CO in OH/IR stars close to the Galactic centre

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.

💡 Research Summary

The paper presents a pilot investigation aimed at detecting circumstellar carbon‑monoxide (CO) emission from three OH/IR stars located in the direction of the Galactic centre, with the broader goal of assessing the feasibility of a large‑scale mass‑loss survey using interferometric facilities such as the Atacama Large Millimeter/sub‑millimeter Array (ALMA). The motivation stems from the fact that asymptotic‑giant‑branch (AGB) stars in the bulge are major contributors of gas and dust to the interstellar medium, yet their individual mass‑loss rates remain poorly constrained because the dense molecular environment toward the centre produces strong, spatially extended interstellar CO that overwhelms single‑dish observations. By employing interferometers as spatial filters, the authors aim to isolate the compact, high‑velocity CO envelopes that surround the evolved stars while suppressing the diffuse background.

Observations were carried out with two millimetre‑wave interferometers. The Nobeyama Millimeter Array (NMA) observed the ¹²CO J = 1 → 0 transition at 115 GHz, delivering a synthesized beam of roughly 0.5 arcsec and a spectral resolution of ∼4 km s⁻¹. The Submillimeter Array (SMA) targeted the ¹²CO J = 2 → 1 line at 230 GHz, achieving a finer beam of ∼0.3 arcsec and ∼2 km s⁻¹ resolution. Each source was integrated for several hours to reach a noise level that would allow detection of line intensities of order a few hundred milli‑Kelvin‑kilometre per second. The data were calibrated using standard procedures, imaged with CLEAN, and examined for compact emission coincident with the known OH maser positions and velocities.

Two of the three targets (hereafter Star A and Star B) displayed clear CO detections. Their line centres matched the OH maser velocities within 1–2 km s⁻¹ (−115 km s⁻¹ for Star A and −98 km s⁻¹ for Star B), and the full‑width at half‑maximum (FWHM) of the profiles was 15–20 km s⁻¹, typical of an expanding AGB wind. The integrated line fluxes were ∼0.4 K km s⁻¹ for the J = 1 → 0 transition and ∼0.6 K km s⁻¹ for J = 2 → 1, providing sufficient signal‑to‑noise for quantitative analysis. Using standard radiative‑transfer prescriptions, these fluxes correspond to mass‑loss rates in the range 10⁻⁶–10⁻⁵ M⊙ yr⁻¹, consistent with expectations for OH/IR stars.

The third source (Star C) also yielded CO emission, but the line profile deviated markedly from the textbook double‑horned or flat‑topped shape of an unresolved, spherically expanding shell. The profile was asymmetric, weaker, and appeared partially resolved by the interferometer, suggesting that a significant fraction of the envelope’s flux was filtered out. The authors propose two interpretations: (1) the CO envelope of Star C is physically larger than the angular scales sampled by the longest baselines, leading to partial spatial filtering; or (2) Star C is not a bulge member but a foreground post‑AGB object whose envelope has expanded beyond the compact scale typical of OH/IR stars. In either case, the anomalous profile underscores the importance of baseline configuration when targeting extended circumstellar material in crowded Galactic regions.

From a methodological standpoint, the study demonstrates that interferometric observations can successfully isolate stellar CO emission even against the bright, confused background of the inner Galaxy. The detection of compact CO in two out of three targets validates the approach and suggests that a systematic survey of hundreds of OH/IR stars is technically viable. The authors argue that ALMA, with its vastly superior sensitivity, flexible baseline configurations, and wide instantaneous bandwidth, would be able to detect similar CO lines in a few minutes per source. Short integrations would yield line profiles of sufficient quality to derive expansion velocities and, via calibrated CO‑to‑H₂ conversion factors, robust mass‑loss rates. Moreover, ALMA’s capability to combine the 12‑m main array with the Atacama Compact Array (ACA) would allow simultaneous recovery of both compact wind emission and any more extended, possibly detached shells, providing a more complete picture of the mass‑loss history.

In conclusion, this pilot project confirms that circumstellar CO can be observed toward the Galactic centre using interferometric spatial filtering, that the resulting line parameters are consistent with typical AGB wind properties, and that the technique is scalable to a large, statistically meaningful sample. A future ALMA‑based survey would dramatically improve our knowledge of the mass‑return budget from evolved stars in the bulge, refine models of Galactic chemical evolution, and help to constrain the dynamical evolution of the central stellar population.