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

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 7σ at a rest velocity v LSR = -40.6 km s −1 and expansion velocity of 3.2 ± ∼0.4 km s −1 . 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 pulsationenhanced 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.

A Census of Thermally Pulsing AGB Stars in the Andromeda Galaxy and a First Estimate of Their Contribution to the Global Dust Budget

“…There have also been empirical studies suggesting that metallicity can have a dramatic impact on the wind speeds of AGB mass outflows, with differing effects for carbon- (Groenewegen 2012) and oxygen-rich (Goldman et al 2017;McDonald et al 2019McDonald et al , 2020 AGB stars. While based on a limited range in metallicity, and small samples, metallicity seems to have a much larger impact on the wind speeds of oxygen-rich AGB stars.…”

The onset of mass loss in evolved stars

“…Strong dust production by oxygen-rich, metal-poor stars is also seen in metal-poor dwarf galaxies (e.g., Boyer et al 2015& Boyer et al 2017 where its onset also appears to correlate with the onset of strong pulsation (McDonald et al 2014). Few measurements of wind velocities and CO-based mass-loss rates exist for marginally dust-producing, metal-poor stars: those few suggest the dust opacity remains the same partly because the wind is slower (e.g., McDonald et al 2019. Infrared spectra also show the presence of a different dust species in metal-poor stars, showing only continuum emission, which is posited to be metallic iron (McDonald et al 2010.…”

Spatially resolving the AGB star V3 in the metal-poor globular cluster 47 Tuc with VLTI/GRAVITY

“…Its low metallicity of [Fe/H] = −0.72 (Harris 1996) is ∼1/5 the solar value, making it ideal for studying the AGB mass loss at low metallicities. McDonald et al (2019) analyzed the radio CO observations of the pulsating AGB star V3 in 47 Tuc and found that the terminal velocity is slower than its Galactic counterparts. This study suggested that the lower metallicity results in smaller grains, which, in turn, slow down the stellar wind.…”