Herschel Hi-GAL imaging of massive young stellar objects
  We used Herschel Hi-GAL survey data to determine whether massive young
stellar objects (MYSOs) are resolved at 70$\mu$m and to study their envelope
density distribution. Our analysis of three relatively isolated sources in the
l=30{\deg} and l=59{\deg} Galactic fields show that the objects are partially
resolved at 70$\mu$m. The Herschel Hi-GAL survey data have a high scan velocity
which makes unresolved and partially resolved sources appear elongated in the
70$\mu$m images. We analysed the two scan directions separately and examine the
intensity profile perpendicular to the scan direction. Spherically symmetric
radiative transfer models with a power law density distribution were used to
study the circumstellar matter distribution. Single dish sub-mm data were also
included to study how different spatial information affects the fitted density
distribution. The density distribution which best fits both the 70$\mu$m
intensity profile and SED has an average index of ~0.5. This index is shallower
than expected and is probably due to the dust emission from bipolar outflow
cavity walls not accounted for in the spherical models. We conclude that 2D
axisymmetric models and Herschel images at low scan speeds are needed to better
constrain the matter distribution around MYSOs.
