- A mechanism to account for well known peculiarities of low mass AGB star nucleosynthesis
- The Evolution of More Massive Stars
- The temperature and chronology of heavy-element synthesis in low-mass stars.
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- The temperature and chronology of heavy-element synthesis in low-mass stars
A mechanism to account for well known peculiarities of low mass AGB star nucleosynthesis
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- Evolution and nucleosynthesis of low-mass stars?
Research Article January 01, Meyer Bradley S. Google Scholar. Stars that formed in the first or second generation are extremely rare objects, and only a few are known. The lack of metals in the gas available in the mini-halos, where the first stars formed, limits radiative cooling, increasing the Jeans mass and shifting the initial mass function to large masses, to the point that perhaps no low-mass stars were formed in the first generation.
The Evolution of More Massive Stars
This picture has been challenged in recent years by the discovery of low-mass stars which show extremely low metallicity and low carbon and nitrogen abundances, suggesting that low-mass stars can form even at such low metallicities. In red, the best model fit. Figure taken from Aguado et al. Large format: [ PNG ]. Since the H- burning shell now advances very quickly because of the combined effects of the very high ZCNO and of the low H mass fraction, it follows that: the maximum temperature increases now very steeply with the He core mass so that the size of the He core mass at the He flash is just 0.
It follows that the time spent by the star on this 2nd RGB phase is extremely short, of the order of 2 Myr.
When MHe reaches 0. Though the ignition point is very similar to the one where the first He core flash occurred, this time the entropy barrier at the H-He interface is large enough to prevent the penetration of the outer border of the He convective shell in the H rich mantle. The He core flash develops as usual through a series of successive He burning ignitions and associated convective shell episodes that lift the electron degeneracy in regions progressively closer to the center.
This star experiences a series of 15 flashes that are shown in Figure 6. During this phase the star rolls in the HR diagram towards lower luminosities and slightly larger effective temperatures. Once the electron degeneracy is fully removed in the He core, the star settles in the point marked by the filled triangle in Figure 2 and 1. Our stellar model has Y greater than 0. As for the percentage time spent by the star at the various effective temperatures, one third of the total central He burning lifetime is spent at the red and one third at the blue side of the HR diagram, the remaining third being spent crossing on a nuclear timescale the HR diagram from the red to blue.
Another thing worth mentioning is that the C mass fraction in the He convective core at the beginning of the central He burning is 0. The reason for both these occurrences is, once again, the very high ZCNO and the low H abundance in the envelope that force the H burning shell to be very efficient in piling up fresh He on the He core mass.
All the key properties of the evolution of this star are reported in the first column of Table 1. After the mass, initial metallicity and He abundance, rows 4 to 17 refer to the surface abundances of the most abundant nuclei. Additional models We have shown in the previous sections that the He core flash of a 0.
The temperature and chronology of heavy-element synthesis in low-mass stars.
Such a phenomenon has very interesting consequences, the most important one being the dredge up of large amounts of C, N, O and Li to the surface. If this peculiar phenomenon were confined to just this mass and to this specific chemical composition, its interest would be largely academical. If, vice versa, it would occur for a reasonably wide range of values of the three parameters M, Y and Z , its reality would be strengthened and the probability of its occurrence in real stars would increase signifi- cantly. Hence, in order to assess the robustness of this phenomenon, we have computed a few further models, i.
The final surface chemical composition after the dredge up is quite similar, the global ZCNO mildly increasing from 1. The explanation for such an occurrence can be easily understood by looking at Figure 7 that shows the trend of the maximum off-center temperature with the He core mass. The zero point which in practice may be identified as the temperature at which it starts moving off center scales inversely with the metallicity actually ZCNO because the larger the ZCNO the earlier the CNO cycle starts dominating the nuclear energy production and hence the cooler will be the interior of the star while it exits the Main Sequence to enter the RGB phase.
The slope of that relation, on the contrary, steepens increasing the metallicity ZCNO because the larger the metallicity the faster the H burning shell advances and hence the faster will be the growth of the He core and its heating. Also in this case we found an extended mixing followed by the dredge up to the surface of the product of the He-H burnings.
The surface chemical composition after the dredge up is shown in the 4th column of Table 1. The influence of the initial mass has been studied by computing two stellar models of masses 1.
Also the 1. To stress simultaneously the effect of the mass and the initial He abundance we have also computed the evolution of a 0.
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Also in this case we found an extended penetration of the He convective shell in the H rich mantle this evolution was stopped as soon as the He convective shell was definitely penetrated in the H rich envelope. All these tests show that a the He- H mixing at the He core flash is a phenomenon that occurs over a reasonably large range of metallicities, initial He abundances and masses and b within the range of parameters addressed in this paper, the final surface ZCNO is only mildly dependent on the initial mass, metallicity and He abundance. Before closing this section let us briefly comment on the comparison between our results and others available in the literature.
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In particular we want to stress here that, in spite of the quantitative differences that come out when closely comparing the models computed by different groups, a few key features seem to be quite well established fortunately. First of all the sequence of events that leads to the surface pollution: firstly, the penetration of the outer border of the He convective shell in the H rich mantle, then, the splitting of the He convective shell in two at the mass location where the H-burst occurs and, eventually, the merging of the H-convective shell with the convective envelope.
Also the limiting metallicity Z actually one should mention ZCNO that allows the He convective shell to penetrate the H rich mantle is quite similar to the one quoted by Schlattl et al. The HE case A large C and N overabundances relative to Iron is a quite common occurrence among the extremely iron poor stars, e. Two stars have been analyzed in detail up to now Schlattl et al. Though these two stars are indeed extremely Fe poor, their Fe abundance is large enough that even a scaled solar ZCNO would bring them outside the range of values for which the hot He-H mixing occurs in the present generation of models.
A very possible O overabundance would even worsen the situation. To overcome such a difficulty, Schlattl et al.
The temperature and chronology of heavy-element synthesis in low-mass stars
Quite recently it has been discovered and quite accurately analyzed the most iron poor star presently known: HE Christlieb et al. The key evolutionary features of a 0. Therefore it is not necessary to invoke for this star any, ad hoc, late accretion of polluted matter. There are many reasons for that, each of which would be sufficient by itself to exclude the autopollution scenario. Let us analyze each of them separately. The black portion of the path refers to the phases in which the surface chemical composition is still the pristine one, while the gray path shows the part of track where the star shows the large overabundances of the elements brought to the surface during the He core flash.
It is strikingly evident that the position of HE is still on the middle of the first RGB, well below the He core flash and well below the central He burning location.