Phosphorus is an essential element for biological systems, but its synthesis and dispersion over the history of the Milky Way has defied galactic evolution models. The molecular hydride form of phosphorus, phosphine (PH3), should be abundant in the atmospheres of low-temperature brown dwarfs and gas giant exoplanets, as it is in Jupiter and Saturn. However, observations to date have found this molecule to be highly depleted compared to vertical mixing models. In this presentation, we report the detection of undepleted phosphine in the atmosphere of the brown dwarf Wolf 1130C, a wide companion to the metal-depleted, thick disk M dwarf-white dwarf binary system Wolf 1130AB. JWST/NIRSpec spectroscopy clearly reveals the 1 and 3 stretch modes of phosphine at 4.3 m, which is visible thanks to the absence of the typically enhanced and overlapping carbon dioxide (CO2) 2 absorption band. Using retrieval models, we infer a phosphine abundance of 0.1 parts per million, 100 times stronger than previous detections and upper limits, but consistent with the disequilibrium chemistry predicted by vertical mixing models. We discuss various possibilities as to why PH3 has a uniquely normal abundance in Wolf 1130C, and this study's implications for measuring phosphorus enrichment throughout the Milky Way.

Based on observations conducted as part of JWST-GO-4668. This work is supported by the Heising-Simons Foundation, the UK Research and Innovation Science and Technology Facilities Council, the Portuguese Foundation for Science and Technology, and NASA and CSA funding through programs JWST-GO-4668 and JWST-AR-2232.