Intermediate mass stars, in their last phases of evolution ("AGB stars"),produce a large number of heavy elements (rich in neutrons), some ofthem radioactive isotopes, such as Rubidium and Technetium. Theseelements are pushed outwards to the surface of the star, and afterwards released into the interstellar medium. Among this type of stars, those least studied have been the more massive ones (between 4 and 8 times the mass of the Sun). Massive AGB stars have been recently identified in our Galaxy and in other nearby galaxies, such as the Magellanic Clouds, thanks to the detection of strong Rubidium overabundances in the spectra of these stars. However, the high abundances of Rubidium observed in these stars were a challenge for the theoretical models, which predicted considerably smaller Rubidium abundances. As apossible cause for this disagreement between theory and observations it was noted that the model atmospheres used previously to derive the chemical abundances were not sufficiently realistic for the AGB stars, because they did not take into account the large envelopes of gas and dust which surround the central star. In this work, we have determined for the first time the abundance of Rubidium taking into account the effect of the circumstellar envelope in a representative sample of massive AGB stars. We find that the Rubidium abundances determinedusing the new model atmospheres are much smaller, showing that our understanding of the nucleosynthesis in massive AGB stars is essentially valid. Given that the AGB stars account for the cosmic origin of more than 50% of all the elements in the Universe heavier than Iron, studying them has important consequences in other fields ofAstrophysics, such as stellar evolution, the chemical evolution of the galaxies, the origin of the globular clusters, or the chemical composition of the Solar System.
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H II regions are ionized nebulae associated with the formation of massive stars. They exhibit a wealth of emission lines in their spectra that form the basis for estimation of chemical composition. The amount of heavy chemical elements is essential to the understanding of important phenomena such as nucleosynthesis, star formation and chemical evolution of galaxies. For over 80 years, however, a discrepancy exists of a factor of around two between heavy-element abundances (the so-called metallicity) derived from the two main kinds of emission lines that can be measured in nebular spectra
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CaII Kgrains, i.e., intermittent, short-lived (about 1 minute), periodic (2-4 minutes), pointlike chromospheric brightenings, are considered to be the manifestations of acoustic waves propagating upward from the solar surface and developing into shocks in the chromosphere. After the simulations of Carlsson and Stein, we know that hot shocked gas moving upward interacting with the downflowing chromospheric gas (falling down after having been displaced upward by a previous shock) nicely reproduces the spectral features of the CaII K profiles observed in such grains, i.e., a narrowband emission
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It is well known that fullerenes – big, complex, and highly resistant carbon molecules with potential applications in nanotechnology – are mostly seen in planetary nebulae (PNe); old dying stars with progenitor masses similar to our Sun. Fullerenes, like C60 and C70, have been detected in PNe whose infrared (IR) spectra are dominated by broad unidentified IR (UIR) plateau emissions. The identification of the chemical species (structure and composition) responsible for such UIR emission widely present in the Universe is a mystery in astrochemistry; although they are believed to be carbon-rich
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