  Phosphorus is a key ingredient in terrestrial biochemistry, but is rarely
observed in the molecular ISM and therefore little is known about how it is
inherited during the star and planet formation sequence. We present
observations of the phosphorus-bearing molecules PO and PN towards the Class I
low-mass protostar B1-a using the IRAM 30m telescope, representing the second
detection of phosphorus carriers in a Solar-type star forming region. The P/H
abundance contained in PO and PN is ~10$^{-10}$-10$^{-9}$ depending on the
assumed source size, accounting for just 0.05-0.5% of the solar phosphorus
abundance and implying significant sequestration of phosphorus in refractory
material. Based on a comparison of the PO and PN line profiles with the shock
tracers SiO, SO$_2$, and CH$_3$OH, the phosphorus molecule emission seems to
originate from shocked gas and is likely associated with a protostellar
outflow. We find a PO/PN column density ratio of ~1-3, which is consistent with
the values measured in the shocked outflow of the low-mass protostar L1157, the
massive star-forming regions W51 and W3(OH), and the galactic center GMC
G+0.693-0.03. This narrow range of PO/PN ratios across sources with a range of
environmental conditions is surprising, and likely encodes information on how
phosphorus carriers are stored in grain mantles.
