Infrared spectra of methane-containing ice mixtures for JWST data analysis

TL;DR

This study addresses the scarcity of low-temperature methane spectra in non-H2O ice matrices, which hampers interpretation of interstellar and protostellar ices observed by JWST. Using the ISEAge setup, the authors obtain FTIR transmission spectra of CH4 in binary mixtures (CH4:H2O, CH4:CO2, CH4:CH3OH, CH4:NH3) at 6.7 K (phase II*) and 10 K (phase II), and provide corresponding band strengths. They demonstrate that CH4 in mixtures exhibits notable spectral differences from pure methane, including a ~20% increase in deformation-band strengths at 10 K, and they present a public spectral library for non-H2O methane environments, enabling more accurate JWST fits. The work is applied to JWST MIRI-MRS data for the protostar B335, revealing that methane occupies both H2O- and CO2-rich ice environments with spatial variation, and highlighting the significance of non-H2O methane reservoirs in cold astrophysical environments. Overall, the dataset and findings improve methane abundance estimates and pave the way for more nuanced interpretation of JWST observations of ices in star-forming regions. The spectra are publicly available on Zenodo for direct use in JWST data fitting and analysis.

Abstract

Context. Solid methane (CH$_4$) is an important molecule in interstellar and planetary environments, serving as a precursor to complex organic compounds and a potential biosignature in exoplanetary studies. Despite its significance, laboratory data on low-temperature phase of methane below 10 K remain limited. Aims. We aim to obtain spectra of methane in binary mixtures at 10 K and compare it to the spectra obtained at 6.7 K. These temperatures correspond to phases II and II* of pure methane and are representative of dark molecular clouds and protostars at early stages. We also aim to test the obtained data applicability to JWST data interpretation. Methods. Laboratory reference spectra were obtained on the ISEAge setup via FTIR spectroscopy in transmission mode. A weighted $χ^2$ minimization is used for the fitting. Results. We present infrared spectra with corresponding band strengths of pure methane and binary mixtures with methane: CH$_4$:H$_2$O,CH$_4$:CO$_2$, CH$_4$:CH$_3$OH, CH$_4$:NH$_3$ at 6.7 K and 10 K showing a 20\% increase in mixtures compared to commonly used 10 K band strength value of pure methane. We also test the usability of the spectra on open JWST data by probing the spatial distribution of methane in B335. We also present additional experiments concerning the phase transition of methane between phase II* and phase II. Conclusions. Our results reveal distinct spectral features for methane in non-H$_2$O environments, enabling more accurate interpretation of JWST observations. The dataset of spectra, publicly available on Zenodo, can be used for fitting JWST data.

Figures (4)

• Figure 1: The main vibrational modes of methane at 6.7 K and 10 K: the stretching mode at 3000 cm$^{-1}$ (3.33 $\mu$m, $\nu_3$) and the deformation mode at 1300 cm$^{-1}$ (7.7 $\mu$m, $\nu_4$), black is the pure methane, yellow is the CH$_4$:H$_2$O = 1:10 mixture, blue is the CH$_4$:CO$_2$ = 1:5 mixture, light blue is the CH$_4$:CO$_2$ = 1:15 mixture, green is the CH$_4$:CH$_3$OH = 1:3 mixture, purple is the CH$_4$:NH$_3$ = 1:3 mixture. Column density of CH$_4$ in all the IR spectra equals to $4.25\times 10^{16}$ cm$^{-2}$.
• Figure 2: B335 intensity map at 8 $\mu$m using the JWST MIRI-MRS spectrograph data. Small circles denote the 1.53$^{\prime\prime}$ apertures, big circle denotes the 4.59$^{\prime\prime}$ aperture. The center coordinates of chosen apertures are listed in Table \ref{['apertures']}.
• Figure 3: Observational data fitted with ISEAge laboratory mixtures (solid lines). Orange color shows CH$_4$:H$_2$O = 1:10 mixture at 10 K, blue color --- CH$_4$:CO$_2$ = 1:15 at 10 K, green color --- CH$_4$:CH$_3$OH = 1:3 at 10 K, and OCN$^-$ is presented in gray color. Final fit is presented in red color.
• Figure 4: Top panel --- selected IR spectra of pure methane deposition from 20th to 65th minutes with 6.7 K temperature (column density (0.7 to 2.3)$\times10^{16}$ cm$^{-2}$); bottom panel --- IR spectra of pure methane deposition from 20th to 65th minutes with 10 K temperature (column density (0.7--2.3)$\times10^{16}$ cm$^{-2}$). The dashed red line shows the peak position of methane in phase II*.