Far and extreme ultraviolet radiation fields and consequent disc
  destruction in star-forming regions
  The first stages of planet formation usually occur when the host star is
still in a (relatively) dense star-forming region, where the effects of the
external environment may be important for understanding the outcome of the
planet formation process. In particular, star-forming regions that contain
massive stars have strong far ultraviolet (FUV) and extreme ultraviolet (EUV)
radiation fields, which can induce mass-loss from protoplanetary discs due to
photoevaporation. In this paper we present a parameter-space study of the
expected FUV and EUV fields in N-body simulations of star-forming regions with
a range of initial conditions. We then use recently published models to
determine the mass-loss due to photoevaporation from protoplanetary discs. In
particular, we focus on the effects of changing the initial degree of spatial
structure and initial virial ratio in the star-forming regions, as well as the
initial stellar density. We find that the FUV fields in star-forming regions
are much higher than in the interstellar medium, even when the regions have
stellar densities as low as in the Galactic field, due to the presence of
intermediate-mass, and massive, stars (>5Msun). These strong radiation fields
lead to the destruction of the gas component in protoplanetary discs within 1
Myr, implying that gas giant planets must either form extremely rapidly (<1
Myr), or that they exclusively form in star-forming regions like Taurus, which
contain no intermediate-mass or massive stars. The latter scenario is in direct
tension with meteoritic evidence from the Solar system that suggests the Sun
and its protoplanetary disc was born in close proximity to massive stars.
