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 (subarcsecond) observations of13CH3OHν= 0 (∼0.″3 and ∼0.″7) and CH3CNν8= 1 (∼0.″2 and ∼0.″9) line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate Orion KL’s structure on small spatial scales (≤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. Subregions of the Hot Core have a higher excitation temperature in a 0.″2 beam than in a 0.″9 beam, indicative of possible internal sources of heating. Furthermore, the velocity field includes a bipolar ∼7–8 km s−1feature with a southeast–northwest orientation against the surrounding ∼4–5 km s−1velocity 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 more »

Authors:
; ;
Publication Date:
NSF-PAR ID:
10361283
Journal Name:
The Astrophysical Journal
Volume:
924
Issue:
1
Page Range or eLocation-ID:
Article No. 4
ISSN:
0004-637X
Publisher:
DOI PREFIX: 10.3847
3. ABSTRACT In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH+ observed along diffuse molecular sightlines. Intermittent high temperatures should also have an impact on H2 line luminosities. We carry out simulations of magnetohydrodynamic (MHD) turbulence in molecular clouds including heating and cooling, and post-process them to study H2 line emission and hot-gas chemistry, particularly the formation of CH+. We explore multiple magnetic field strengths and equations of state. We use a new H2 cooling function for $n_{\text{H}}\le 10^5\, {\text{cm}}^{-3}$, $T\le 5000\, {\text{K}}$, and variable H2 fraction. We make two important simplifying assumptions: (i) the H2/H fraction is fixed everywhere and (ii) we exclude from our analysis regions where the ion–neutral drift velocity is calculated to be greater than 5 km s−1. Our models produce H2 emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic root-mean-square (rms) magnetic field strengths (≈10 μG) and velocity dispersions, we reproduce observed CH+ abundances. These findings contrast with those of Valdivia et al. (2017) Comparison of predicted dust polarization with observations by Planck suggests thatmore »