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- Magrini, L., et al.
(författare)
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The Gaia -ESO survey: Mixing processes in low-mass stars traced by lithium abundance in cluster and field stars
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 651
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to constrain the mixing processes in low-mass stars by investigating the behaviour of the Li surface abundance after the main sequence. We take advantage of the data from the sixth internal data release of Gaia-ESO, IDR6, and from the Gaia Early Data Release 3, EDR3s. Methods. We selected a sample of main-sequence, sub-giant, and giant stars in which the Li abundance is measured by the Gaia-ESO survey. These stars belong to 57 open clusters with ages from 130 Myr to about 7 Gyr and to Milky Way fields, covering a range in [Fe/H] between -1.0and +0.5 dex, with few stars between -1.0 and -2.5dex. We studied the behaviour of the Li abundances as a function of stellar parameters. We inferred the masses of giant stars in clusters from the main-sequence turn-off masses, and for field stars through comparison with stellar evolution models using a maximum likelihood technique. We compared the observed Li behaviour in field giant stars and in giant stars belonging to individual clusters with the predictions of a set of classical models and of models with mixing induced by rotation and thermohaline instability. Results. The comparison with stellar evolution models confirms that classical models cannot reproduce the observed lithium abundances in the metallicity and mass regimes covered by the data. The models that include the effects of both rotation-induced mixing and thermohaline instability account for the Li abundance trends observed in our sample in all metallicity and mass ranges. The differences between the results of the classical models and of the rotation models largely differ (up to 2 dex), making lithium the best element with which to constrain stellar mixing processes in low-mass stars. We discuss the nature of a sample of Li-rich stars. Conclusions. We demonstrate that the evolution of the surface abundance of Li in giant stars is a powerful tool for constraining theoretical stellar evolution models, allowing us to distinguish the effect of different mixing processes. For stars with well-determined masses, we find a better agreement of observed surface abundances and models with rotation-induced and thermohaline mixing. Rotation effects dominate during the main sequence and the first phases of the post-main-sequence evolution, and the thermohaline induced mixing after the bump in the luminosity function.
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| 2. |
- da Silva, R., et al.
(författare)
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Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 678, s. A195-A195
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Tidskriftsartikel (refereegranskat)abstract
- Context. Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Dating back to the beginning of the 20th century, they have been safely adopted to trace the rotation, kinematics, and chemical enrichment history of the Galactic thin disk.Aims. The main aim of this investigation is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs analyzed so far (1118 spectra of 356 objects). The current sample, containing 70 CCs for which spectroscopic metal abundances are provided for the first time, covers a wide range in galactocentric distances, pulsation modes, and pulsation periods.Methods. Optical high-resolution spectra with a high signal-to-noise ratio that were collected with different spectrographs were adopted to provide homogeneous estimates of the atmospheric parameters (effective temperature, surface gravity, and microturbulent velocity) that are required to determine the abundance. Individual distances were based either on trigonometric parallaxes by the Gaia Data Release 3 (Gaia DR3) or on distances based on near-infrared period-luminosity relations.Results. We found that iron and α-element radial gradients based on CCs display a well-defined change in the slope for galactocentric distances larger than ~12 kpc. We also found that logarithmic regressions account for the variation in [X/H] abundances from the inner to the outer disk. Radial gradients for the same elements, but based on open clusters covering a wide range in cluster ages, display similar trends. This means that the flattening in the outer disk is an intrinsic feature of the radial gradients because it is independent of age. Empirical evidence indicates that the S radial gradient is steeper than the Fe radial gradient. The difference in the slope is a factor of two in the linear fit (−0.081 vs. −0.041 dex kpc−1) and changes from −1.62 to −0.91 in the logarithmic distance. Moreover, we found that S (explosive nucleosynthesis) is underabundant on average when compared with O (hydrostatic nucleosynthesis). The difference becomes clearer in the metal-poor regime and for the [O/Fe] and [S/Fe] abundance ratios. We performed a detailed comparison with Galactic chemical evolution models and found that a constant star formation efficiency for galactocentric distances larger than 12 kpc accounts for the flattening observed in both iron and α-elements. To further constrain the impact of the predicted S yields for massive stars on radial gradients, we adopted a toy model and found that the flattening in the outermost regions requires a decrease of a factor of four in the current S predictions.Conclusions. CCs are solid beacons for tracing the recent chemical enrichment of young stellar populations. Sulfur photospheric abundances, when compared with other α-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular sulfur abundances in external galaxies.
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