  Mass loss at the asymptotic giant branch (AGB) plays an important role not
only in the final fates of stars, but also in the chemical evolution of
galaxies. Nevertheless, the metallicity effects on AGB mass loss are not yet
fully understood. We present spatially resolved observations of an AGB star,
V3, in the metal-poor globular cluster 47 Tuc (NGC 104). The AGB star 47 Tuc V3
was observed using the GRAVITY instrument at ESO's Very Large Telescope
Interferometer (VLTI) at 2-2.45 micron with a projected baseline length of up
to 96 m. The object 47 Tuc V3 has been spatially resolved and stands as the
first to attempt to spatially resolve an individual star in a globular cluster.
The uniform-disk fit to the observed data results in an angular diameter of
~0.7 mas. Our modeling of the spectral energy distribution and near-infrared
interferometric GRAVITY data suggests that the observed data can be explained
by an optically thin dust shell with a 0.55 micron optical depth of 0.05-0.25,
consisting of metallic iron grains, likely together with effects of the
extended atmosphere of the central star. The dust temperature at the inner
shell boundary is 500-800 K (corresponding to 23-90 stellar radii),
significantly lower than observed in nearby oxygen-rich AGB stars. Radiation
pressure on small (< 0.05 micron) iron grains is not sufficient to drive
stellar winds. Therefore, iron grains may grow to larger sizes, even in the
metal-poor environment. Alternatively, it is possible that the observed iron
grain formation is a result of the mass outflow initiated by some other
mechanism(s). The sensitivity and angular resolution of VLTI provides a new
window onto spatially resolving individual stars in metal-poor globular
clusters. This allows us to improve subsequent studies of the metallicity
dependence of dust formation and mass loss.
