$^{12}$CO $J$=3--2 Observations of Tycho's supernova remnant: constraints on the environmental gas properties

TL;DR

This study uses multi-transition CO observations and radiative transfer modeling to characterize the molecular environment around Tycho's supernova remnant, revealing cold, dense gas properties and highlighting the challenge of detecting shock-heated gas.

Contribution

First detailed multi-transition CO analysis constraining physical properties of molecular clouds around Tycho's SNR, emphasizing the limitations in detecting thin shocked gas layers.

Findings

Northern cloud has high molecular column density (up to 4.5×10^{22} cm^{-2})

Cloud temperatures range from 9 to 22 K, with densities of 20–700 cm^{-3}

Shocked molecular gas layer is too thin to be detected with current CO observations.

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 $N({\rm H}_{2})=0.5$ -- $4.5\times 10^{22}$ cm$^{-2}$, while other regions have $N({\rm H}_{2})=0.2$ -- $3.9\times10^{21}$ cm$^{-2}$; the kinetic temperatures $T_{\rm k}$ of these clouds are in the range of 9 -- 22 K and the volume densities $n({\rm H}_{2})$ 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 $0.2''$ 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.