${}^{12}$CO $J$=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 multi-wavelength morphology. To constrain the physical properties of environmental MCs and search for heated gas, we perform a JCMT ${}^{12}$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 (Radiative transfer code in non-Local Thermodynamic Equilibrium) models, we constrain the physical properties of molecular gas surrounding Tycho: the northern cloud has a molecular column density of $N({\mathrm{H}}_2)=0.5$ -- $4.5\times 10^{22}$ cm${}^{-2}$, while other regions have $N({\mathrm{H}}_2)=0.2$ -- $3.9\times 10^{21}$ cm${}^{-2}$; the kinetic temperatures $T_{\mathrm{k}}$ of these clouds are in the range of 9 -- 22 K and the volume densities $n({\mathrm{H}}_2)$ 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 $0.2^{\prime \prime }$ 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.

11 pages, 5 figures. This article is accepted for publication in ApJ