ATOMIUM: Halide molecules around the S-type AGB star W Aquilae
  S-type asymptotic giant branch (AGB) stars are thought to be intermediates in
the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of
their circumstellar envelopes are also intermediate, but have not been studied
in as much detail as their carbon- and oxygen-rich counterparts. We aim to
determine the abundances of AlCl and AlF from rotational lines, which have been
observed for the first time towards an S-type AGB star, W Aql. In combination
with models based on PACS observations, we aim to update our chemical kinetics
network based on these results. We analyse ALMA observations towards W Aql of
AlCl in the ground and first two vibrationally excited states and AlF in the
ground vibrational state. Using radiative transfer models, we determine the
abundances and spatial abundance distributions of Al$^{35}$Cl, Al$^{37}$Cl, and
AlF. We also model HCl and HF emission and compare these models to PACS spectra
to constrain the abundances of these species. AlCl is found in clumps very
close to the star, with emission confined within 0.1$^{\prime\prime}$ of the
star. AlF emission is more extended, with faint emission extending
0.2$^{\prime\prime}$ to 0.6$^{\prime\prime}$ from the continuum peak. We find
peak abundances, relative to H$_2$, of $1.7\times 10^{-7}$ for Al$^{35}$Cl,
$7\times 10^{-8}$ for Al$^{37}$Cl and $1\times 10^{-7}$ for AlF. From the PACS
spectra, we find abundances of $9.7\times 10^{-8}$ and $\leq 10^{-8}$, relative
to H$_2$, for HCl and HF, respectively. The AlF abundance exceeds the solar F
abundance, indicating that fluorine synthesised in the AGB star has already
been dredged up to the surface of the star and ejected into the circumstellar
envelope. From our analysis of chemical reactions in the wind, we conclude that
AlF may participate in the dust formation process, but we cannot fully explain
the rapid depletion of AlCl seen in the wind.
