NICMOS Observations of Shocked H_2 in Orion

TL;DR

This study uses NICMOS images to analyze shocked molecular hydrogen in Orion, revealing detailed shock front structures and comparing observed line ratios with shock models to understand the physical conditions.

Contribution

It provides a detailed, quantitative analysis of H_2 emission in Orion, demonstrating that C-shock models better explain the data than J-shocks, with specific shock velocities and densities.

Findings

Most data fit C-shock models with velocities of 20-45 km/s.

Shock velocities tend to be lower at higher densities.

NICMOS images resolve individual shock fronts in Orion.

Abstract

HST NICMOS narrowband images of the shocked molecular hydrogen emission in OMC-1 are analyzed to reveal new information on the BN/KL outflow. The outstanding morphological feature of this region is the array of molecular hydrogen ``fingers'' emanating from the general vicinity of IRc2 and the presence of several Herbig-Haro objects. The NICMOS images appear to resolve individual shock fronts. This work is a more quantitative and detailed analysis of our data from a previous paper (Schultz etal. 1999). Line strengths for the H_2 1--0 S(4) plus 2--1 S(6) lines at 1.89 micron are estimated from measurements with the Paschen_alpha continuum filter F190N at 1.90 micron, and continuum measurements at 1.66 and 2.15 micron. We compare the observed H_2 line strengths and ratios of the 1.89 micron and 2.12 micron 1--0 S(1) lines with models for molecular cloud shock waves. Most of the data cannot be fit by J-shocks, but are well matched by C-shocks with shock velocities in the range of 20--45 km/s and preshock densities of 10^{4} - 10^{6} cm^{-3}, similiar to values obtained in larger beam studies which averaged over many shocks. There is also some evidence that shocks with higher densities have lower velocities.