Phosphorus in the Diffuse Interstellar Medium

We present FUSE and HST/STIS measurements of the PII column density toward Galactic stars. We analyzed PII through the profile fitting of the unsaturated $\lambda$1125 and $\lambda$1533 lines and derived column densities integrated along the sightlines as well as in individual resolved components. We find that phosphorus is not depleted along those sightlines sampling the diffuse neutral gas. We also investigate the correlation existing between PII and OI column densities and find that there is no differential depletion between these two specie. Furthermore, the ratio N(PII)/N(OI) is consistent with the solar P/O value, implying that PII and OI coexist in the same gaseous phase and are likely to evolve in parallel since the time they are produced in stars. We argue that phosphorus, as traced by PII, is an excellent neutral oxygen tracer in various physical environments, except when ionization corrections are a significant issue. Hence, PII lines (observable with FUSE, HST/STIS, or with VLT/UVES for the QSO sightlines) reveal particularly useful as a proxy for OI lines when these are saturated or blended.