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

Sendi Bo, Yu Huang, Ping Zhou, Tian-Yu Tu, Samar Safi-Harb, Zhi-Yu Zhang, Yang Chen, Hidetoshi Sano

TL;DR

Tycho's SNR likely expands into a molecular cavity wall, making it essential to constrain the surrounding gas to understand its evolution and emission. The authors combine JCMT $^{12}$CO J=3-2 data with archival CO transitions and RADEX-based MCMC modeling to derive the physical properties of nearby MCs, finding predominantly cold ($T_k$ ≈ 9–22 K) and modestly dense gas ($n({ m H_2})$ ≈ 20–700 cm$^{-3}$) with north-region $N({ m H_2})$ ≈ 0.4–4.5×10$^{22}$ cm$^{-2}$. They do not detect compelling evidence of shocked, hot molecular gas within the current angular resolution, arguing that the potentially very thin shocked layer (~0.003 pc, ≈0.2 arcsec at 2.5 kpc) is unresolved, and that beam dilution by unshocked gas dominates the signal. The results support the cavity-wall interpretation and motivate higher-resolution follow-up (e.g., interferometry) to directly detect a thin shocked layer and to better connect the molecular environment with Tycho's dynamical evolution and progenitor scenario.

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.

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

TL;DR

Tycho's SNR likely expands into a molecular cavity wall, making it essential to constrain the surrounding gas to understand its evolution and emission. The authors combine JCMT CO J=3-2 data with archival CO transitions and RADEX-based MCMC modeling to derive the physical properties of nearby MCs, finding predominantly cold ( ≈ 9–22 K) and modestly dense gas ( ≈ 20–700 cm) with north-region ≈ 0.4–4.5×10 cm. They do not detect compelling evidence of shocked, hot molecular gas within the current angular resolution, arguing that the potentially very thin shocked layer (~0.003 pc, ≈0.2 arcsec at 2.5 kpc) is unresolved, and that beam dilution by unshocked gas dominates the signal. The results support the cavity-wall interpretation and motivate higher-resolution follow-up (e.g., interferometry) to directly detect a thin shocked layer and to better connect the molecular environment with Tycho's dynamical evolution and progenitor scenario.

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 CO =3--2 observation and compare with previous CO =2--1, CO =1--0 and CO =1--0 data. We present the CO =3--2 map toward Tycho and show that the CO =3--2 spatial distribution and line profiles are similar to those of the lower- 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 -- cm, while other regions have -- cm; the kinetic temperatures of these clouds are in the range of 9 -- 22 K and the volume densities are 20 -- cm. 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 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.

Paper Structure

This paper contains 13 sections, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. JCMT observations
  4. Other data
  5. IRAM 30m data
  6. MWISP Data
  7. Chandra image
  8. Results
  9. Discussion
  10. Physical Properties of the Molecular Clouds
  11. On the lack of MC-shock interaction evidence
  12. Summary
  13. RADEX parameters and MCMC calculation

Figures (5)

  • Figure 1: The $^{12}$CO $J$=3--2 velocity-averaged intensity maps with a step of 2 $\,{\rm km}\,{\rm s}^{-1}$ ranging from $-71$$\,{\rm km}\,{\rm s}^{-1}$ to $-53$$\,{\rm km}\,{\rm s}^{-1}$, overlaid with contours of the Chandra X-ray map. To increase the signal-to-noise ratio, the maps use the smoothed data with an angular resolution of 30$"$ and a pixel size of $15"$. Data with the intensity above $2\sigma$ are shown in the plot.
  • Figure 2: The main panel shows a grid map of the line profiles of $^{12}$CO $J$=3--2, $^{12}$CO $J$=2--1 and $^{12}$CO $J$=1--0 toward Tycho, overlaid with the Chandra hard X-ray flux in 4.1 -- 6.1 keV. The black, yellow and red lines represent $^{12}$CO $J$=1--0, $^{12}$CO $J$=2--1 and $^{12}$CO $J$=3--2 line profiles, respectively. The zoomed-in panels illustrate spectra from selected regions "N", "NE", "SE", "SW", with $^{13}$CO $J$=1--0 spectra (blue lines) added. The solid lines are the spectra we observed while the dashed lines represent fit results using the Gaussian function. The gray shadows denote the velocity components we use to study molecular gas properties with RADEX.
  • Figure 3: The left panel displays the intensity ratio maps of $^{12}$CO $J$=3--2 to $^{12}$CO $J$=2--1 from $V_{\rm LSR}=-71~\,{\rm km}\,{\rm s}^{-1}$ to $V_{\rm LSR}=-52~\,{\rm km}\,{\rm s}^{-1}$, while the right panel shows the ratio maps of $^{12}$CO $J$=2--1 to $^{12}$CO $J$=1--0. The spatial resolutions of the two maps are $30"$ and $55"$, respectively. The Chandra X-ray contours are overlaid. The red squares mark the four regions where we later calculate gas parameters using RADEX (see Table \ref{['tab:position']}).
  • Figure 4: MCMC corner plots based on multiple transitions of CO data and RADEX models. The dashed lines indicate the 0.16, 0.5, 0.84 quantiles of 1D PDFs.
  • Figure A.1: The comparison between posterior distribution of line ratios in selected regions and the observed ones. The violin plots show the posterior distribution values from RADEX. The errorbars show the observed values.