A Multi-Epoch Study of Radio Continuum Emission from MassiveProtostars
  We report the results of the Jansky Very Large Array (JVLA) observation of
five massive protostars at 6 and 22.2 GHz. The aim of the study was to compare
their current fluxes and positions with previous observations to search for
evidence of variability. Most of the observed sources present the morphologies
of a thermal core, hosting the protostar and exhibiting no proper motion, and
associated non-thermal radio lobes that are characterised by proper motions and
located away from the thermal core. Some of the protostars drive jets whose
lobes have dissimilar displacement vectors, implying precession of the jets or
the presence of multiple jet drivers. The jets of the protostars were found to
have proper motions that lie in the range 170$ \leq v \leq $650 kms$^{-1}$, and
precessions of periods 40$\leq p \leq$50 years and angles 2$\leq \alpha
\leq$10$^\circ$, assuming that their velocities $v=$500 kms$^{-1}$. The core of
one of the sources, S255 NIRS 3, which was in outburst at the time of our
observations, showed a significant change in flux compared to the other
sources. Its spectral index decreased during the outburst, consistent with the
model of an expanding gas bubble. Modelling the emission of the outburst as
that of a new non-thermal lobe that is emerging from a thermal core whose
emission enshrouds that of the lobe also has the potential to account for the
increase in flux and a decrease in the spectral index of the source's outburst.
