A chemical census of volatile ices in protostellar envelopes

We propose to obtain an inventory of bulk volatiles in the youngest protostellar envelopes, just prior to the final assembly of protoplanetary disks. This will trace late-stage chemical evolution of young stellar envelopes, and establish the initial conditions of water and organics for comparison to observations of protoplanetary disk chemistry. To accomplish this goal, we will take advantage of the dense cluster of class 0 protostellar envelopes in the Serpens Main cloud. This cluster is sufficiently dense, and is located at a near-optimum distance and low galactic latitude, to allow us to use the multi-object spectroscopy mode of NIRSpec to obtain 2-5 micron ice absorption spectra toward ~150 highly extincted field stars, including at least ~30 sightlines directly through the protostellar envelopes. Using the incredible sensitivity of NIRSpec, we will be able to probe extinctions as high as Av~75 mag needed to reveal rare ice species. We estimate that we will obtain ~5 lines of sight through each of 6-8 protostellar envelopes, providing radial maps of relative ice abundances on scales of a few thousand au. The column densities of bulk ices are readily observed in the 2-5 micron region, including much of the oxygen-bearing inventory, as well as simple organics (e.g, H2O, CO2, CO, CH3OH). Particularly methanol ice is a critical, yet poorly understood, cornerstone of pre-planetary organic chemistry. The high column densities also allows for sensitive searches for the rare isotopologues 13CO, 13CO2, and HDO. The two former enables studies of dust grain shapes, while the latter may yield the first robust measurements of the deuteration fraction of pre-planetary material.