This thesis addresses several topics in the study of the more massive and O-rich asymptotic giant branch (AGB) stars in our Galaxy. In particular, the main goals are the exploration of the circumstellar effects on the abundance determination in solar metallicity massive AGB stars by using more realistic pseudo-dynamical model atmospheres and the obtention, for the first time, of the CNO elemental abundances and isotopic ratios in truly massive Galactic AGB stars.

First, we have determined the Rb and Zr abundances in a sample of massive Galactic AGB stars, previously studied with hydrostatic models, by using extended model atmospheres that consider the presence of a circumstellar envelope with a radial wind. The new Rb abundances are much lower than those obtained from hydrostatic models, while the Zr abundances are close to the hydrostatic ones because the 6474A ZrO bandhead used to derive the Zr
abundance is formed deeper in the atmosphere and is less affected, by the circumstellar effects, than the 7800A Rb I line. The new [Rb/Fe] abundances and [Rb/Zr] ratios are in much better agreement with the theoretical predictions, significantly resolving the previous mismatch between the observations and the nucleosynthesis models in the more massive AGB stars and confirming that the 22Ne(alpha, n)25Mg reaction is the main neutron source in these stars.

Second, we have reported new hydrostatic and pseudo-dynamic abundances of Li and Ca in a sample of massive Galactic O-rich AGB stars by using the new extended (pseudo-dynamical) model atmospheres. The Li abundances from extended atmosphere models are very similar to those obtained from hydrostatic models, confirming the Li-rich (and super Li-rich in some stars) character of our sample stars and the strong activation of the hot bottom burning (HBB) process in massive AGB stars. This is in good agreement with most of the AGB nucleosynthesis models. The Ca abundances, which have been derived here for the first time in massive Galactic AGB stars, are consistent with the predictions from the s-process nucleosynthesis models for massive AGB stars at solar metallicity. A minority of stars show a significant Ca depletion and possible explanations are offered to explain their apparent and unexpected Ca depletion.

Finally, we have obtained, for the first time by spectral synthesis, the CNO abundances and isotopic ratios in a small sample of truly massive Galactic O-rich AGB stars at the beginning of the thermally pulsing (TP) phase. Moreover, we have found that the presence of a circumstellar envelope and radial wind is not affecting the determination of the CNO elemental/isotopic abundances in the near-infrared wavelength range. The derived CNO elemental/isotopic abundances are in general good agreement with the theoretical predictions for very massive AGB stars (around7-8 solar masses) at solar metallicity that experience HBB in the early TPs. In particular, the large N enhancements and the low 12C/13C ratios confirm the HBB activation independently of the Li overabundances. In addition, two sample stars seem to be descendants of8 Msun HBB progenitors that have not experienced any TP and they thus represent very good candidates for truly super-AGB stars. We have also compared our O isotopic ratios with the measurements made on presolar grains. The O isotopic composition in two
sample stars reinforce the idea that the more massive AGB stars could be the formation site of the Group II grains. However, the large uncertainties and our conservative lower limits for the O isotopic ratios in the rest of sample stars do not permit us to reach a definitive answer about the origin of the Group II grains.