  As a galaxy evolves, its chemical composition changes and the abundance
ratios of different elements are powerful probes of the underlying evolutionary
processes. Phosphorous is an element whose evolution has remained quite elusive
until now, because it is difficult to detect in cool stars. The infrared weak P
I lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators
of the presence of phosphorus. The availability of CRIRES at VLT has permitted
access to this wavelength range in stellar spectra.We attempt to measure the
phosphorus abundance of twenty cool stars in the Galactic disk. The spectra are
analysed with one-dimensional model-atmospheres computed in Local Thermodynamic
Equilibrium (LTE). The line formation computations are performed assuming LTE.
The ratio of phosphorus to iron behaves similarly to sulphur, increasing
towards lower metallicity stars. Its ratio with respect to sulphur is roughly
constant and slightly larger than solar, [P/S]=0.10+- 0.10. We succeed in
taking an important step towards the understanding of the chemical evolution of
phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance
ratio is steeper than predicted by Galactic chemical evolution model model
developed by Kobayashi and collaborators. Phosphorus appears to evolve
differently from the light odd-Z elements sodium and aluminium. The constant
value of [P/S] with metallicity implies that P production is insensitive to the
neutron excess, thus processes other than neutron captures operate. We suggest
that proton captures on 30Si and alpha captures on $27Al are possibilities to
investigate. We see no clear distinction between our results for stars with
planets and stars without any detected planet.
