Shaping the structure of a GMC with radiation and winds
  We study the effect of stellar feedback (photodissociation/ionization,
radiation pressure and winds) on the evolution of a Giant Molecular Cloud
(GMC), by means of a 3D radiative transfer, hydro-simulation implementing a
complex chemical network featuring ${\rm H}_2$ formation and destruction. We
track the formation of individual stars with mass $M>1\,{\rm M}_\odot$ with a
stochastic recipe. Each star emits radiation according to its spectrum, sampled
with 10 photon bins from near-infrared to extreme ultra-violet bands; winds are
implemented by energy injection in the neighbouring cells. We run a simulation
of a GMC with mass $M=10^5\,{\rm M}_\odot$, following the evolution of
different gas phases. Thanks to the simultaneous inclusion of different stellar
feedback mechanisms, we identify two stages in the cloud evolution: (1)
radiation and winds carve ionized, low-density bubbles around massive stars,
while FUV radiation dissociates most ${\rm H}_2$ in the cloud, apart from
dense, self-shielded clumps; (2) rapid star formation (SFR$\simeq 0.1\,{\rm
M}_\odot\,{\rm yr}^{-1}$) consumes molecular gas in the dense clumps, so that
UV radiation escapes and ionizes the remaining HI gas in the GMC. ${\rm H}_2$
is exhausted in $1.6$ Myr, yielding a final star formation efficiency of 36 per
cent. The average intensity of FUV and ionizing fields increases almost
steadily with time; by the end of the simulation ($t=2.5$ Myr) we find $\langle
G_0 \rangle \simeq 10^3$ (in Habing units), and a ionization parameter $\langle
U_{\rm ion} \rangle \simeq 10^2$, respectively. The ionization field has also a
more patchy distribution than the FUV one within the GMC. Throughout the
evolution, the escape fraction of ionizing photons from the cloud is $f_{\rm
ion, esc} < 0.03$.
