Detection of CO$_2$ ice in the planetary nebula NGC 6302

Abstract

Using JWST/MIRI observations, we report the detection of CO$_2$ ice in the dusty torus of the planetary nebula NGC 6302, an environment generally considered hostile to fragile molecular species and ices due to intense UV irradiation. This detection accompanies cold (20-50 K) gas-phase CO$_2$ along the same sightlines. The ice absorption profile exhibits a double-peak profile, a characteristic of pure, crystalline CO$_2$ ice. The CO$_2$ gas-to-ice ratio is more than an order of magnitude higher than in young stellar objects, pointing to distinct ice formation or processing mechanisms in evolved stellar environments. This discovery demonstrates that the dusty torus provides sufficient shielding to harbour ice chemistry, and that ice-mediated surface reactions must be incorporated into chemical models of planetary nebulae.

Figures (6)

• Figure 1: Detection of CO2 ice and gas. The black curve on top is the JWST/MIRI spectrum at the position indicated by the dot in Fig. \ref{['fig:HST']}. Uncertainties on the surface brightness are $\sim \pm$ 40 MJy sr$^{-1}$ . The best-fit gas-phase model ($T_{\rm gas}=30$ K and $\log N$=16.8 cm$^{-2}$) is shown in blue. We divided out this model, resulting in the brown curve. The orange curve shows a single-temperature model for clinoenstatite emission. The grey dashed line is the continuum we adopt to analyse the ice band. The green curve shows the normalized spectrum using that continuum. The red curve is a laboratory spectrum for CO2 ice (CO:CO$_2$ 2:1 ice mixture at 80 K); and the bottom panel shows the residuals between the normalized spectrum (green) and the ice model (red).
• Figure 2: Comparison of observational and laboratory ice spectra. (a) Co-added CO2 absorption profile across all pixels with detectable CO2 (gray), with the smoothed profile after masking the H i line at 14.96 $\mu$m overplotted in black. Laboratory spectra: (b) CO:CO$_2$ (2:1) at 80 K (green); (c)H2O:CH3OH:CO2 (0.8:0.9:1) at 125 K (blue); (d) pure CO2 at 25 K (brown); (e) pure CO2 on amorphous silicate spheres at 15 K (red); (f)H2O:CO2 (1:6) at 75 K (pink).
• Figure 3: Location of CO2 ice in NGC 6302. The image shows HST/WFC3 observations featuring filter F656N 2022ApJ...927..100K, which traces H$\alpha$ emission. The JWST MIRI mosaic is indicated by the white frame. Contours show the column density of gas-phase CO2, with corresponding log N values (cm$^{-2}$) provided in the lower left. The yellow dot marks the pixel position (R.A. $=$ 17$^h$:13$^m$:44.402$^s$, Dec. $=$ -37$^\circ$:06$'$:10.23 (J2000))used to extract the spectra shown in the Figs. \ref{['fig:spectrum']}, \ref{['fig:iceprofiles']}, \ref{['fig:12CO2_and_13CO2fit']}, \ref{['fig:clinoenstatite']}.
• Figure 4: Spatial distribution of excitation temperature of CO2 gas. The derived best-fit temperature of gas-phase CO2 is mapped on the ALMA $^{12}$CO J$=$2--1 map 2026ApJ...998....7M. The temperature map reveals two blobs of colder CO2 gas, which might indicate clumps in the torus, but needs further investigation.
• Figure 5: A representative fit of $^{12}$CO$_2$ and $^{13}$CO$_2$. Normalized observed spectrum towards the northern part of the torus facing us (R.A. $=$ 17$^h$:13$^m$:44.402$^s$, Dec. $=$ -37$^\circ$:06$'$:10.23$"$ (J2000)) is shown in black and best-fit $^{12}$CO$_2$ model and $^{13}$CO$_2$ model are shown in blue and red, respectively.