Unveiling the molecular gas composition in young protostellar systems with JWST/MIRI

We will present the results on the molecular gas content of young embedded protostellar systems derived from the JWST Observations of Young protoStars (JOYS) program. Due to the unprecedented resolution and sensitivity of MIRI-MRS, we resolve the emission lines of several molecules including CO, H2O, CO2, C2H2, CH4, and even SiO toward more than a dozen low-mass protostars.

Molecular emission is commonly observed around protostellar systems with millimeter interferometric observatories such as ALMA, originating from all the protostellar components (i.e., envelope, disk, jet/outflow). However, ALMA detects only rotational transitions emerging mostly from colder (< few hundred K) regions. Moreover, ALMA cannot detect important molecules that lack a permanent dipole moment such as CO2, C2H2, and CH4. Therefore, to study the warm and hot regions of protostellar systems (i.e., inner disk, jet shocks), one needs observations of molecules in their ro-vibrational bands at infrared wavelengths.

With JWST/MIRI-MRS, this is now possible for low-mass protostellar systems due to the higher spectral and spatial resolution and sensitivity than previous space observatories such as Spitzer. Through the detection of multiple transitions per molecule, we can perform an excitation analysis to derive column densities (and their ratios) and determine their physical origin. These range from hot (>500 K) shocked regions in the outflow and jet to the warm (~100-300 K) inner regions of the protostellar disk and its surface layers. The derived column densities can be compared to those detected in the ices in absorption toward the same sources (e.g., for CO2, CH4) to determine whether their chemical origin lies in solid-state or gas-phase chemistry and to constrain the C/O ratios in the gas at this earliest planet formation stage. Furthermore, molecular emission observed in the outflows can be compared to those detected at millimeter wavelengths with ALMA to further determine the chemistry in outflows.