3He in stars of low and intermediate mass (original) (raw)
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Compulsory Deep Mixing of 3 He and CNO Isotopes in the Envelopes of Low‐Mass Red Giants
The Astrophysical Journal, 2008
Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3 He(3 He,2p) 4 He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3 He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism operates rapidly once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M develop 12 C/ 13 C ratios of 14.5 ± 1.5, while Pop II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M , this mechanism also destroys 90% to 95% of the 3 He produced on the main sequence.
Astronomy and Astrophysics
. We performed detailed nucleosynthesis calculations in hydrogen and helium burning shells as well as for hot bottom burning at the base of convective envelope in a low-metallicity (Z = 10 Gamma4 ) intermediate-mass star during thermalpulsing AGB evolution. Based on complete stellar models, up-to-date simple analytical expressions were used to describe the model star and its evolution. Our study concentrated on surface abundances of light elements, such as C, N, O, Na and Al, and their isotopes in order to test a hypothesis of Cottrell and Da Costa (1981) frequently invoked to explain star-to-star abundance variations in globular-cluster red giants. It is shown that this hypothesis of primordial contamination of intracluster matter by nuclear products from early generation AGB stars fails to reproduce the observed O depletion and Al enhance- Send offprint requests to: A. Weiss ment. We propose an alternative mechanism which combines some primordial compositio...
On the Need for Deep Mixing in AGB Stars of Low Mass
2010
The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the 26 Al/ 27 Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the 26 Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of population I stars above about 1.2 M ⊙ , this consumes 3 He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.
ON THE NEED FOR DEEP-MIXING IN ASYMPTOTIC GIANT BRANCH STARS OF LOW MASS
The Astrophysical Journal, 2010
The photospheres of low-mass red giants show CNO isotopic abundances that are not satisfactorily accounted for by canonical stellar models. The same is true for the measurements of these isotopes and of the 26 Al/ 27 Al ratio in presolar grains of circumstellar origin. Non-convective mixing, occurring during both red giant branch (RGB) and asymptotic giant branch (AGB) stages, is the explanation commonly invoked to account for the above evidence. Recently, the need for such mixing phenomena on the AGB was questioned, and chemical anomalies usually attributed to them were suggested to be formed in earlier phases. We have therefore re-calculated extra-mixing effects in low-mass stars for both the RGB and AGB stages, in order to verify the above claims. Our results contradict them; we actually confirm that slow transport below the convective envelope occurs also on the AGB. This is required primarily by the oxygen isotopic mix and the 26 Al content of presolar oxide grains. Other pieces of evidence exist, in particular from the isotopic ratios of carbon stars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC grains of AGB origin. We further show that, when extra-mixing occurs in the RGB phases of Population I stars above about 1.2 M , this consumes 3 He in the envelope, probably preventing the occurrence of thermohaline diffusion on the AGB. Therefore, we argue that other extra-mixing mechanisms should be active in those final evolutionary phases.
Monthly Notices of the Royal Astronomical Society, 2015
Abundances of the proton-capture elements and their isotopes in globular-cluster stars correlate with each other in such a manner as if their variations were produced in high-temperature hydrogen burning at the same time in the past. In addition to these primordial abundance variations, the RGB stars in globular clusters, like their field counterparts, show the evolutionary variations of the C and N abundances and 12 C/ 13 C isotopic ratio. The latter are caused by extra mixing operating in the RGB star's radiative zone that separates the H-burning shell from the bottom of its convective envelope. We demonstrate that among the potential sources of the primordial abundance variations in globular-cluster stars proposed so far, such as the hot-bottom burning in massive AGB stars and H burning in the convective cores of supermassive and fastrotating massive MS stars, only the supermassive MS stars with M > 10 4 M can explain all the abundance correlations without any fine-tuning of free parameters. We use our assumed chemical composition for the pristine gas in M13 (NGC 6205) and its mixtures with 50% and 90% of the material partially processed in H burning in the 6 × 10 4 M MS model star as the initial compositions for the normal, intermediate and extreme populations of low-mass stars in this globular cluster, as suggested by its O-Na anti-correlation. We evolve these stars from the zero-age MS to the RGB tip with the thermohaline and parametric prescriptions for the RGB extra mixing. We find that the 3 He-driven thermohaline convection cannot explain the evolutionary decline of [C/Fe] in M 13 RGB stars, which, on the other hand, is well reproduced with the universal values for the mixing depth and rate calibrated using the observed decrease of [C/Fe] with M V in the globular cluster NGC5466 that does not have the primordial abundance variations.
Deep Mixing of 3 He: Reconciling Big Bang and Stellar Nucleosynthesis
Science, 2006
Low-mass stars, ∼1 to 2 solar masses, near the Main Sequence are efficient at producing the helium isotope 3 He, which they mix into the convective envelope on the giant branch and should distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the low observed cosmic abundance of 3 He with the predictions of both stellar and Big Bang nucleosynthesis. Here we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell, where a nuclear reaction lowers the mean molecular weight slightly. Thus, we are able to remove the threat that 3 He production in low-mass stars poses to the Big Bang nucleosynthesis of 3 He.
Arxiv preprint astro-ph/ …, 1997
Star-to-star abundance variations of C, N, O, Na and Al in globular-cluster red giants have been recently supplemented by the finding that [Mg/Fe] is depleted in stars with extremely large [Al/Fe] (Shetrone 1996a). To find out which of the magnesium isotopes is responsible for the observed depletion of [Mg/Fe] Shetrone (1996b) also undertook an isotopic analysis of Mg and found that it is 24 Mg which is depleted in Al-rich giants. On the other hand, demonstrated that even in the massive globular cluster ω Cen which has intrinsic spreads in both [Fe/H] and the abundances of the s-process elements, [O/Fe] anticorrelates with [Na/Fe] and [Al/Fe] as in "normal" monometallic clusters. These new spectroscopic results allow us to test current models of stellar evolution and nucleosynthesis, as well as those of the formation and chemical enrichment of globular clusters. In an effort to explain self-consistently these observations we have considered two possibilities: (1) a deep mixing scenario which assumes that in red giants some kind of (extra)mixing transports products of nuclear reactions from the hydrogen burning shell (HBS) to the base of the convective envelope; and (2) a combination of primordial and deep mixing scenarios. It is shown that (1) cannot account for the anticorrelation of [Mg/Fe] vs. [Al/Fe] without additional ad hoc assumptions, among which we identify a strong but still undetected low energy resonance in the reaction 24 Mg(p,γ) 25 Al, and episodical increases of the HBS temperature up to the value T ≈ 74 10 6 K. In (2) intermediate mass AGB stars are assumed to produce the decreased 24 Mg and increased 25 Mg initial abundances in some globular-cluster low mass stars and Al is synthesized at the expense of 25 Mg in the HBS and transported to the surface of the red giant by extramixing. We discuss advantages and deficiencies of both scenarios and propose some observational tests.
Nucleosynthesis of Light-Element Isotopes in Evolved Stars Experiencing Extended Mixing
Publications of the Astronomical Society of Australia, 2009
We present computations of nucleosynthesis in red giants and Asymptotic Giant Branch (AGB) stars of Population I experiencing extended mixing. The assumed physical cause for mass transport is the buoyancy of magnetized structures, according to recent suggestions. The peculiar property of such a mechanism is to allow for both fast and slow mixing phenomena, as required for reproducing the spread in Li abundances displayed by red giants and as discussed in an accompanying paper. We explore here the effects of this kind of mass transport on CNO and intermediate-mass nuclei and compare the results with the available evidence from evolved red giants and from the isotopic composition of presolar grains of AGB origin. It is found that a good general accord exists between predictions and measurements; in this framework we also show which type of observational data best constrains the various parameters. We conclude that magnetic buoyancy, allowing for mixing at rather different speeds, can ...
Early chemical enrichment of the universe and the role of very massive population III stars
Monthly Notices of the Royal Astronomical Society, 2005
In this paper the role of very massive pop III stars in the chemical enrichment of the early universe is discussed. We first compare our predictions with the abundance ratios measured in the high redshift Lymanα forest to check whether they are compatible with the values predicted by assuming that the early universe was enriched by massive pop III stars. We conclude that to explain the observed C/Si ratio in the intergalactic medium, a contribution from pop II stars to carbon enrichment is necessary, already at redshift z=5. We then evaluate the number of Pair-Instability Supernovae (SN γγ ) required to enrich the universe to the critical metallicity Z cr , i.e. the metallicity value which causes the transition from a very massive star regime (m > 100M ⊙ ) to a lower mass regime, similar to the one characteristic of the present time (m < 100M ⊙ ). It is found that between 110 and 115 SN γγ are sufficient to chemically enrich a cubic megaparsec of the intergalactic medium at high redshift for a variety of initial mass functions. The number of ionizing photons provided by these SN γγ and also by the pop III stars ending as black holes was computed and we conclude that there are not enough photons to reionize the universe, being down by at least a factor of ∼ 3. Finally, we calculate the abundance ratios generated by pop III stars and compare it with the ones observed in low metallicity Damped Lyman-α systems (DLAs). We suggest that pop III stars alone cannot be responsible for the abundance ratios in these objects and that intermediate mass pop II stars must have played an important role especially in enriching DLAs in nitrogen.
Nucleosynthesis and mixing on the asymptotic giant branch. I - MS and S stars with and without TC
The Astrophysical Journal, 1992
We present the results of s-process nucleosynthesis calculations for Asymptotic Giant Branch (AGB) stars of different metallicities and different initial stellar masses (1.5 and 3 M ⊙ ) and comparisons of them with observational constraints from high resolution spectroscopy of evolved stars over a wide metallicity range. The computations were based on previously published stellar evolutionary models that account for the third dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13 C(α,n) 16 O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13 C is manufactured locally by proton captures on the abundant 12 C; a few protons are assumed to penetrate from the convective envelope into the radiative layer at any third dredge up episode, when a chemical discontinuity is established between the convective envelope and the He-and Crich zone. A weaker neutron release is also guaranteed by the marginal activation of the reaction 22 Ne(α,n) 25 Mg during the convective thermal pulses. Owing to the lack of a consistent model for 13 C formation, the abundance of 13 C burnt per cycle is allowed to vary as a free parameter over a wide interval (a factor of 50). The s-enriched material is subsequently mixed with the envelope by the third dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or 'hs') and that of the Zr-peak ones (light s, or 'ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency (e.g. of the neutron exposure). Our model predictions for this parameter show a complex trend versus metallicity. Especially noteworthy is the prediction that the flow along the s path at low metallicities drains the Zr-peak and Ba-peak and builds an excess at the doubly-magic 208 Pb, at the termination of the s path. We then discuss the effects on the models of variations in the crucial parameters of the 13 C pocket, finding that they are not critical for interpreting the results.