Theoretical analysis of surface brightness-colour relations for
  late-type stars using MARCS model atmospheres
  Surface brightness-colour relations (SBCRs) are largely used for general
studies in stellar astrophysics and for determining extragalactic distances.
Based on simulated spectra of late-type stars using MARCS model atmospheres,
our aim is to analyse the effect of stellar fundamental parameters on the
surface brightness. We also compare theoretical and recent empirical SBCRs. We
used MARCS model atmospheres to compute spectra and the surface brightness of
stars. We first explored the parameter space of MARCS (i.e. effective
temperature, $\log g$, $\mathrm{[Fe/H]}$, microturbulence, and mass) in order
to quantify their impact on the surface brightness. Then we considered a
relation between the effective temperature and $\log g$ for late dwarfs and
giants, as well as a solar metallicity, in order to allow a consistent
comparison of theoretical and empirical SBCRs. We find that the SBCR is not
sensitive to the microturbulence and mass. The effect of metallicity on the
SBCR is found to be larger for dwarfs than for giants. It is also larger when
considering larger $V-K_s$ values. We also find that a difference of 0.5 dex in
metallicity between Galactic and LMC SBCRs does not affect the recent LMC
distance determination, based on eclipsing binaries, by more than 0.4%. By
comparing theoretical with empirical SBCRs, we find a good agreement of less
than 2$\sigma$ for F5-K7 dwarfs and giants stars, while a larger discrepancy is
found for M dwarfs and giants (about 4-6$\sigma$). The surface gravity
properties, as modelled in MARCS, explain the differences in the empirical
SBCRs in terms of class. We finally find that theoretical and empirical SBCRs
for Cepheids are consistent. Carefully considering metallicity and $\log g$ is
mandatory when calibrating or using SBCRs.
