  Stellar activity patterns are responsible for jitter effects that are
observed at different timescales and amplitudes. These effects are currently in
the focus of many exoplanet search projects, since the lack of a well-defined
characterization and correction strategy hampers the detection of the signals
associated with small exoplanets. Accurate simulations of the stellar
photosphere can provide synthetic time series data. These may help to
investigate the relation between activity jitter and stellar parameters when
considering different active region patterns. Moreover, jitters can be analysed
at different wavelength scales in order to design strategies to remove or
minimize them. In this work we present the StarSim tool, which is based on a
model for a spotted rotating photosphere built from the integration of the
spectral contribution of a fine grid of surface elements. The model includes
all significant effects affecting the flux intensities and the wavelength of
spectral features produced by active regions and planets. A specific
application for the characterization and modelling of the spectral signature of
active regions is considered, showing that the chromatic effects of faculae are
dominant for low temperature contrasts of spots. Synthetic time series are
modelled for HD 189733. Our algorithm reproduces both the photometry and the
RVs to good precision, generally better than the studies published to date. We
evaluate the RV signature of the activity in HD 189733 by exploring a grid of
solutions from the photometry. We find that the use of RV data in the inverse
problem could break degeneracies and allow for a better determination of some
stellar and activity parameters. In addition, the effects of spots are studied
for a set of simulated transit photometry, showing that these can introduce
variations which are very similar to the signal of an atmosphere dominated by
dust.
