2026ApJ...997..154B

Recent observations suggest that Tycho's supernova remnant (SNR; SN 1572) is expanding into a cavity wall of molecular clouds (MCs), which decelerate the SNR and influence its multiwavelength morphology. To constrain the physical properties of environmental MCs and search for heated gas, we perform a James Clerk Maxwell Telescope ¹²CO J = 3-2 observation and compare with previous ¹²CO J = 2-1, ¹²CO J = 1-0 and ¹³CO J = 1-0 data. We present the ¹²CO J = 3-2 map toward Tycho and show that the ¹²CO J = 3-2 spatial distribution and line profiles are similar to those of the lower-J CO lines. By comparing the multiple transitions of CO and the RADEX models, we constrain the physical properties of molecular gas surrounding Tycho: the northern cloud has a molecular column density of N(H₂) = 0.5-4.5 × 10²² cm^–2, while other regions have N(H₂) = 0.2-3.9 × 10²¹ cm^–2; the kinetic temperatures T_k of these clouds are in the range of 9-22 K, and the volume densities n(H₂) are 20-700 cm^–3. We also discuss the difficulty in finding hot molecular gas shocked by such a young SNR. We estimate that the shocked molecular layer can be as thin as 0.003 pc, corresponding to 0$\mathop{.}\limits^{\unicode{x02033}}$2 at the distance of 2.5 kpc, which is 2 orders of magnitude smaller than the angular resolution of current CO observations. Therefore, our molecular observations are largely insensitive to the thin shocked gas layer; instead, they detect the environmental gas.