  Phosphorus abundance is crucial for DNA-based extraterrestrial life in
exoplanets. Atomic data for observed spectral lines of P-ions are needed for
its accurate determination. We present the first calculations for collision
strengths for the forbidden PIII fine structure transition $3s^23p
(^2P^o_{1/2-3/2})$ within the ground state at 17.9 $\mu$m, as well as allowed
UV transitions in the $3s^23p (^2P^o_{1/2,3/2}) \rightarrow 3s3p^2
(^2D_{3/2,5/2}, ^2S_{1/2}, ^2P_{1/2,3/2})$ multiplets between 915-1345 $\AA$.
Collision strengths are computed using the Breit-Pauli R-Matrix method
including the first 18 levels, and they exhibit extensive auto-ionizing
resonance structures. In particular, the Maxwellian averaged effective
collision strength for the FIR 17.9 $\mu$m transition shows a factor 3
temperature variation broadly peaking at typical nebular temperatures. Its
theoretical emissivity with solar phosphorus abundance is computed relative to
H$\beta$ and found to be similar to observed intensties from planetary nebulae;
the abundances derived in earlier works are 3-5 times sub-solar. The results
pertain to the reported paucity of phosphorus from preferred production sites
in supernovae, and abundances in planetary nebulae and supernova remnants.

Solar and Stellar Astrophysics