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Title:
Infrared Imaging and Spectroscopy of the Molecular Shock in IC 443
Authors:
Richter, Matthew J.; Graham, James R.; Wright, Gillian S.
Publication:
Astrophysical Journal v.454, p.277 (ApJ Homepage)
Publication Date:
11/1995
Origin:
APJ; KNUDSEN
ApJ Keywords:
INFRARED: ISM: LINES AND BANDS, SHOCK WAVES, ISM: SUPERNOVA REMNANTS, ISM: INDIVIDUAL ALPHANUMERIC: IC 443, ISM: MOLECULES
DOI:
10.1086/176481
Bibliographic Code:
1995ApJ...454..277R

Abstract

We present observations of H_{2 }emission from the molecular shock in the supernova remnant IC 443. The 1-0 5(1) line of H_{2 }at 2.122 microns has been imaged at an angular resolution of 2" over a 10' x 10' area including the peak H2 emission near the southeast edge of the remnant and at 0"7 resolution over an area of 50" x 65" including the emission peak. At high resolution, the previously known sinuous ridge of emission is shown to contain a wealth of structure, consisting of features with sizes ranging from 1" up to scales extending beyond the edges of the image. Unlike the optical emission from supernova remnant shocks, the small-scale H2 emission cannot be well described as a filamentary structure, but is better portrayed as an assemblage of knots.

Long-slit spectroscopy from 2 to 4 microns is used to examine the ro-vibrational excitation of H2. The H2 1-0 5(1): 2-1 5(1) line ratio shows little variation over a large range of emission strength, suggesting that column density variations are primarily responsible for brightness differences. The lowest surface brightness emission may include a significant contribution from formation-pumped H2. Upper limits on Brγ emission provide an upper limit to the shock speed, ruling out low Alfvén Mach number bow shocks. Comparing data from different regions of IC 443 with each other and with published Orion Peak 1 data, we find the range of gas temperatures producing emission to be surprisingly uniform. This uniformity rules out the two-shock model of Wang & Scoville which would produce distinctly different line ratios. Comparisons with calculations show that the postshock gas is likely to be in LTE. Arguments based on pressure balance within the remnant require that nH2 ≍4 × 103 cm-3 depending on the dissociation fraction and compression ratio. This preshock density is much lower than previously considered in shock models of H2 emission, but similar to estimates based on the CO observations of van Dishoeck et al. To reach LTE at such a low density, H must be the dominant collision partner for H2, indicating that the shocks in IC 443 must be dissociative.

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