Spitzer Observations of Molecular Hydrogen in Interacting Supernova Remnants

With Spitzer IRS we have obtained sensitive low-resolution spectroscopy from 5 to 35 microns for six supernova remnants (SNRs) that show evidence of shocked molecular gas: Kes 69, 3C 396, Kes 17, G346.6-0.2, G348.5-0.0 and G349.7+0.2. Bright, pure-rotational lines of molecular hydrogen are detected at the shock front in all remnants, indicative of radiative cooling from shocks interacting with dense clouds. We find the excitation of H2 S(0)-S(7) lines in these SNRs requires two non-dissociative shock components: a slow, 10 km/s C- shock through clumps of density 10^6 cm^-3, and a faster, 40-70 km/s C- shock through a medium of density 10^4 cm^-3. The ortho-to-para ratio for molecular hydrogen in the warm shocked gas is typically found to be much less than the LTE value, suggesting that these SNRs are propagating into cold quiescent clouds. Additionally a total of thirteen atomic fine-structure transitions of Ar+, Ar++, Fe+, Ne+, Ne++, S++, and Si+ are detected. The ionic emitting regions are spatially segregated from the molecular emitting regions within the IRS slits. The presence of ionic lines with high appearance potential requires the presence of much faster, dissociative shocks through a lower density medium. The IRS slits are sufficiently wide to include regions outside the SNR which permits emission from diffuse gas around the remnants to be separated from the shocked emission. We find the diffuse molecular hydrogen gas projected outside the SNR is excited to a temperature of 100 to 300 K with a warm gas fraction of 0.5 to 15 percent along the line of sight.

💡 Research Summary

This paper presents low‑resolution Spitzer IRS spectroscopy (5–35 µm) of six supernova remnants (SNRs) that exhibit clear signatures of shocked molecular gas: Kes 69, 3C 396, Kes 17, G346.6‑0.2, G348.5‑0.0, and G349.7+0.2. The authors obtained spectra with both the Short‑Low (SL) and Long‑Low (LL) modules, using slit widths large enough to encompass both the shocked regions and the surrounding diffuse medium. In every remnant, bright pure‑rotational lines of molecular hydrogen from S(0) through S(7) were detected. By constructing H₂ excitation diagrams and fitting them with shock models, the authors demonstrate that a single shock component cannot reproduce the observed line intensities. Instead, two non‑dissociative C‑type shocks are required: (1) a slow (≈10 km s⁻¹) shock propagating through very dense clumps (n ≈ 10⁶ cm⁻³) and (2) a faster (40–70 km s⁻¹) shock moving through a more tenuous medium (n ≈ 10⁴ cm⁻³). Both shocks are non‑dissociative, heating the gas to several hundred to a few thousand kelvin, which accounts for the observed S‑branch line ratios.

A striking result is the ortho‑to‑para ratio (OPR) of the warm H₂, which is systematically below the LTE value of 3, typically ranging from 0.5 to 1.5. This low OPR indicates that the pre‑shock molecular gas was very cold (∼10 K) and that the shock has heated the gas faster than the spin conversion processes can equilibrate, implying that the SNRs are currently propagating into quiescent, cold molecular clouds.

In addition to molecular lines, the spectra reveal thirteen atomic fine‑structure transitions, including lines from Ar⁺, Ar²⁺, Fe⁺, Ne⁺, Ne²⁺, S²⁺, and Si⁺. These ions have high excitation potentials (≥20 eV), requiring the presence of much faster, dissociative J‑type shocks in lower‑density gas. Spatial analysis within the IRS slits shows that the ionic emission is offset from the molecular H₂ emission, confirming that the two shock regimes occupy distinct zones: dense clumps experience gentle C‑shocks, while surrounding lower‑density material is swept up by faster, ionizing shocks.

Because the IRS slits extend beyond the SNR boundaries, the authors were able to separate emission from the diffuse interstellar medium projected along the line of sight. The diffuse H₂ component outside the remnants exhibits temperatures of 100–300 K and accounts for only 0.5–15 % of the total column of warm gas, consistent with typical Galactic diffuse molecular clouds.

The paper discusses the implications of these findings for the interaction of SNRs with the interstellar medium (ISM). The coexistence of multiple shock components demonstrates that SNRs can simultaneously compress and heat dense molecular clumps while also driving high‑velocity, ionizing shocks into more tenuous gas. The low OPR provides a diagnostic of the pre‑shock conditions, suggesting that many SNRs encounter cold, previously undisturbed molecular material. Moreover, the detection of both molecular and ionic lines within the same remnants offers a powerful tool for probing the physical structure, energetics, and chemical evolution of shocked regions.

In conclusion, the Spitzer IRS observations establish that the six studied SNRs host a complex shock environment characterized by (i) a slow, high‑density C‑shock responsible for bright H₂ rotational emission, (ii) a faster, moderate‑density C‑shock that contributes to higher‑J H₂ lines, and (iii) fast, dissociative shocks that generate the observed ionic fine‑structure lines. The ability to separate shocked emission from background diffuse gas further refines our understanding of the energy budget and the role of SNRs in processing the Galactic ISM. These results underscore the importance of mid‑infrared spectroscopy for dissecting the multi‑phase nature of supernova‑driven feedback in galaxies.