Molecular Gas toward Supernova Remnant Cassiopeia A

We mapped 12CO J = 1–0, 12CO J = 2–1, 13CO J = 1–0, and 13CO J = 2–1 lines toward supernova remnant (SNR) Cassiopeia A with the IRAM 30 m telescope. The molecular clouds (MCs) along the line of sight of Cas A do not show optically thin, shock-broadened 12CO lines (${\rm{\Delta }}V\leqslant 7\,\mathrm{km}\,{{\rm{s}}}^{-1}$ toward Cas A), or high-temperature features from shock heating (${T}_{{\rm{k}}}\leqslant 22\,{\rm{K}}$ toward Cas A). Therefore, we suggest that there is no physical evidence to support that the SNR is impacting the molecular gas. All the detected MCs are likely in front of Cas A, as implied by the HCO+ absorption line detected in the same velocity ranges. These MCs contribute H2 column densities of $5\times {10}^{21}\,\,{\mathrm{cm}}^{-2}$, $5\times {10}^{21}\,\,{\mathrm{cm}}^{-2}$, and $2\times {10}^{21}\,\,{\mathrm{cm}}^{-2}$ in the west, south, and center of the SNR, respectively. The 20 K warm gas at ${V}_{\mathrm{LSR}}\sim -47\,\mathrm{km}\,{{\rm{s}}}^{-1}$ is distributed along a large-scale molecular ridge in the south of Cas A. Part of the gas is projected onto Cas A, providing a foreground H2 mass of $\sim 200{(d/3\mathrm{kpc})}^{2}\,{M}_{\odot }$, consistent with the mass of cold dust (15–20 K; 2–4 ${M}_{\odot }$) found in front of the SNR. We suggest that the 20 K warm gas is heated by background cosmic rays with an ionization rate of $\zeta ({{\rm{H}}}_{2})\sim 2\times {10}^{-16}\,{{\rm{s}}}^{-1}$. The cosmic rays and X-ray emission from Cas A are excluded as the heating sources of the clouds.