OH/IR stars in the inner Galactic bulge: I. Colour, CO line, and stellar light curve characteristics

Context. Stars on the asymptotic giant branch (AGB) play important roles in a number of astronomical contexts. To quantify these roles, it is necessary to establish the mass-loss characteristics of stars as they evolve up and beyond the AGB. Aims. We used an equidistant sample of 77 AGB stars in the inner Galactic bulge, selected on the existence and strength of OH 1612 MHz maser emission, to obtain information on the mass loss of O-rich AGB stars, and on its transformations in geometry and kinematics that occur at the tip of the AGB. Methods. Observations of circumstellar lines from several rotational transitions of ¹²CO, ¹³CO, and C¹⁸O were performed with the Atacama Large Millimeter/submillimeter Array (ALMA), and, for a sub-sample, with the Atacama Pathfinder EXperiment telescope (APEX). The ALMA observations also provide continuum data. Existing infrared photometry was used to estimate colours and produce stellar light curves. Results. Based on mid-infrared colour, CO line, and near-infrared variability characteristics, the objects were divided into four categories of distinct stellar and/or circumstellar properties. Various circumstellar CO line characteristics are presented and compared between the categories, such as morphologies and extents of brightness distributions (BDs), line profiles (LPs), line intensity ratios, and kinematics. A majority of the objects form a homogenous group with sharply, centrally peaked BDs and LPs of the soft-parabola type, while the rest show extended and complex BDs and/or LPs with high-velocity wings. The C¹⁸O line and ALMA continuum detection rates vary significantly between the categories. CO line emission is also detected from interaction zones where the expanding circumstellar gas meets the interstellar medium. Conclusions. These data provide the foundation for more detailed studies, including radiative transfer analyses of the CO line and photometry data, on the evolution of stellar and circumstellar characteristics of O-rich stars on the upper AGB and beyond.