Gas and dust in the star-forming region rho Oph A: II. The gas in the
  PDR and in the dense cores
  We investigate to what degree local physical and chemical conditions are
related to the evolutionary status of various objects in star-forming media.
rho Oph A displays the entire sequence of low-mass star formation in a small
volume of space. Using spectrophotometric line maps of H2, H2O, NH3, N2H+, O2,
OI, CO, and CS, we examine the distribution of the atomic and molecular gas in
this dense molecular core. The physical parameters of these species are
derived, as are their relative abundances in rho Oph A. Using radiative
transfer models, we examine the infall status of the cold dense cores from
their resolved line profiles of the ground state lines of H2O and NH3, where
for the latter no contamination from the VLA 1623 outflow is observed and line
overlap of the hyperfine components is explicitly taken into account. The
stratified structure of this photon dominated region (PDR), seen edge-on, is
clearly displayed. Polycyclic aromatic hydrocarbons (PAHs) and OI are seen
throughout the region around the exciting star S1. At the interface to the
molecular core 0.05 pc away, atomic hydrogen is rapidly converted into H2,
whereas OI protrudes further into the molecular core. This provides oxygen
atoms for the gas-phase formation of O2 in the core SM1, where X(O2)~ 5.e-8.
There, the ratio of the O2 to H2O abundance [X(H2O)~ 5.e-9] is significantly
higher than unity. Away from the core, O2 experiences a dramatic decrease due
to increasing H2O formation. Outside the molecular core, on the far side as
seen from S1, the intense radiation from the 0.5 pc distant early B-type star
HD147889 destroys the molecules. Towards the dark core SM1, the observed
abundance ratio X(O2)/X(H2O)>1, which suggests that this object is extremely
young, which would explain why O2 is such an elusive molecule outside the solar
system.
