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Atomic Diffusion

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N. Piskunov – One of the best experts on this subject based on the ideXlab platform.

  • Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397
    The Astrophysical Journal, 2007
    Co-Authors: Andreas Korn, Olivier Richard, Lyudmila Mashonkina, Frank Grundahl, B. Gustafsson, P. Barklem, R. Collet, N. Piskunov

    Abstract:

    We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Stromgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of Atomic Diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that Atomic Diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of Atomic Diffusion in old metal-poor stars are also discussed.

  • Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397
    The Astrophysical Journal, 2007
    Co-Authors: A. J. Korn, O. Richard, B. Gustafsson, F. Grundahl, P. Barklem, R. Collet, L. Mashonkina, N. Piskunov

    Abstract:

    We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H]~-2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Strömgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of Atomic Diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that Atomic Diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of Atomic Diffusion in old metal-poor stars are also discussed. Based on observations carried out at the European Southern Observatory (ESO), Paranal, Chile, under program ID 075.D-0125(A).

  • New Abundances for Old Stars – Atomic Diffusion at Work in NGC 6397
    The Messenger, 2006
    Co-Authors: Andreas Korn, O. Richard, Lyudmila Mashonkina, F. Grundahl, P. Barklem, R. Collet, N. Piskunov, B. Gustafsson

    Abstract:

    A homogeneous spectroscopic analysis of unevolved and evolved stars in the metal-poor globular cluster NGC 6397 with FLAMES-UVES reveals systematic trends of stellar surface abundances that are likely caused by Atomic Diffusion. This finding helps to understand, among other issues, why the lithium abundances of old halo stars are significantly lower than the abundance found to be produced shortly after the Big Bang.

O. Richard – One of the best experts on this subject based on the ideXlab platform.

  • Hydrodynamical Instabilities Induced by Atomic Diffusion in F and A Stars
    , 2017
    Co-Authors: M. Deal, O. Richard, S. Vauclair

    Abstract:

    Atomic Diffusion, including the effect of radiative accelerations on individual elements, leads to strong variations of the chemical composition inside the stars as well as of the evolution of surface abundances. Accumulation in specific element layers, which are the main contributors to the local opacity, leads to hydrodynamical instabilities that modify the internal stellar structure and surface abundances. We show that the variations of the chemical composition induced by Atomic Diffusion can lead to an increase of the Rosseland opacity in some layers by up to a factor of three, and have important effects on the internal stellar mixing. These processes can no longer be neglected in stellar evolution models as the observations are more and more precise, especially with the future space missions TESS and PLATO.

  • Atomic Diffusion and lithium processing in old metal poor stars
    , 2012
    Co-Authors: O. Richard

    Abstract:

    Due to nuclear reactions within their interior, stars are the generators of the chemical evolution of galaxies. In this context, it is important to recognize that the chemical abundances observed in stellar surfaces are most often not the original ones. In fact, due to Atomic Diffusion, the Atomic species in stars move either downwards or upwards. For solar metallicity stars, it is well accepted within the international community that stellar modeling should include Atomic Diffusion. However it is not yet considered as a standard process in lower metallicity stars. In this paper I will present the effect of Atomic Diffusion on surface abundances and on abundance profile in metal poor stars. The effect of the initial metallicity will also be addressed.

  • Lithium in globular clusters: significant systematics. Atomic Diffusion, the temperature scale, and pollution in NGC 6397
    , 2012
    Co-Authors: T. Nordlander, O. Richard, A. J. Korn, K. Lind

    Abstract:

    We describe our latest investigation of stars in NGC 6397, as an update to Korn et al. There, effects of Atomic Diffusion were found to significantly affect surface abundances of lithium, as well as iron and magnesium. In this updated analysis, we have adopted a new, hotter, temperature scale. Additionally, we now analyze lithium in NLTE, have added chromium in NLTE to the analysis, and corrected magnesium abundances for effects of cluster-internal pollution. We determine abundances in six chemical elements. Significant evolutionary variations on the 3 sigma level are still identified in the iron abundances. Variations in all six elements match model predictions only if Atomic Diffusion processes are moderated by turbulent mixing of moderate efficiency. We infer an intial cluster lithium abundance abundance Li = 2.57 ± 0.10, which agrees with WMAP-calibrated BBN predictions, abundance Li = 2.71 ± 0.06.

Olivier Richard – One of the best experts on this subject based on the ideXlab platform.

  • Important consequences of Atomic Diffusion inside main-sequence stars: opacities, extra-mixing, oscillations
    EPJ Web of Conferences, 2017
    Co-Authors: M. Deal, Olivier Richard, S. Vauclair, M. Catelan, W. Gieren

    Abstract:

    Atomic Diffusion, including the effects of radiative accelerations on individual elements, leads to important variations of the chemical composition inside stars. The accumulation of important elements in specific layers leads to a local increase of the average opacity and to hydrodynamic instabilities that modify the internal stellar structure. This can also have important consequences for asteroseismology.

  • Asteroseismology of the exoplanet-host F-type star 94 Ceti: Impact of Atomic Diffusion on the stellar parameters
    Astronomy and Astrophysics, 2017
    Co-Authors: M. Deal, S. Vauclair, M. E. Escobar, G. Vauclair, A. Hui-bon-hoa, Olivier Richard

    Abstract:

    Context. A precision of the order of one percent is needed on the parameters of exoplanet-hosts stars to correctly characterise the planets themselves. This can be achieved by asteroseismology. It is important in this context to test the influence of introducing Atomic Diffusion with radiative accelerations in the models upon the derived parameters. In this paper, we begin this study with the case of the star 94 Ceti A.Aims. We aim to perform a complete asteroseismic analysis of the exoplanet-host F-type star 94 Ceti A, from the first radial-velocity observations with the High Accuracy Radial velocity Planet Searcher (HARPS) up to the final computed best models. We also aim to test the influence of Atomic Diffusion, including radiative accelerations, upon the computed frequencies and on the determined stellar parameters. 94 Ceti A is hot enough to suffer these effects. We also aim to test the effect of including a complete atmosphere in the stellar models.Methods. The radial velocity observations were performed with HARPS in 2007. The low degree modes were derived and identified using classical methods and compared with the results obtained from stellar models computed with the Toulouse Geneva Evolution Code (TGEC).Results. We obtained precise parameters for the star 94 Ceti A. We showed that including Atomic Diffusion with radiative accelerations can modify the age by a few percent, whereas adding a complete atmosphere does not change the results by more than one percent. Conclusions. Atomic Diffusion including radiative accelerations should be taken into account in all the computations of stellar models with masses larger than 1.3 M ⊙ in order to determine accurate parameters for observed stars.

  • Atomic Diffusion and mixing in old stars VI. The lithium content of M30
    Astronomy and Astrophysics, 2016
    Co-Authors: Pieter Gruyters, Olivier Richard, Martin Asplund, Karin Lind, Frank Grundahl, Luca Casagrande, Corinne Charbonnel, Antonino Milone, Francesca Primas, Andreas Korn

    Abstract:

    Context. The prediction of the Planck-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium. Aims. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined. Methods. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. Non-local thermal equilibrium (NLTE) abundances for Li, Ca, and Fe were derived where possible by fitting spectra obtained with VLT/FLAMES-GIRAFFE using the quantitative-spectroscopy package SME. Stellar parameters were derived by matching isochrones to the observed V vs. V – I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-Teff calibrations to the broadband photometry. Results. Li abundances of the turnoff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including Atomic Diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by Atomic Diffusion and additional mixing, but we were unable to determine the efficiency of the additional mixing precisely. This is the fourth globular cluster (after NGC 6397, NGC 6752, and M4) in which Atomic Diffusion signatures are detected. After applying a conservative correction (T6.0 model) for Atomic Diffusion, we find an initial Li abundance of A(Li) = 2.48 ± 0.10 for the globular cluster M30. We also detected a Li-rich SGB star with a Li abundance of A(Li) = 2.39. The finding makes Li-rich mass transfer a likely scenario for this star and rules out models in which its Li enhancement is created during the RGB bump phase.