Solid and Gaseous Methane in IRAS 23385+6053 as seen with Open JWST Data

Abstract: We present a new description of the 7.7~$\mu$m region towards the high-mass star-forming region IRAS 23385+6053 taken from open James Webb Space Telescope Mid-Infrared Instrument Medium Resolution Spectrometer (JWST MIRI/MRS) data. This area is commonly attributed to the $\nu_4$ deformation mode of methane ice. For the first time gaseous and solid methane were analyzed simultaneously in IRAS 23385+6053. The band at 7.58--7.8 $\mu$m (1320--1280 cm$^{-1}$) is interpreted as a wide solid absorption methane feature overlapped by the sharp features of the methane emission. We report the detection of gaseous methane and estimate its emitting area radius~$R$, temperature~$T$ and column density~$N$ as $R=2940$~au, $T=103^{+13}_{-11}$~K, and $N=0.78_{+6.18}^{-0.64}\times10^{17}$~cm$^{-2}$, correspondingly. The ice content was analyzed with the laboratory spectra dataset of methane in different molecular environments obtained on the Ice Spectroscopy Experimental Aggregate (ISEAge). We were able to describe the wide feature of solid methane with the following laboratory spectra: CH$4$~:~CO$_2$~=~1~:~5 (at $27.4^{+6.0}{-10.8}$~K) and CH$4$~:~H$_2$O~=~1~:~10 (at $8.4^{+16.4}{-1.7}$~K) deposited at 6.7~K and warmed up at a rate of 0.5 K per minute. The derived column densities are $N_{\text{CH}4}$(CO$_2$)~=~$2.97^{+0.37}{-0.57}\times10^{17}$~cm$^{-2}$ and $N_{\text{CH}4}$(H$_2$O)~=~$1.02^{+0.46}{-0.27}\times10^{17}$~cm$^{-2}$. According to the best fit solid methane is mostly surrounded by CO$_2$ rather than H$_2$O. The residuals analysis reveals the unassigned region at 1283--1297~cm$^{-1}$ (7.71--7.79~$\mu$m) which is tentatively assigned to nitrous oxide (N$_2$O) in various environments.