Thermohaline instability and rotation-induced mixing

Charbonnel, C.; Lagarde, N.

doi:10.1051/0004-6361/201014432

Cited by 282 publications

(488 citation statements)

References 93 publications

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“…It is well known that extra-mixing processes are needed to explain the abundance patterns in first red giant branch (RGB) stars located above the RGB bump (Charbonnel & Zahn 2007;Charbonnel & Lagarde 2010;Wachlin et al 2011;Lagarde et al 2012;Maeder et al 2013). We refer here to mixing processes in addition to overshooting, such as rotationally induced mixing 9 or thermohaline mixing 10 .…”

Section: Discussionmentioning

confidence: 99%

On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

Henry

Stephenson

Bertolami

et al. 2017

Monthly Notices of the Royal Astronomical Society

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The primary goal of this paper is to make a direct comparison between the measured and modelpredicted abundances of He, C and N in a sample of 35 well-observed Galactic planetary nebulae (PN). All observations, data reductions, and abundance determinations were performed in house to ensure maximum homogeneity. Progenitor star masses (M ≤ 4 M ⊙ ) were inferred using two published sets of post-AGB model tracks and L and T ef f values. We conclude the following: 1) the mean values of N/O across the progenitor mass range exceeds the solar value, indicating significant N enrichment in the majority of our objects; 2) the onset of hot bottom burning appears to begin around 2 M ⊙ , i.e., lower than ∼ 5 M ⊙ implied by theory; 3) most of our objects show a clear He enrichment, as expected from dredge-up episodes; 4) the average sample C/O value is 1.23, consistent with the effects of third dredge-up; and 5) model grids used to compare to observations successfully span the distribution over metallicity space of all C/O and many He/H data points but mostly fail to do so in the case of N/O. The evident enrichment of N in PN and the general discrepancy between the observed and model-predicted N/O abundance ratios signal the need for extra-mixing as an effect of rotation and/or thermohaline mixing in the models. The unexpectedly high N enrichment that is implied here for low mass stars, if confirmed, will likely impact our conclusions about the source of N in the Universe.

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Section: Discussionmentioning

confidence: 99%

On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

Henry

Stephenson

Bertolami

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…An empirically derived value of Ct = 1000 can reproduce abundance patterns in both globular cluster stars (Angelou et al 2011(Angelou et al , 2012 and field stars (Charbonnel & Lagarde 2010) as well as the dichotomy between carbon-normal and carbon-enhanced metalpoor stars ). Denissenkov (2010) and Cantiello & Langer (2010) prefer a lower value of Ct = 12 on theoretical grounds, which is also supported by 3D hydrodynamical models that suggest the mixing is inefficient on the RGB (Denissenkov & Merryfield 2011;Traxler, Garaud & Stellmach 2011;Brown, Garaud & Stellmach 2013).…”

Section: Overview Of the Stellar Modelsmentioning

confidence: 93%

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Angelou

D’Orazi

Constantino

et al. 2015

Monthly Notices of the Royal Astronomical Society

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We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest-metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turnoff and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter ∆M V MSTO bump . We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but ∆M V MSTO bump , which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of ∆M V MSTO bump .

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“…For these low-mass stars, classical models do not predict any dredge-up of fresh Na produced in situ by proton-capture on pristine 22 Ne (see e.g. Charbonnel & Lagarde 2010), so the reason for the apparent Na enhancement must lie elsewhere. A similar difference was identified by D 'Orazi et al (2010), between dwarfs and giants in 47 Tuc, which they propose to be an artificial result, stemming from the assumption of Gaussian line profiles when determining the equivalent widths of the 818.3/819.4 nm lines.…”

Section: O Na Mg and Almentioning

confidence: 99%

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

Lind

Charbonnel

Decressin

et al. 2011

A&A

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Context. The chemical compositions of globular clusters provide important information on the star formation that occurred at very early times in the Galaxy. In particular the abundance patterns of elements with atomic number z ≤ 13 may shed light on the properties of stars that early on enriched parts of the star-forming gas with the rest-products of hydrogen-burning at high temperatures. Aims. We analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars in the metal-poor globular cluster NGC 6397. We compare the derived abundance patterns with theoretical predictions in the framework of the "wind of fast rotating massive star"-scenario. Methods. High-resolution spectra were obtained with the FLAMES/UVES spectrograph on the VLT. We determined non-LTE abundances of Na, and LTE abundances for the remaining 21 elements, including O (from the [OI] line at 630 nm), Mg, Al, α, iron-peak, and neutron-capture elements, many of which had not been previously analysed for this cluster. We also considered the influence of possible He enrichment in the analysis of stellar spectra. Results. We find that the Na abundances of evolved, as well as unevolved, stars in NGC 6397 show a distinct bimodality, which is indicative of two stellar populations: one primordial stellar generation of composition similar to field stars, and a second generation that is polluted with material processed during hydrogen-burning, i.e., enriched in Na and Al and depleted in O and Mg. The red giant branch exhibits a similar bimodal distribution in the Strömgren colour index c y = c 1 − (b − y), implying that there are also large differences in the N abundance. The two populations have the same composition for all analysed elements heavier than Al, within the measurement uncertainty of the analysis, with the possible exception of [Y/Fe]. Using two stars with almost identical stellar parameters, one from each generation, we estimate the difference in He content, ΔY = 0.01 ± 0.06, given the assumption that the mass fraction of iron is the same for the stars. Conclusions. NGC 6397 hosts two stellar populations that have different chemical compositions of N, O, Na, Mg, and probably Al. The cluster is dominated (75%) by the second generation. We show that massive stars of the first generation can be held responsible for the abundance patterns observed in the second generation long-lived stars of NGC 6397. We estimate that the initial mass of this globular cluster is at least ten times higher than its present-day value.

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Thermohaline instability and rotation-induced mixing

Cited by 282 publications

References 93 publications

On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

Diagnostics of stellar modelling from spectroscopy and photometry of globular clusters

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

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