ALMA reveals sunburn: CO dissociation around AGB stars in the globular
  cluster 47 Tucanae
  ALMA observations show a non-detection of carbon monoxide around the four
most luminous asymptotic giant branch (AGB) stars in the globular cluster 47
Tucanae. Stellar evolution models and star counts show that the mass-loss rates
from these stars should be ~1.2-3.5 x 10^-7 solar masses per year. We would
naively expect such stars to be detectable at this distance (4.5 kpc). By
modelling the ultraviolet radiation field from post-AGB stars and white dwarfs
in 47 Tuc, we conclude CO should be dissociated abnormally close to the stars.
We estimate that the CO envelopes will be truncated at a few hundred stellar
radii from their host stars and that the line intensities are about two orders
of magnitude below our current detection limits. The truncation of CO envelopes
should be important for AGB stars in dense clusters. Observing the CO (3-2) and
higher transitions and targeting stars far from the centres of clusters should
result in the detections needed to measure the outflow velocities from these
stars.
