CO in OH/IR stars close to the Galactic centre

TL;DR

This study demonstrates the feasibility of detecting circumstellar CO emission from OH/IR stars near the Galactic centre, paving the way for large-scale mass-loss rate surveys with ALMA to better understand Galactic bulge evolution.

Contribution

First observational evidence showing that ALMA can detect circumstellar CO in stars near the Galactic centre, enabling future large-scale mass-loss studies.

Findings

CO emission detected in two stars with consistent velocities

One star's CO profile suggests it is a foreground object

ALMA can measure mass-loss rates efficiently

Abstract

Aims: A pilot project has been carried out to measure circumstellar CO emission from three OH/IR stars close to the Galactic centre. The intention was to find out whether it would be possible to conduct a large-scale survey for mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA). Such a survey would increase our understanding of the evolution of the Galactic bulge. Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is necessary as a `spatial filter' in this region of space because of the confusion with interstellar CO emission. Results: Towards two of the stars, CO emission was detected with positions and radial velocities coinciding within the statistical errors with the corresponding data of the associated OH sources. However, for one of the stars the line profile is not what one expects for an unresolved expanding circumstellar envelope. We believe that this CO envelope is partially resolved and that this star therefore is a foreground star not belonging to the bulge. Conclusions: The results of the observations have shown that it is possible to detect line profiles of circumstellar CO from late-type stars both within and in the direction of the Galactic bulge. ALMA will be able to detect CO emission in short integrations with sensitivity sufficient to estimate mass-loss rates from a large number of such stars.