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تسجيل مستخدم جديدThe connection between missing AGB stars and extended horizontal branches
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R.G. Gratton
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Recent surveys confirm early results about a deficiency or even absence of CN-strong stars on the asymptotic giant branch (AGB) of globular clusters (GCs), although with quite large cluster-to-cluster variations. In general, this is at odds with the distribution of CN band strengths among first ascent red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong stars in some clusters is a consequence of a smaller mass of these stars that cannot evolve through the full AGB phase. In this short paper we found that the relative frequency of AGB stars can change by a factor of two between different clusters. We also find a very good correlation between the minimum mass of stars along the horizontal branch (Gratton et al. 2010) and the relative frequency of AGB stars, with a further dependence on metallicity. We conclude that indeed the stars with the smallest mass on the HB cannot evolve through the full AGB phase, being AGB-manque. These stars likely had large He and N content, and large O-depletion. We then argue that there should not be AGB stars with extreme O depletion, and few of them with a moderate one.
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[Abridged] In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, as it is now thought that basically all GCs host multiple stellar populations, each with its own chemical abundance pattern and
colour-magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the GC NGC6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first-generation stars, and that all second-generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second-generation horizontal branch stars possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the AGB phase, thus explaining at the same time the low value of the ratio between HB and AGB stars (the R_2 parameter) observed in NGC6752. We have critically discussed this scenario, finding that the required mass-loss rates are of the order of 10^{-9} Mo/yr, significantly higher than current theoretical and empirical constraints. By making use of synthetic HB simulations, we demonstrate that our modelling predicts correctly the R_2 parameter for NGC6752, without the need to invoke very efficient mass loss during the core He-burning stage. Our simulations for NGC6752 HB predict however the presence of a significant fraction - at the level of about 50% - second generation stars along the cluster AGB. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle.
We present recent studies using the near-infrared instrument AMBER of the VLT Interferometer (VLTI) to investigate the structure and shaping processes within the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER observations o
f the Mira variable S Ori have revealed wavelength-dependent apparent angular sizes. These data were successfully compared to dynamic model atmospheres, which predict wavelength-dependent radii because of geometrically extended molecular layers. Most recently, AMBER closure phase measurements of several AGB stars have also revealed wavelength-dependent deviations from 0/180 deg., indicating deviations from point symmetry. The variation of closure phase with wavelength indicates a complex non-spherical stratification of the extended atmosphere, and may reveal whether observed asymmetries are located near the photosphere or in the outer molecular layers. Concurrent observations of SiO masers located within the extended molecular layers provide us with additional information on the morphology, conditions, and kinematics of this shell. These observations promise to provide us with new important insights into the shaping processes at work during the AGB phase. With improved imaging capabilities at the VLTI, we expect to extend the successful story of imaging studies of planetary nebulae to the photosphere and extended outer atmosphere of AGB stars.
We report the discovery of nine metal-poor stars with high levels of r-process enhancement (+0.81<[Eu/Fe]<+1.13), including six subgiants and three stars on the red horizontal branch. We also analyze four previously-known r-process-enhanced metal-poo
r red giants. From this sample of 13 stars, we draw the following conclusions. (1) High levels of r-process enhancement are found in a broad range of stellar evolutionary states, reaffirming that this phenomenon is not associated with a chemical peculiarity of red giant atmospheres. (2) Only 1 of 10 stars observed at multiple epochs shows radial velocity variations, reaffirming that stars with high levels of r-process enhancement are not preferentially found among binaries. (3) Only 2 of the 13 stars are highly-enhanced in C and N, indicating that there is no connection between high levels of r-process enhancement and high levels of C and N. (4) The dispersions in [Sr/Ba] and [Sr/Eu] are larger than the dispersions in [Ba/Eu] and [Yb/Eu], suggesting that the elements below the second r-process peak do not always scale with those in the rare earth domain, even within the class of highly-r-process-enhanced stars. (5) The light-element (12<Z<30) abundances of highly-r-process-enhanced stars are indistinguishable from those with normal levels of r-process material at the limit of our data, 3.5 per cent (0.015 dex) on average. The nucleosynthetic sites responsible for the large r-process enhancements did not produce any detectable light-element abundance signatures distinct from normal core-collapse supernovae.
We study the effects of neutron captures in AGB stars on oq Fe-groupcqb elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni in solar-system materials, which are commonly attributed to SNe
. However, as large fractions of the interstellar medium (ISM) were reprocessed in AGB stars, these elements were reprocessed, too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through 1.5msb and 3msb AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of $^{54}$Cr, $^{58}$Fe, and $^{64}$Ni, while the other isotopes are little altered; hence, the observations may be explained by AGB processing. The results are robust and not dependent on the detailed initial isotopic composition. Consequences for other oq Fe groupcqb elements are then explored. They include $^{50}$Ti excesses, and some production of $^{46,47,49}$Ti. In many circumstellar condensates, Ti quantitatively reflects these effects of AGB neutron captures. Scatter in the data results from small variations (granularity) in the isotopic composition of the local ISM. For Si, the main effects are instead due to variations in the local ISM from different SNe sources. The problem of Ca is discussed, particularly with regard to $^{48}$Ca. The measured data are usually represented assuming terrestrial values for $^{42}$Ca/$^{44}$Ca. Materials processed in AGB stars or sources with variable initial $^{42}$Ca/$^{44}$Ca ratios can give apparent $^{48}$Ca excesses/deficiencies, attributed to SNe. The broader issue of Galactic Chemical Evolution is also discussed in view of the isotopic granularity in the ISM. end{abstract}
There is ample evidence for strong magnetic fields in the envelopes of (Post-)Asymptotic Giant Branch (AGB) stars as well as supergiant stars. The origin and role of these fields are still unclear. This paper updates the current status of magnetic fi
eld observations around AGB, post-AGB stars and describes their possible role during these stages of evolution. The discovery of magnetically aligned dust around a supergiant star is also highlighted. In our search for the origin of the magnetic fields, recent observations show the signatures of possible magnetic activity and rotation, indicating that the magnetic fields might be intrinsic to the AGB stars.
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