Mapping physical parameters in Orion KL at high spatial resolution

Abstract

The Orion Kleinmann-Low nebula (Orion KL) is notoriously complex and exhibits a range of physical and chemical components. We conducted high angular resolution (sub-arcsecond) observations of $^13$CH$_{3}$OH $\nu=0$ ($\sim$0.3$^{\prime\prime}$ and $\sim$0.7$^{\prime\prime}$) and CH$_3$CN $\nu_8=1$ ($\sim$0.2$^{\prime\prime}$ and $\sim$0.9$^{\prime\prime}$) line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate Orion KL's structure on small spatial scales (${\le}350$ au). Gas kinematics, excitation temperatures, and column densities were derived from the molecular emission via a pixel-by-pixel spectral line fitting of the image cubes, enabling us to examine the small-scale variation of these parameters. Sub-regions of the Hot Core have a higher excitation temperature in a 0.2$^{\prime\prime}$ beam than a 0.9$^{\prime\prime}$ beam, indicative of possible internal sources of heating. Furthermore, the velocity field includes a bipolar ${\sim}7{-}8$ km s$^{-1}$ feature with a southeast-northwest orientation against the surrounding ${\sim}4{-}5$ km s$^{-1}$ velocity field, which may be due to an outflow. We also find evidence of a possible source of internal heating toward the Northwest Clump, since the excitation temperature there is higher in a smaller beam versus a larger beam. Finally, the region southwest of the Hot Core (Hot Core-SW) presents itself as a particularly heterogeneous region bridging the Hot Core and Compact Ridge. Additional studies to identify the (hidden) sources of luminosity and heating within Orion KL are necessary to better understand the nebula and its chemistry.