Biosignature Line Ratios of [P II] in Exoplanetary and Nebular Environments

Abstract

Being the backbone element of DNA, phosphorus is a key component in the search for life in the Universe. To aid in its detection, we present line emissivity ratios for the five lowest-lying forbidden [P~II] transitions, namely those among the levels $3s^23p^2(^3P_0,^3P_1, ^3P_2,^1D_2,^1S_0)$. The wavelengths range between 0.44-70 \mum, and several lie within the spectroscopic domain observable with the James Webb Space Telescope (JWST). These line ratios have been calculated using a new collisional-radiative-recombination (CRR) model combining calculated collision strengths and level-specific recombination rate coefficients; with both datasets computed using the accurate Breit-Pauli R-Matrix method. The CRR model includes a new scheme for \eion recombination to emission line formation. We compare its effect to models incorporating only electron impact excitation and spontaneous radiative decay. We find that electron-ion recombination has a significant impact on all line ratios, and represents a major improvement in physical accuracy of emission line models.