First Detections of PN, PO, and PO+ toward a Shocked Low-mass Starless Core

Abstract

Phosphorus is a key element that plays an essential role in biological processes important for living organisms on Earth. The origin and connection of phosphorus-bearing molecules to early Solar system objects and star-forming molecular clouds is therefore of great interest, yet there are limited observations throughout different stages of low-mass ($M<$ a few M${}_{\odot }$) star formation. Observations from the Yebes 40 m and IRAM 30 m telescopes detect for the first time in the 7mm, 3mm, and 2mm bands multiple transitions of PN and PO, as well as a single transition of PO${}^{+}$, toward a low-mass starless core. The presence of PN, PO and PO${}^{+}$ is kinematically correlated with bright SiO(1-0) emission. Our results reveal not only that shocks are the main driver of releasing phosphorus from dust grains and into the gas-phase, but that the emission originates from gas not affiliated with the shock itself, but quiescent gas that has been shocked in the recent past. From radiative transfer calculations, the PO/PN abundance ratio is found to be $3.1_{-0.6}^{+0.4}$, consistent with other high-mass and low-mass star-forming regions. This first detection of PO${}^{+}$ toward any low-mass star-forming region reveals a PO${}^{+}$/PO ratio of $0.0115_{-0.0009}^{+0.0008}$, a factor of ten lower than previously determined from observations of a Galactic Center molecular cloud, suggesting its formation can occur under more standard Galactic cosmic-ray ionization rates. These results motivate the need for additional observations that can better disentangle the physical mechanisms and chemical drivers of this precursor of prebiotic chemistry.

12 pages, 6 figures, 3 tables; Accepted for publication in ApJL; Replacement to match publication version

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