Some interesting OH/H₂O masers

Abstract

These six sources exhibit many of the characteristics common to the “complex” class of OH/H₂O maser associations found in the VLA survey. The tendency for H₂O and OH masers to occur in linear arrangements suggests that they may be associated with shock-compressed boundary layers, dense molecular disks, or jet-like ejecta from young stellar objects. If a proper interpretation of these maser spots can be arrived at, masers should prove to be excellent probes of the dense and energetic environment within a few milliparsecs of massive young stars.

In most cases the maser radial velocities do not show systematic gradients. This is particularly true of H₂O masers, but also holds in general for OH masers. This suggests that the wide velocity spreads observed in many H₂O maser groups is caused by shocks or impact phenomena. The range of velocities observed in a compact maser group frequently contains both ambient and high-velocity components, consistent with maser formation at the interface between dense ambient gas and high-velocity shocks or ejecta from a central source.

Finally, maser spot maps of these regions are often complex and difficult to interpret. It seems likely that many of the process inferred to be going on in these regions, e.g. expanding shocks, rotating disks, energetic outflows, gravitational infall, etc., may all be occurring together. It is therefore not surprising that a complex and somewhat chaotic distribution of maser spots is often found in massive star forming regions.