An analysis of high-resolution near-infrared spectra of a sample of 45 asymptotic giant branch ( AGB ) stars towards the Galactic bulge is presented .
The sample consists of two subsamples , a larger one in the inner and intermediate bulge , and a smaller one in the outer bulge .
The data are analysed with the help of hydrostatic model atmospheres and spectral synthesis .
We derive the radial velocity of all stars , and the atmospheric chemical mix ( [ Fe/H ] , C/O , ^ { 12 } C/ ^ { 13 } C , Al , Si , Ti , and Y ) where possible .
Our ability to model the spectra is mainly limited by the ( in ) completeness of atomic and molecular line lists , at least for temperatures down to T _ { eff } \approx 3100 K. We find that the subsample in the inner and intermediate bulge is quite homogeneous , with a slightly sub-solar mean metallicity and only few stars with super-solar metallicity , in agreement with previous studies of non-variable M-type giants in the bulge .
All sample stars are oxygen-rich , C/O < 1.0 .
The C/O and carbon isotopic ratios suggest that third dredge-up ( 3DUP ) is absent among the sample stars , except for two stars in the outer bulge that are known to contain technetium .
These stars are also more metal-poor than the stars in the intermediate or inner bulge .
Current stellar masses are determined from linear pulsation models .
The masses , metallicities and 3DUP behaviour are compared to AGB evolutionary models .
We conclude that these models are partly in conflict with our observations .
Furthermore , we conclude that the stars in the inner and intermediate bulge belong to a more metal-rich population that follows bar-like kinematics , whereas the stars in the outer bulge belong to the metal-poor , spheroidal bulge population .