JCMT ¹² CO <i>J</i> = 3–2 Observations of Tycho’s Supernova Remnant: Constraints on the Environmental Gas Properties

Abstract 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 12 CO J = 3–2 observation and compare with previous 12 CO J = 2–1, 12 CO J = 1–0 and 13 CO J = 1–0 data. We present the 12 CO J = 3–2 map toward Tycho and show that the 12 CO 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 (H 2 ) = 0.5–4.5 × 10 22 cm −2 , while other regions have N (H 2 ) = 0.2–3.9 × 10 21 cm −2 ; the kinetic temperatures T k of these clouds are in the range of 9–22 K, and the volume densities n (H 2 ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>″</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 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.