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Journal ArticleDOI

Line-profile variations of stochastically excited oscillations in four evolved stars

01 Jun 2010-Astronomy and Astrophysics (EDP Sciences)-Vol. 515, pp 1-9

AbstractContext. 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.

Topics: Solar-like oscillations (63%), Stars (51%), Amplitude (50%)

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Citations
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Journal ArticleDOI
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.

318 citations


Journal ArticleDOI
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.

122 citations


Journal ArticleDOI
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.

98 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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Journal ArticleDOI
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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Journal ArticleDOI
Abstract: Comparing models and data of pulsating stars is a powerful way to understand the stellar structure better.\beta Hyi is an evolved solar-type pulsator with mixed modes in its frequency spectrum, making it very interesting for asteroseismic studies.The goal of this work is to search for the best model of the solar-type star \beta Hyi, based on up-to-date non-seismic and seismic data.We present a revised list of frequencies for 33 modes, which we produced by analysing the power spectrum of the published observations again using a new weighting scheme that minimises the daily sidelobes.We ran several grids of evolutionary models with different input parameters and different physics, using the stellar evolutionary code ASTEC.For the models that are inside the observed error box of \beta Hyi, we computed their frequencies with the pulsation code ADIPLS.We used two approaches to find the model that oscillates with the frequencies that are closest to the observed frequencies of \beta Hyi:(i)we assume that the best model is the one that reproduces the star's interior based on the radial oscillation frequencies alone, to which we have applied the correction for the near-surface effects;(ii)we assume that the best model is the one that produces the lowest value of the chi-square (\chi2), i.e. that minimises the difference between the observed frequencies of all available modes and the model predictions, after all model frequencies are corrected for near-surface effects.We show that after applying this correction to the frequencies of the best models, we can reproduce the observed modes well, including the mixed modes.The model that gives the lowest value of the \chi2 is a post-main-sequence model with a mass of 1.04 MSun and a metallicity slightly lower than that of the Sun.Our results underscore the importance of having individual frequencies to constrain the properties of the stellar model.

57 citations


Cites background from "Line-profile variations of stochast..."

  • ...From their analysis, Hekker & Aerts (2010) attempted to determine the inclination angle,i, of β Hyi, suggesting a value of 55± 17◦ for this star....

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  • ...More recently, Bruntt et al. (2010) foundvsini = 2.7 ± 0.6 km s−1, and Hekker & Aerts (2010) from spectroscopic line-profile analysis, foundvsini = 4.3 km s−1....

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References
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"Line-profile variations of stochast..." refers methods in this paper

  • ...Hekker et al. (2006) used the INTER-TACOS (INTERpreter for the Treatment, the Analysis and the COrrelation of Spectra)software package developed at Geneva Observatory (Baranne et al. 1996) to compute cross-correlation functions (CCFs) of theobserved spectra....

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Journal ArticleDOI
Abstract: We present stellar parameters and metallicities, obtained from a detailed spectroscopic analysis, for a large sample of 98 stars known to be orbited by planetary mass companions (almost all known targets), as well as for a volume-limited sample of 41 stars not known to host any planet. For most of the stars the stellar parameters are revised versions of the ones presented in our previous work. However, we also present parameters for 18 stars with planets not previously published, and a compilation of stellar parameters for the remaining 4 planet-hosts for which we could not obtain a spectrum. A comparison of our stellar parameters with values of T eff , log g, and [Fe/H] available in the literature shows a remarkable agreement. In particular, our spectroscopic log g values are now very close to trigonometric log g estimates based on Hipparcos parallaxes. The derived [Fe/H] values are then used to confirm the previously known result that planets are more prevalent around metal-rich stars. Furthermore, we confirm that the frequency of planets is a strongly rising function of the stellar metallicity, at least for stars with [Fe/H] > 0. While only about 3% of the solar metallicity stars in the CORALIE planet search sample were found to be orbited by a planet, this number increases to more than 25% for stars with [Fe/H] above +0.3. Curiously, our results also suggest that these percentages might remain relatively constant for values of [Fe/H] lower than about solar, increasing then linearly with the mass fraction of heavy elements. These results are discussed in the context of the theories of planetary formation.

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Journal ArticleDOI

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"Line-profile variations of stochast..." refers methods in this paper

  • ...This proof is based on the determination of the harmonic degrees, using the asymptotic relationfor high-order low-degree modes (Tassoul 1980)....

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  • ...For β Hydri the degrees of the modes with highest power have been identified using the asymptotic relation (Tassoul 1980) by Bedding et al. (2007)....

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