2018MNRAS.478.1017O

The existence of multiple layers in the inner Orion nebula has been revealed using data from an Atlas of spectra at 2 arcsec and 12 km s^–1 resolution. These data were sometimes grouped over Samples of 10 arcsecx10 arcsecto produce high signal-to-noise spectra and sometimes grouped into sequences of pseudo-slit Spectra of 12^′′_.8-39 arcsec width for high spatial resolution studies. Multiple velocity systems were found: V_MIF traces the Main Ionization Front (MIF), V_scat arises from back-scattering of V_MIF emission by particles in the background Photon Dissociation Region (PDR), V_low is an ionized layer in front of the MIF and if it is the source of the stellar absorption lines seen in the Trapezium stars, it must lie between the foreground Veil and those stars, V_new,[OIII] may represent ionized gas evaporating from the Veil away from the observer. There are features such as the Bright Bar where variations of velocities are due to changing tilts of the MIF, but velocity changes above about 25 arcsec arise from variations in velocity of the background PDR. In a region 25 arcsec ENE of the Orion-S Cloud one finds dramatic changes in the [OIII] components, including the signals from the V_low,[OIII] and V_MIF,[OIII] becoming equal, indicating shadowing of gas from stellar photons of >24.6 eV. This feature is also seen in areas to the west and south of the Orion-S Cloud.