# Circumstellar CO in metal-poor stellar winds: the highly irradiated globular cluster star 47 Tucanae V3

## TL;DR

This study reports the first detection of circumstellar CO in a globular cluster star, revealing insights into mass-loss processes in metal-poor stellar winds influenced by radiation and dust interactions.

## Contribution

It provides the first observational evidence of circumstellar CO in a globular cluster, highlighting the effects of low metallicity on stellar wind properties and mass-loss mechanisms.

## Key findings

- Detection of CO and SiO lines around V3 in 47 Tucanae.
- The stellar wind is slower and denser compared to Galactic AGB stars.
- The CO line strength supports models of CO dissociation by cluster radiation.

## Abstract

We report the first detection of circumstellar CO in a globular cluster. Observations with ALMA have detected the CO J=3-2 and SiO v=1 J=8-7 transitions at 345 and 344 GHz, respectively, around V3 in 47 Tucanae (NGC 104; [Fe/H] = -0.72 dex), a star on the asymptotic giant branch. The CO line is detected at 7 sigma at a rest velocity v_LSR = -40.6 km/s and expansion velocity of 3.2 +/- ~0.4 km/s. The brighter, asymmetric SiO line may indicate a circumstellar maser. The stellar wind is slow compared to similar Galactic stars, but the dust opacity remains similar to Galactic comparisons. We suggest that the mass-loss rate is set by the levitation of material into the circumstellar environment by pulsations, but that the terminal wind-expansion velocity is determined by radiation pressure on the dust: a pulsation-enhanced dust-driven wind. We suggest the metal-poor nature of the star decreases the grain size, slowing the wind and increasing its density and opacity. Metallic alloys at high altitudes above the photosphere could also provide an opacity increase. The CO line is weaker than expected from Galactic AGB stars, but its strength confirms a model that includes CO dissociation by the strong interstellar radiation field present inside globular clusters.