Water maser variability in a high-mass YSO outburst -- VERA and ALMA
  observations of S255 NIRS 3
  We carried out observations of the 22 GHz H2O masers in a high-mass protostar
S255 NIRS 3 by using VERA. We measured the proper motions of the 22 GHz H2O
masers associated with a bipolar outflow. The expansion velocity of the
blueshifted bow shock traced by the 22 GHz H2O masers was 28 km s-1
corresponding to a dynamical timescale of 60 years. The direction of the maser
outflow is slightly tilted compared with the radio jet, which could suggest a
more recent ejection episode during the accretion burst event. The total flux
density of the 22 GHz H2O masers has gradually increased from early 2017 and
has become almost constant in 2018. For the first time, we reveal extended H2O
maser emission at 22 GHz in a star forming region, which is partly resolved out
by VERA and even by the most extended VLA configurations. We find that the flux
variation of such an extended component is similar to that of the unresolved
maser emission. We also conducted observations of the submillimeter continuum
and the 321 GHz H2O masers with ALMA at Band 7. The continuum emission does not
show significant variations compared with the previous observations performed 5
months before. We mapped the 321 GHz H2O masers in S255 NIRS 3 providing the
fourth example, for this maser, of the spatial distribution in a high-mass
star-forming region. The lower ratio of the 22 GHz/321 GHz maser luminosity in
the blueshifted bow shock suggests a temperature (>1000 K), higher than for the
other maser features in this region. We conclude that the bow shock structure
traced by the 22 GHz H2O maser features is unlikely to be originating at the
interface between the radio jet powered by the recent accretion outburst and
the surrounding medium. The brightening of the 22 GHz H2O masers could be due
to radiative excitation by photons form the infrared outburst escaping along
the cavity created by the newly ejected material.
