maarten Mooij for useful discussions and Wolfgang Zima for his help with FAMIAS....
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...Send offprint requests to: S. Hekker, email: saskia@bison.ph.bham.ac.uk ⋆ The software package FAMIAS developed in the framework of the FP6 European Coordination Action HELAS (http://www.helas-eu.org) has been used in this research....
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...So forβ Hydri we find a surface rotational frequency ranging between 3.6µHz and 5.5µHz for inclination angles between 72◦ and 38◦ and a projected rotational velocity of 4.3 km s−1 as obtained with FAMIAS....
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TL;DR: In this article, the Fourier parameter fit method was used to identify stellar radial and non-radial pulsation modes based on the high-resolution time-series spectroscopy of absorption-line profiles.
Abstract: We present the Fourier parameter fit method, a new method for spectroscopically identifying stellar radial and non-radial pulsation modes based on the high-resolution time-series spectroscopy of absorption-line profiles. In contrast to previous methods this one permits a quantification of the statistical significance of the computed solutions. The application of genetic algorithms in seeking solutions makes it possible to search through a large parameter space. The mode identification is carried out by minimizing chi-square, using the observed amplitude and phase across the line profile and their modeled counterparts. Computations of the theoretical line profiles are based on a stellar displacement field, which is described as superposition of spherical harmonics and that includes the first order effects of the Coriolis force. We made numerical tests of the method on a grid of different mono- and multi-mode models for 0 <= l <= 4 in order to explore its capabilities and limitations. Our results show that whereas the azimuthal order m can be unambiguously identified for low-order modes, the error of l is in the range of pm 1. The value of m can be determined with higher precision than with other spectroscopic mode identification methods. Improved values for the inclination can be obtained from the analysis of non-axisymmetric pulsation modes. The new method is ideally suited to intermediatley rotating Delta Scuti and Beta Cephei stars.
60 citations
TL;DR: The G9.5 giant ϵ Oph as mentioned in this paper shows evidence of radial p-mode pulsations in both radial velocity and luminosity, and the observed frequencies are matched to both radial and non-radial modes in the best model fit.
Abstract: The G9.5 giant ϵ Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10–20 d. The best fit model itself, constrained only by the observed frequencies, lies within ± of ϵ Oph's position in the HR-diagram and the interferometrically determined radius.
60 citations
TL;DR: In this article, the G9.5 giant eps Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity, and the observed frequencies are matched to both radial and non-radial modes in the best model fit.
Abstract: The G9.5 giant eps Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10-20d. The best fit model itself, constrained only by the observed frequencies, lies within 1 sigma of eps Oph's position in the HR-diagram and the interferometrically determined radius.
57 citations