Molecular clumps photoevaporation in ionized regions
  We study the photoevaporation of molecular clumps exposed to a UV radiation
field including hydrogen-ionizing photons ($h\nu > 13.6$ eV) produced by
massive stars or quasars. We follow the propagation and collision of shock
waves inside clumps and take into account self-shielding effects, determining
the evolution of clump size and density with time. The structure of the
ionization-photodissociation region (iPDR) is obtained for different initial
clump masses ($M=0.01 - 10^4\,{\rm M}_\odot$) and impinging fluxes ($G_0=10^2 -
10^5$ in units of the Habing flux). The cases of molecular clumps engulfed in
the HII region of an OB star and clumps carried within quasar outflows are
treated separately. We find that the clump undergoes in both cases an initial
shock-contraction phase and a following expansion phase, which lets the
radiation penetrate in until the clump is completely evaporated. Typical
evaporation time-scales are $\simeq 0.01$ Myr in the stellar case and 0.1 Myr
in the quasar case, where the clump mass is 0.1 ${\rm M}_\odot$ and $10^3\,{\rm
M}_\odot$ respectively. We find that clump lifetimes in quasar outflows are
compatible with their observed extension, suggesting that photoevaporation is
the main mechanism regulating the size of molecular outflows.
