  We aim to develop the Orion B Giant Molecular Cloud (GMC) as a local template
for interpreting extra-galactic molecular line observations. We use the
wide-band receiver at the IRAM-30m to spatially and spectrally resolve the
Orion B GMC. The observations cover almost 1 square degree at 26" resolution
with a bandwidth of 32 GHz from 84 to 116 GHz in only two tunings. Among the
mapped spectral lines are the 12CO, 13CO, C18O, C17O, HCN, HNC, 12CN, CCH,
HCO+, N2H+ (1-0), and 12CS, 32SO, SiO, c-C3H2, CH3OH (2-1) transitions. We
introduce the molecular anatomy of the Orion B GMC, including relations between
line intensities and gas column density or far-UV radiation fields, and
correlations between selected line and line ratios. We also obtain a
dust-traced gas mass that is less than about one third the CO-traced mass,
using the standard Xco conversion factor. The presence of overluminous CO can
be traced back to the dependence of the CO intensity on UV illumination. In
fact, while most lines show some dependence on the UV radiation field, CN and
CCH are the most sensitive. Moreover dense cloud cores are almost exclusively
traced by N2H+. Other traditional high density tracers, such as HCN (1-0), are
also easily detected in extended translucent regions at a typical density of
about 500 H2 cm-3. In general, we find no straightforward relation between line
critical density and the fraction of the line luminosity coming from dense gas
regions. Our initial findings demonstrate that the relations between line
(ratio) intensities and environment in GMCs are more complicated than often
assumed. Sensitivity (i.e., the molecular column density), excitation, and
above all chemistry contribute to the observed line intensity distributions.
They must be considered together when developing the next generation of
extra-galactic molecular line diagnostics of mass, density, temperature and
radiation field.
