The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
Dennis Stello - One of the best experts on this subject based on the ideXlab platform.
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asteroseismology can reveal strong internal magnetic fields in Red Giant Stars
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: R A Garcia, Jim Fuller, Matteo Cantiello, Dennis Stello, Lars BildstenAbstract:Internal stellar magnetic fields are inaccessible to direct observations and little is known about their amplitude, geometry and evolution. We demonstrate that strong magnetic fields in the cores of Red Giant Stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, which arises from a magnetic greenhouse effect that scatters and traps oscillation mode energy within the core of the star. The Kepler satellite has observed a few dozen Red Giants with depressed dipole modes which we interpret as Stars with strongly magnetized cores. We find field strengths larger than $\sim\! 10^5 \,{\rm G}$ may produce the observed depression, and in one case we infer a minimum core field strength of $\approx \! \! 10^7 \,{\rm G}$.
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Asteroseismology can reveal strong internal magnetic fields in Red Giant Stars.
Science, 2015Co-Authors: Jim Fuller, Rafael A. García, Matteo Cantiello, Dennis Stello, Lars BildstenAbstract:Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores of Red Giant Stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen Red Giants with depressed dipole modes, which we interpret as Stars with strongly magnetized cores. We find that field strengths larger than ~105 gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈107 gauss.
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Multisite campaign on the open cluster M67 – II. Evidence for solar-like oscillations in Red Giant Stars
Monthly Notices of the Royal Astronomical Society, 2007Co-Authors: Dennis Stello, H. Bruntt, Hans Kjeldsen, Timothy R. Bedding, T. Arentoft, R. L. Gilliland, J. Nuspl, S.-l. Kim, Y. B. Kang, Jae-rim KooAbstract:Measuring solar-like oscillations in an ensemble of Stars i n a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field Stars. The most ambitious attempt to pursue these prospects was by Gilliland et al. (1993) who targeted 11 turn-off Stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (< 20µmag) to obtain unambiguous detections. Like Gilliland et al. (1993) we also aim at detecting solar-like oscillations in M67, but we target Red Giant Stars with expected amplitudes in the range 50‐500µmag and periods of 1 to 8 hours. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compaRed with simulations and with estimated properties of the stellar oscillations. Noise le vels in the Fourier spectra as low as 27µmag are obtained for single sites, while the combined data reach 19µmag, making this the best photometric time series of an ensemble of Red Giant Stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of Stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequ ency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.
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multisite campaign on the open cluster m67 ii evidence for solar like oscillations in Red Giant Stars
Monthly Notices of the Royal Astronomical Society, 2007Co-Authors: Dennis Stello, H. Bruntt, Hans Kjeldsen, Timothy R. Bedding, T. ArentoftAbstract:Measuring solar-like oscillations in an ensemble of Stars i n a cluster, holds promise for testing stellar structure and evolution more stringently than just fitting parameters to single field Stars. The most ambitious attempt to pursue these prospects was by Gilliland et al. (1993) who targeted 11 turn-off Stars in the open cluster M67 (NGC 2682), but the oscillation amplitudes were too small (< 20µmag) to obtain unambiguous detections. Like Gilliland et al. (1993) we also aim at detecting solar-like oscillations in M67, but we target Red Giant Stars with expected amplitudes in the range 50‐500µmag and periods of 1 to 8 hours. We analyse our recently published photometry measurements, obtained during a six-week multisite campaign using nine telescopes around the world. The observations are compaRed with simulations and with estimated properties of the stellar oscillations. Noise le vels in the Fourier spectra as low as 27µmag are obtained for single sites, while the combined data reach 19µmag, making this the best photometric time series of an ensemble of Red Giant Stars. These data enable us to make the first test of the scaling relations (used to estimate frequency and amplitude) with an homogeneous ensemble of Stars. The detected excess power is consistent with the expected signal from stellar oscillations, both in terms of its frequ ency range and amplitude. However, our results are limited by apparent high levels of non-white noise, which cannot be clearly separated from the stellar signal.
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oscillation mode lifetimes in xi hydrae will strong mode damping limit asteroseismology of Red Giant Stars
Astronomy and Astrophysics, 2006Co-Authors: Dennis Stello, Hans Kjeldsen, Timothy R. Bedding, Derek L. BuzasiAbstract:We introduce a new method to measure frequency separations and mode lifetimes of stochastically excited and damped oscillations, so-called solar-like oscillations. Our method shows that velocity data of the Red Giant star ξ Hya (Frandsen et al. 2002) support a large frequency separation between modes of roughly 7 µ Hz. We also conclude that the data are consistent with a mode lifetime of 2 days, which is so short relative to its pulsation period that none of the observed frequencies are unambiguous. Hence, we argue that the maximum asteroseismic output that can be obtained from these data is an average large frequency separation, the oscillation amplitude and the average mode lifetime. However, the significant discrepancy between the theoretical calculations of the mode lifetime (Houdek & Gough 2002) and our result based on the observations of ξ Hya, implies that Red Giant Stars can help us better understand the damping and driving mechanisms of solar-like p-modes by convection.
Martin Asplund - One of the best experts on this subject based on the ideXlab platform.
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3d lte spectral line formation with scattering in Red Giant Stars
arXiv: Solar and Stellar Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:We investigate the effects of coherent isotropic continuum scattering on the formation of spectral lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model atmospheres of Red Giant Stars. Continuum flux levels, spectral line profiles and curves of growth for different species are compaRed with calculations that treat scattering as absorption. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE lines: continua gain more brightness than line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the line profiles, leading to larger abundance corrections for stronger lines, which reach -0.5 dex at 3000 A for Fe II lines in 3D with excitation potential 2 eV at [Fe/H]=-3.0. The corrections are less severe for low-excitation lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. 1D calculations exhibit similar scattering abundance corrections for weak lines, but those for strong lines are generally smaller compaRed to 3D models and depend on the choice of microturbulence.
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3D LTE spectral line formation with scattering in Red Giant Stars
Astronomy & Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:Aims. We investigate the effects of coherent isotropic continuum scattering on the formation of spectral lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and ID hydrostatic model atmospheres of Red Giant Stars. Methods. Detailed radiative transfer with coherent and isotropic continuum scattering is computed for 3D hydrodynamical and 1D hydrostatic models of late-type stellar atmospheres using the SCATE code. Opacities are computed in LTE, while a coherent and isotropic scattering term is added to the continuum source function. We investigate the effects of scattering by comparing continuum flux levels, spectral line profiles and curves of growth for different species with calculations that treat scattering as absorption. Results. Rayleigh scattering is the dominant source of scattering opacity in the continuum of Red Giant Stars. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of λ ≲ 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE lines: continua gain more brightness than line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the line profiles, leading to larger abundance corrections for stronger lines, which reach -0.5 dex at 3000 A for Fe II lines in 3D with excitation potential χ = 2 eV at [Fe/H] = -3.0. The corrections are less severe for low-excitation lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. ID calculations exhibit similar scattering abundance corrections for weak lines, but those for strong lines are generally smaller compaRed to 3D models and depend on the choice of microturbulence. Conclusions. Continuum scattering should be taken into account for computing realistic spectral line profiles at wavelengths λ ≲ 4000 A in metal-poor Giant Stars. Profile shapes are strongly affected by velocity fields and horizontal inhomogeneities, requiring a treatment based on 3D hydrodynamical rather than classical 1D hydrostatic model atmospheres.
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Three-dimensional hydrodynamical simulations of Red Giant Stars: semi-global models for interpreting interferometric observations
Astronomy & Astrophysics, 2010Co-Authors: Andrea Chiavassa, Remo Collet, Luca Casagrande, Martin AsplundAbstract:Context. Theoretical pRedictions from models of Red Giant branch Stars are a valuable tool for various applications in astrophysics ranging from galactic chemical evolution to studies of exoplanetary systems. Aims. We use the radiative transfer code Optim3D and realistic 3D radiative-hydrodynamical (RHD) surface convection simulations of Red Giants to explore the impact of granulation on interferometric observables. We assess how 3D simulations of surface convection can be validated against observations. Methods. We computed intensity maps for the 3D simulation snapshots in two filters, the optical at 5000 ± 300 A and the K band 2.14±0.26 μm FLUOR filter, corresponding to the wavelength-range of instruments mounted on the CHARA interferometer. From the intensity maps, we constructed images of the stellar disks and account for center-to-limb variations. We then derived interferometric visibility amplitudes and phases. We study their behavior with position angle and wavelength, and compare them with CHARA observations of the Red Giant star HD 214868. Results. We provide average limb darkening coefficients for different metallicities and wavelengths ranges. We explain prospects for detecting and characterizing granulation and center-to-limb variations of Red Giant Stars with today’s interferometers. Regarding interferometric observables, we find that the effect of convective-related surface structures depends on metallicity and surface gravity. We provide theoretical closure-phases that should be incorporated into the analysis of Red Giant planet companion closure phase signals. We estimate 3D−1D corrections to stellar radii determination: 3D models are ∼3.5% smaller to ∼1% larger in the optical than 1D, and roughly 0. 5t o 1.5% smaller in the infraRed. Even if these corrections are small, they are needed to properly set the zero point of effective temperature scale derived by interferometry and to strengthen the confidence of existing Red Giant catalogs of calibrating Stars for interferometry. Finally, we show that our RHD simulations provide an excellent fit to the Red Giant HD 214868 even though more observations are needed at higher spatial frequencies and shorter wavelength.
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Chemical similarities between the Galactic bulge and local thick disk Red Giant Stars: analysis from optical data
Proceedings of the International Astronomical Union, 2009Co-Authors: A. Alves-brito, Jorge Meléndez, Martin AsplundAbstract:AbstractThe Galactic structure and composition remain as one of the greatest open problems in modern astrophysics. We show here that there are chemical similarities between the Galactic bulge and local thick disk Red Giant Stars. This finding puts strong constraints on the IMF, SFR and chemical enrichment timescale of the bulge and thick disk. Our results are based upon a detailed elemental abundance analysis of 80 high S/N and high resolution optical spectra of Giant Stars, in the range −1.5 < [Fe/H] < +0.5.
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Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars
Astronomy & Astrophysics, 2008Co-Authors: Jorge Meléndez, Cristina Chiappini, A. Alves-brito, Martin Asplund, Katia Cunha, Beatriz Barbuy, Michael S. Bessell, Kenneth C. Freeman, Ivan Ramirez, Verne V. SmithAbstract:Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge Red Giant Stars. We also completed an identical study of local thin disk, thick disk and halo Giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infraRed spectra of 19 bulge Giants and 49 comparison Giants in the solar neighborhood were acquiRed with Gemini/Phoenix. All Stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk Stars ensuRed that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf Stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shaRed, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.
Verne V. Smith - One of the best experts on this subject based on the ideXlab platform.
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Lithium in Red Giant Stars: Constraining non-standard mixing with large surveys in the Gaia era
Astronomy & Astrophysics, 2020Co-Authors: Corinne Charbonnel, Verne V. Smith, Nadège Lagarde, Gérard Jasniewicz, Pierre North, Matthew D. Shetrone, J. Krugler Hollek, R. Smiljanic, A. Palacios, G. OttoniAbstract:Lithium is extensively known to be a good tracer of non-standard mixing processes occurring in stellar interiors. We present the results of a new large Lithium survey in Red Giant Stars and combine it with surveys from the literature to probe the impact of rotation-induced mixing and thermohaline double-diffusive instability along stellar evolution. We determined the surface Li abundance for a sample of 829 Giant Stars with accurate Gaia parallaxes for a large sub-sample (810 Stars) complemented with accurate Hipparcos parallaxes (19 Stars). The spectra of our sample of northern and southern Giant Stars were obtained in three ground-based observatories (OHP, ESO-La Silla, Mc Donald). We determined the atmospheric parameters (Teff, log(g), [Fe/H]), and the Li abundance. We used Gaia parallaxes and photometry to determine the luminosity of our objects and we estimated the mass and evolution status of each sample star with a maximum-likelihood technique using stellar evolution models computed with the STAREVOL code. We compaRed the observed Li behaviour with pRedictions from stellar models, including rotation and thermohaline mixing. The same approach was used for Stars from selected Li surveys from the literature. Rotation-induced mixing accounts nicely for the lithium behaviour in Stars warmer than about 4200K, independently of the mass domain. For Stars with masses lower than 2Msun thermohaline mixing leads to further Li depletion below the Teff of the RGB bump (about 4000K), and on the early AGB, as observed. Depending on the definition we adopt, we find between 0.8 and 2.2% of Li-rich Giants in our new sample. Gaia puts a new spin on the understanding of mixing processes in Stars, and our study confirms the importance of rotation-induced processes and of thermohaline mixing. However asteroseismology is requiRed to definitively pinpoint the actual evolution status of Li-rich Giants.
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OBSERVABLE SIGNATURES OF PLANET ACCRETION IN Red Giant Stars. I. RAPID ROTATION AND LIGHT ELEMENT REPLENISHMENT
The Astrophysical Journal, 2012Co-Authors: Joleen K. Carlberg, Verne V. Smith, Katia Cunha, Steven R. MajewskiAbstract:The orbital angular momentum of a close-orbiting Giant planet can be sufficiently large that, if transferRed to the envelope of the host star during the Red Giant branch (RGB) evolution, it can spin-up the star's rotation to unusually large speeds. This spin-up mechanism is one possible explanation for the rapid rotators detected among the population of generally slow-rotating Red Giant Stars. These rapid rotators thus comprise a unique stellar sample suitable for searching for signatures of planet accretion in the form of unusual stellar abundances due to the dissemination of the accreted planet in the stellar envelope. In this study, we look for signatures of replenishment in the Li abundances and (to a lesser extent) 12C/13C, which are both normally loweRed during RGB evolution. Accurate abundances were measuRed from high signal-to-noise echelle spectra for samples of both slow and rapid rotator Red Giant Stars. We find that the rapid rotators are on average enriched in lithium compaRed to the slow rotators, but both groups of Stars have identical distributions of 12C/13C within our measurement precision. Both of these abundance results are consistent with the accretion of planets of only a few Jupiter masses. We also explore alternative scenarios for understanding the most Li-rich Stars in our sample—particularly Li regeneration during various stages of stellar evolution. Finally, we find that our stellar samples show non-standard abundances even at early RGB stages, suggesting that initial protostellar Li abundances and 12C/13C may be more variable than originally thought.
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Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars
Astronomy & Astrophysics, 2008Co-Authors: Jorge Meléndez, Cristina Chiappini, A. Alves-brito, Martin Asplund, Katia Cunha, Beatriz Barbuy, Michael S. Bessell, Kenneth C. Freeman, Ivan Ramirez, Verne V. SmithAbstract:Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge Red Giant Stars. We also completed an identical study of local thin disk, thick disk and halo Giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infraRed spectra of 19 bulge Giants and 49 comparison Giants in the solar neighborhood were acquiRed with Gemini/Phoenix. All Stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk Stars ensuRed that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf Stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shaRed, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.
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Chemical similarities between Galactic bulge and local thick disk Red Giant Stars
Astronomy & Astrophysics, 2008Co-Authors: Jorge Meléndez, Cristina Chiappini, A. Alves-brito, Martin Asplund, Katia Cunha, Beatriz Barbuy, Michael S. Bessell, Kenneth C. Freeman, Ivan Ramirez, Verne V. SmithAbstract:The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge Red Giant Stars. We also completed an identical study of local thin disk, thick disk and halo Giants to establish the chemical differences and similarities between the various populations. High-resolution infraRed spectra of 19 bulge Giants and 49 comparison Giants in the solar neighborhood were acquiRed with Gemini/Phoenix. All Stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk Stars ensuRed that systematic errors were minimized. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf Stars in the solar neighborhood. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shaRed, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.
Andrea Miglio - One of the best experts on this subject based on the ideXlab platform.
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Insights from the APOKASC Determination of the Evolutionary State of Red-Giant Stars by consolidation of different methods
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Y. Elsworth, Saskia Hekker, Andrea Miglio, Benoit Mosser, Jennifer A. Johnson, Thomas Kallinger, Marc H. Pinsonneault, Marc Hon, James S. Kuszlewicz, Aldo SerenelliAbstract:The internal working of low-mass Stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal structure of Stars and allows for the determination of the evolutionary state -- i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from Red-Giant Stars in a combination (known as APOKASC) of asteroseismology (from the \textit{Kepler} mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual Stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these Stars based on comparison of observables with stellar evolutionary models (`grid-based modeling') and as a training set for machine-learning and other data-driven methods of evolutionary state determination
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Insights from the APOKASC determination of the evolutionary state of Red-Giant Stars by consolidation of different methods
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: Y. Elsworth, Saskia Hekker, Andrea Miglio, Benoit Mosser, Jennifer A. Johnson, Thomas Kallinger, Marc H. Pinsonneault, Marc Hon, James S. Kuszlewicz, Aldo SerenelliAbstract:ABSTRACT The internal working of low-mass Stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal structure of Stars and allows for the determination of the evolutionary state – i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from Red-Giant Stars in a combination (known as APOKASC) of asteroseismology (from the Kepler mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual Stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these Stars based on comparison of observables with stellar evolutionary models (‘grid-based modelling’) and as a training set for machine-learning and other data-driven methods of evolutionary state determination.
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Testing the cores of first ascent Red Giant Stars using the period spacing of g modes
Monthly Notices of the Royal Astronomical Society: Letters, 2016Co-Authors: Nadège Lagarde, Diego Bossini, Andrea Miglio, M. Vrard, Benoit MosserAbstract:In the context of the determination of stellar properties using asteroseismology, we study the influence of rotation and convective-core overshooting on the properties of Red-Giant Stars. We used models in order to investigate the effects of these mechanisms on the asymptotic period spacing of gravity modes ($\Delta \Pi_1$) of Red-Giant Stars that ignite He burning in degenerate conditions (M$\lesssim$2.0 M$_{\odot}$). We also compare the pRedictions of these models with Kepler observations. For a given $\Delta\nu$, $\Delta \Pi_1$ depends not only on the stellar mass, but also on mixing processes that can affect the structure of the core. We find that in the case of more evolved Red-Giant-branch (RGB) Stars and regardless of the transport processes occurring in their interiors, the observed $\Delta \Pi_1$ can provide information as to their stellar luminosity, within ~10-20%. In general, the trends of $\Delta \Pi_1$ with respect to mass and metallicity that are observed in Kepler Red-Giant Stars are well reproduced by the models.
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Asteroseismology of Red-Giant Stars as a novel approach in the search for gravitational waves
Proceedings of the International Astronomical Union, 2015Co-Authors: Tiago L. Campante, Andrea Miglio, Diego Bossini, Ilídio Lopes, William J. ChaplinAbstract:Stars are massive resonators that may be used as gravitational-wave (GW) detectors with isotropic sensitivity. New insights on stellar physics are being made possible by asteroseismology, the study of Stars by the observation of their natural oscillations. The continuous monitoring of oscillation modes in Stars of different masses and sizes (e.g., as carried out by NASA’s Kepler mission) opens the possibility of surveying the local Universe for GW radiation. Red-Giant Stars are of particular interest in this regard. Since the mean separation between Red Giants in open clusters is small (a few light years), this can in principle be used to look for the same GW imprint on the oscillation modes of different Stars as a GW propagates across the cluster. Furthermore, the frequency range probed by oscillations in Red Giants complements the capabilities of the planned eLISA space interferometer. We propose asteroseismology of Red Giants as a novel approach in the search for gravitational waves.
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Prospects for asteroseismic inference on the envelope helium abundance in Red Giant Stars
Monthly Notices of the Royal Astronomical Society, 2014Co-Authors: Anne-marie Broomhall, William J. Chaplin, Y. Elsworth, Andrea Miglio, Josefina Montalbán, Patrick Eggenberger, Richard Scuflaire, Paolo Ventura, G. A. VernerAbstract:Regions of rapid variation in the internal structure of a star are often referRed to as acoustic glitches since they create a characteristic periodic signature in the frequencies of p modes. Here we examine the localized disturbance arising from the helium second ionization zone in Red Giant branch and clump Stars. More specifically, we determine how accurately and precisely the parameters of the ionization zone can be obtained from the oscillation frequencies of stellar models. We use models produced by three different generation codes that not only cover a wide range of stages of evolution along the Red Giant phase but also incorporate different initial helium abundances. To study the acoustic glitch caused by the second ionization zone of helium we have determined the second differences in frequencies of modes with the same angular degree, l, and then we fit the periodic function described by Houdek & Gough to the second differences. We discuss the conditions under which such fits robustly and accurately determine the acoustic radius of the second ionization zone of helium. When the frequency of maximum amplitude of the p-mode oscillations was greater than 40 μHz a robust value for the radius of the ionization zone was recoveRed for the majority of models. The determined radii of the ionization zones as inferRed from the mode frequencies were found to be coincident with the local maximum in the first adiabatic exponent described by the models, which is associated with the outer edge of the second ionization zone of helium. Finally, we consider whether this method can be used to distinguish Stars with different helium abundances. Although a definite trend in the amplitude of the signal is observed any distinction would be difficult unless the Stars come from populations with vastly different helium abundances or the uncertainties associated with the fitted parameters can be Reduced. However, application of our methodology could be useful for distinguishing between different populations of Red Giant Stars in globular clusters, where distinct populations with very different helium abundances have been observed.
Derek L. Buzasi - One of the best experts on this subject based on the ideXlab platform.
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oscillation mode lifetimes in xi hydrae will strong mode damping limit asteroseismology of Red Giant Stars
Astronomy and Astrophysics, 2006Co-Authors: Dennis Stello, Hans Kjeldsen, Timothy R. Bedding, Derek L. BuzasiAbstract:We introduce a new method to measure frequency separations and mode lifetimes of stochastically excited and damped oscillations, so-called solar-like oscillations. Our method shows that velocity data of the Red Giant star ξ Hya (Frandsen et al. 2002) support a large frequency separation between modes of roughly 7 µ Hz. We also conclude that the data are consistent with a mode lifetime of 2 days, which is so short relative to its pulsation period that none of the observed frequencies are unambiguous. Hence, we argue that the maximum asteroseismic output that can be obtained from these data is an average large frequency separation, the oscillation amplitude and the average mode lifetime. However, the significant discrepancy between the theoretical calculations of the mode lifetime (Houdek & Gough 2002) and our result based on the observations of ξ Hya, implies that Red Giant Stars can help us better understand the damping and driving mechanisms of solar-like p-modes by convection.
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Oscillation mode lifetimes in $\xi\,$Hydrae: will strong mode damping limit asteroseismology of Red Giant Stars?
Astronomy & Astrophysics, 2006Co-Authors: Dennis Stello, Hans Kjeldsen, Timothy R. Bedding, Derek L. BuzasiAbstract:We introduce a new method to measure frequency separations and mode lifetimes of stochastically excited and damped oscillations, so-called solar-like oscillations. Our method shows that velocity data of the Red Giant star ξ Hya (Frandsen et al. 2002) support a large frequency separation between modes of roughly 7 µ Hz. We also conclude that the data are consistent with a mode lifetime of 2 days, which is so short relative to its pulsation period that none of the observed frequencies are unambiguous. Hence, we argue that the maximum asteroseismic output that can be obtained from these data is an average large frequency separation, the oscillation amplitude and the average mode lifetime. However, the significant discrepancy between the theoretical calculations of the mode lifetime (Houdek & Gough 2002) and our result based on the observations of ξ Hya, implies that Red Giant Stars can help us better understand the damping and driving mechanisms of solar-like p-modes by convection.
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Oscillation mode lifetimes in ksi Hydrae: Will strong mode damping limit asteroseismology of Red Giant Stars?
Astronomy & Astrophysics, 2006Co-Authors: Dennis Stello, Timothy R. Bedding, H. Kjeldsen, Derek L. BuzasiAbstract:We introduce a new method to measure frequency separations and mode lifetimes of stochastically excited and damped oscillations, so-called solar-like oscillations. Our method shows that velocity data of the Red Giant star ksi Hya (Frandsen et al. 2002) support a large frequency separation between modes of roughly 7 microHz. We also conclude that the data are consistent with a mode lifetime of 2 days, which is so short relative to its pulsation period that none of the observed frequencies are unambiguous. Hence, we argue that the maximum asteroseismic output that can be obtained from these data is an average large frequency separation, the oscillation amplitude and the average mode lifetime. However, the significant discrepancy between the theoretical calculations of the mode lifetime (Houdek & Gough 2002) and our result based on the observations of ksi Hya, implies that Red Giant Stars can help us better understand the damping and driving mechanisms of solar-like p-modes by convection.Comment: 9 pages, 11 figures, accepted for publication in A&
