Herschel Hi-GAL imaging of massive young stellar objects

We used Herschel Hi-GAL (Herschel infrared Galactic Plane survey) data to determine whether massive young stellar objects (MYSOs) are resolved at 70 μm and to study their envelope density distribution. Our analysis of three relatively isolated sources in the l = 30° and 59° Galactic fields show that the objects are partially resolved at 70 μm. The Herschel Hi-GAL survey data have a high scan velocity which makes unresolved and partially resolved sources appear elongated in the 70 μm 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 submm data were also included to study how different spatial information affects the fitted density distribution. The density distribution which best fits both the 70 μm intensity profile and spectral energy distribution 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.