Spectral line survey of the ultracompact HII region Mon R2
  Ultracompact (UC) HII regions constitute one of the earliest phases in the
formation of a massive star and are characterized by extreme physical
conditions (Go>10^5 Habing field and n>10^6 cm^-3). The UC HII Mon R2 is the
closest one and therefore an excellent target to study the chemistry in these
complex regions.
  We carried out a 3mm and 1mm spectral survey using the IRAM 30-m telescope
towards three positions that represent different physical environments in Mon
R2: (i) the ionization front (IF) at (0",0"); two peaks in the molecular cloud
(ii) MP1 at the offset (+15",-15") and (iii) MP2 at the farther offset
(0",40"). In addition, we carried out extensive modeling to explain the
chemical differences between the three observed regions.
  We detected more than thirty different species. We detected SO+ and C4H
suggesting that UV radiation plays an important role in the molecular chemistry
of this region. We detected the typical PDR molecules CN, HCN, HCO, C2H, and
c-C3H2. While the IF and the MP1 have a chemistry similar to that found in high
UV field and dense PDRs like the Orion Bar, the MP2 is more similar to lower
UV/density PDRs like the Horsehead nebula.
  We also detected complex molecules that are not usually found in PDRs (CH3CN,
H2CO, HC3N, CH3OH and CH3C2H). Sulfur compounds CS, HCS+, C2S, H2CS, SO and SO2
and the deuterated species DCN and C2D were also identified. [DCN]/[HCN]=0.03
and [C2D]/[C2H]=0.05, are among the highest in warm regions.
  Our results show that the high UV/dense PDRs present a different chemistry
from that of the low UV case. Abundance ratios like [CO+]/[HCO+] or
[HCO]/[HCO+] are good diagnostics to differentiate between them. In Mon R2 we
have the two classes of PDRs, a high UV PDR towards the IF and the adjacent
molecular bar and a low-UV PDR which extends towards the north-west following
the border of the cloud.
