Abstract :
We report on the successful 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 asymptotic giant branch stars with the Atacama Pathfinder EXperiment telescope and detected seven. The sources were selected at a sufficiently 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 spectral energy distribution (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 (MLRs). 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 L☉ and periods are shorter than 700 d. The total MLR ranges between 10-5 and 10-4 M☉ yr-1. Comparison with evolutionary models shows that the progenitor mass ≈1.5 M☉, similar to the bulge Miras 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 disc. One star, IRAS 17347-2319, has a very short period of approximately 300 d that 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.

Keywords : stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; galaxy: bulge; radio lines: stars; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies
DOI : 10.1093/mnras/sty1663 
Bibcode : 2018MNRAS.479.3545B