IRC+10216 mass loss properties through the study of $\lambda$3mm
  emission: Large spatial scale distribution of SiO, SiS, and CS
  The study of the gas in the envelopes surrounding asymptotic giant branch
(AGB) stars through observations in the millimetre wavelength range provides
information about the history and nature of these molecular factories. Here we
present ALMA observations at subarsecond resolution, complemented with IRAM-30m
data, of several lines of SiO, SiS, and CS towards the best-studied AGB
circumstellar envelope, IRC+10216. We aim to characterise their spatial
distribution and determine their fractional abundances mainly through radiative
transfer and chemical modelling. The three species display extended emission
with several enhanced emission shells. CS displays the most extended
distribution reaching distances up to approximately 20''. SiS and SiO emission
have similar sizes of approximately 11'', but SiS emission is slightly more
compact. We have estimated fractional abundances relative to H$_2$, which on
average are equal to f(SiO)$\sim$10$^{-7}$, f(SiS)$\sim$10$^{-6}$, and
f(CS)$\sim$10$^{-6}$ up to the photo-dissociation region. The observations and
analysis presented here show evidence that the circumstellar material displays
clear deviations from an homogeneous spherical wind, with clumps and low
density shells that may allow UV photons from the interstellar medium (ISM) to
penetrate deep into the envelope, shifting the photo-dissociation radius
inwards. Our chemical model predicts photo-dissociation radii compatible with
those derived from the observations, although it is unable to predict abundance
variations from the starting radius of the calculations ($\sim$10$R_{*}$),
which may reflect the simplicity of the model. We conclude that the spatial
distribution of the gas proves the episodic and variable nature of the mass
loss mechanism of IRC+10216, on timescales of hundreds of years.
