  We present the new ARTEMIS emulator suite of high resolution (baryon mass of
$2.23 \times 10^{4}$ $h^{-1}$M$_{\odot}$) zoom-in simulations of Milky Way mass
systems. Here, three haloes from the original ARTEMIS sample have been rerun
multiple times, systematically varying parameters for the stellar feedback
model, the density threshold for star formation, the reionisation redshift and
the assumed warm dark matter (WDM) particle mass (assuming a thermal relic).
From these simulations emulators are trained for a wide range of statistics
that allow for fast predictions at combinations of parameters not originally
sampled, running in $\sim 1$ms (a factor of $\sim 10^{11}$ faster than the
simulations). In this paper we explore the dependence of the central haloes'
stellar mass on the varied parameters, finding the stellar feedback parameters
to be the most important. When constraining the parameters to match the
present-day stellar mass halo mass relation inferred from abundance matching we
find that there is a strong degeneracy in the stellar feedback parameters,
corresponding to a freedom in formation time of the stellar component for a
fixed halo assembly history. We additionally explore the dependence of the
satellite stellar mass function, where it is found that variations in stellar
feedback, the reionisation redshift and the WDM mass all have a significant
effect. The presented emulators are a powerful tool which allows for
fundamentally new ways of analysing and interpreting cosmological hydrodynamic
simulations. Crucially, allowing their free (subgrid) parameters to be varied
and marginalised, leading to more robust constraints and predictions.
