  In 2020 the European Space Agency selected Ariel as the next mission to join
the space fleet of observatories to study planets outside our Solar System.
Ariel will be devoted to the characterisation of a thousand planetary
atmospheres, for understanding what exoplanets are made of, how they formed and
how they evolve. To achieve the last two goals all planets need to be studied
within the context of their own host stars, which in turn have to be analysed
with the same technique, in a uniform way. We present the spectro-photometric
method we have developed to infer the atmospheric parameters of the known host
stars in the Tier 1 of the Ariel Reference Sample. Our method is based on an
iterative approach, which combines spectral analysis, the determination of the
surface gravity from {\em Gaia} data, and the determination of stellar masses
from isochrone fitting. We validated our approach with the analysis of a
control sample, composed by members of three open clusters with well-known ages
and metallicities. We measured effective temperature, Teff, surface gravity,
logg, and the metallicity, [Fe/H], of 187 F-G-K stars within the Ariel
Reference Sample. We presented the general properties of the sample, including
their kinematics which allows us to separate them between thin and thick disc
populations. A homogeneous determination of the parameters of the host stars is
fundamental in the study of the stars themselves and their planetary systems.
Our analysis systematically improves agreement with theoretical models and
decreases uncertainties in the mass estimate (from 0.21+/-0.30 to 0.10+/-0.02
M_sun), providing useful data for the Ariel consortium and the astronomical
community at large.
