First Images of Phosphorus Molecules toward a Protosolar Analog

The chemistry of phosphorus in star- and planet-forming regions is poorly understood, despite the central role of phosphorus in terrestrial biochemistry. We present Atacama Large Millimeter/submillimeter Array band 3 and 4 observations of PO and PN toward the Class I protostar B1-a, representing the first spatially resolved observations of phosphorus carriers toward a solar-type star-forming region. The phosphorus molecules emit from two distinct clumps, which coincide with regions where the protostellar outflow (traced by SiO) interacts with a filament of dense gas (traced by CCS). Thus, the gas-phase phosphorus seems to originate from the shocking of dense interstellar clumps. Based on the observed emission patterns, PO and PN appear to be daughter products of a solid phosphorus carrier with an intermediate volatility between ices and silicate grains. Interstellar shocks may therefore play an important role in converting semi-refractory phosphorus to a more volatile form prior to incorporation into cometary ices. Indeed, the (PO+PN)/CH₃OH ratio is similar in B1-a and comet 67P, implying a comparable reservoir of volatile phosphorus. The PO/PN ratio ranges from ~1-8 across B1-a. The northern emission clump exhibits a lower PO/PN ratio and weaker ¹³CH₃OH emission than southern clump, indicating distinct shock physics and chemistry at the two positions. Resolved observations of P carriers toward additional sources are needed to better understand what regulates such variations in the PO/PN ratio in protostellar environments.