WARPFIELD 2.0: Feedback-regulated minimum star formation efficiencies of
  giant molecular clouds
  Star formation is an inefficient process and in general only a small fraction
of the gas in a giant molecular cloud (GMC) is turned into stars. This is
partly due to the negative effect of stellar feedback from young massive star
clusters. In a recent paper, we introduced a novel 1D numerical treatment of
the effects of stellar feedback from young massive clusters on their natal
clouds, which we named WARPFIELD. Here, we present version 2 of the WARPFIELD
code, containing improved treatments of the thermal evolution of the gas and
the fragmentation of the feedback-driven shell. As part of this update, we have
produced new cooling and heating tables that account for the combined effects
of photoionization and collisional ionization on the cooling rate of the gas,
which we now make publically available. We employ our updated version of
WARPFIELD to investigate the impact of stellar feedback on GMCs with a broad
range of masses and surface densities and a variety of density profiles. We
show that the minimum star formation efficiency $\epsilon_{\mathrm{min}}$, i.e.
the star formation efficiency above which the cloud is destroyed by feedback
and further star formation is shut off, is mainly set by the average cloud
surface density. A star formation efficiency of 1-6 % is generally sufficient
to destroy a GMC. We also find star formation efficiencies per free-fall time
$\epsilon_{\mathrm{ff}} \sim 0.3$ %, in good agreement with recent
observations. Our results imply that stellar feedback alone is sufficient to
explain the low observed star formation efficiencies of GMCs. Finally, we show
that very massive clouds with steep density profiles - possible proxies of the
giant clumps observed in galaxies at $z \approx 2$ - are more resilient to
feedback than typical GMCs, with $\epsilon_{\mathrm{min}}$ between 1 and 12 %.
