Deep Search for Phosphine in a Prestellar Core

TL;DR

This study used ALMA observations to search for phosphine in a prestellar core, setting upper limits on its abundance and suggesting most phosphorus exists in refractory forms, impacting theories of phosphorus chemistry in star formation.

Contribution

First sensitive search for phosphine in a prestellar core, providing upper limits on its abundance and constraining phosphorus chemical forms in star-forming regions.

Findings

Phosphine was not detected, with an upper abundance limit of 5×10⁻¹².

The volatile phosphorus abundance in L1544 is significantly lower than in comet 67P.

Most phosphorus in the core likely exists in refractory forms.

Abstract

Understanding in which chemical forms phosphorus exists in star- and planet-forming regions and how phosphorus is delivered to planets are of great interest from the viewpoint of the origin of life on Earth. Phosphine (PH3) is thought to be a key species to understanding phosphorus chemistry, but never has been detected in star- and planet-forming regions. We performed sensitive observations of the ortho-PH3 $1_0-0_0$ transition (266.944 GHz) toward the low-mass prestellar core L1544 with the ACA stand-alone mode of ALMA. The line was not detected down to 3$\sigma$ levels in 0.07 km s$^{-1}$ channels of 18 mK. The non-detection provides the upper limit to the gas-phase PH3 abundance of $5\times10^{-12}$ with respect to H2 in the central part of the core. Based on the gas-ice astrochemical modeling, we find the scaling relationship between the gas-phase PH3 abundance and the volatile (gas and ice with larger volatility than water) P elemental abundance for given physical conditions. This characteristic and well-constrained physical properties of L1544 allow us to constrain the upper limit to the volatile P elemental abundance of $5\times10^{-9}$, which is a factor of 60 lower than the overall P abundance in the ISM. Then the majority of P should exist in refractory forms. The volatile P elemental abundance of L1544 is smaller than that in the coma of comet 67P/C-G, implying that the conversion of refractory phosphorus to volatile phosphorus could have occurred along the trail from the presolar core to the protosolar disk through e.g., sputtering by accretion/outflow shocks.