We report on the succesful search for CO (2-1) and (3-2) emission associated with OH/IR stars in the Galactic Bulge. We observed a sample of eight extremely red AGB stars with the APEX telescope and detected seven. The sources were selected at sufficient high galactic latitude to avoid interference by interstellar CO, which hampered previous studies of inner galaxy stars. To study the nature of our sample and the mass loss we constructed the SEDs from photometric data and Spitzer IRS spectroscopy. In a first step we apply radiative transfer modelling to fit the SEDs and obtain luminosities and dust mass loss rates (MLR). Through dynamical modelling we then retrieve the total MLR and the gas-to-dust ratios. We derived variability periods of our stars. The luminosities range between approximately 4000 and 5500 Lsun and periods are below 700 days. The total MLR ranges between 1E-5 and 1E-4 Msun/yr. Comparison with evolutionary models shows that the progenitor mass is approximately 1.5 Msun, similar to the Bulge Miras and are of intermediate age (3 Gyr). The gas-to-dust ratios are between 100 and 400 and are similar to what is found for OH/IR stars in the galactic Disk. One star, IRAS 17347-2319, has a very short period of approximately 300 days which may be decreasing further. It may belong to a class of Mira variables with a sudden change in period as observed in some Galactic objects. It would be the first example of an OH/IR star in this class and deserves further follow-up observations.
Download