The first steps of Interstellar Phosphorus Chemistry
  Phosphorus-bearing species are an essential key to form life on Earth,
however they have barely been detected in the interstellar medium. Since only
PN and PO have been identified so far towards star-forming regions, the
chemical formation pathways of P-bearing molecules are not easy to constrain
and are thus highly debatable. An important factor still missing in the
chemical models is the initial elemental abundance of phosphorus, i.e. the
depletion level of P. In order to overcome this problem, we study P-bearing
species in diffuse/translucent clouds. In these objects phosphorus is mainly in
the gas phase and therefore the elemental initial abundance needed in our
chemical simulations corresponds to the cosmic one and is thus well
constrained. An advanced chemical model with an updated P-chemistry network has
been used. Single-pointing observations were performed with the IRAM 30m
telescope towards the line of sight to the blazar B0355+508 aiming for the
(2-1) transitions of PN, PO, HCP and CP. This line of sight incorporates five
diffuse/translucent clouds. The (2-1) transitions of the PN, PO, HCP and CP
were not detected. We report detections of the (1-0) lines of
$\mathrm{^{13}CO}$, HNC and CN along with a first detection of
$\mathrm{C^{34}S}$. We have reproduced the observations of HNC, CN, CS and CO
in every cloud by applying typical conditions for diffuse/translucent clouds.
According to our best-fit model, the most abundant P-bearing species are HCP
and CP ($\sim10^{-10}$), followed by PN, PO and $\mathrm{PH_3}$
($\sim10^{-11}$). We show that the production of P-bearing species is favoured
towards translucent rather than diffuse clouds, where the environment provides
a stronger shielding from the interstellar radiation. Based on our improved
model, the (1-0) transitions of HCP, CP, PN and PO are expected to be
detectable with estimated intensities up to $\sim200$ mK.
