CORINOS II. JWST-MIRI detection of warm molecular gas from an embedded, disk-bearing protostar

We present James Webb Space Telescope (JWST) Mid-InfraRed Instrument (MIRI) observations of warm CO and H$_2$O gas in emission toward the low-mass protostar IRAS 15398-3359, observed as part of the CORINOS program. The CO is detected via the rovibrational fundamental band and hot band near 5 $μ$m, whereas the H$_2$O is detected in the rovibrational bending mode at 6-8 $μ$m. Rotational analysis indicates that the CO originates in a hot reservoir of $1598\pm118$ K, while the water is much cooler at $204\pm 7$ K. Neither the CO nor the H$_2$O line images are significantly spatially extended, constraining the emission to within $\sim$40 au of the protostar. The compactness and high temperature of the CO are consistent with an origin in the embedded protostellar disk, or a compact disk wind. In contrast, the water must arise from a cooler region and requires a larger emitting area (compared to CO) to produce the observed fluxes. The water may arise from a more extended part of the disk, or from the inner portion of the outflow cavity. Thus, the origin of the molecular emission observed with JWST remains ambiguous. Better constraints on the overall extinction, comparison with realistic disk models, and future kinematically-resolved observations may all help to pinpoint the true emitting reservoirs.