Protoplanetary disc truncation mechanisms in stellar clusters: comparing
  external photoevaporation and tidal encounters
  Most stars form and spend their early life in regions of enhanced stellar
density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such
stars are subject to the influence of other members of the cluster. Physically,
PPDs might be truncated either by photoevaporation due to ultraviolet flux from
massive stars, or tidal truncation due to close stellar encounters. Here we aim
to compare the two effects in real cluster environments. In this vein we first
review the properties of well studied stellar clusters with a focus on stellar
number density, which largely dictates the degree of tidal truncation, and far
ultraviolet (FUV) flux, which is indicative of the rate of external
photoevaporation. We then review the theoretical PPD truncation radius due to
an arbitrary encounter, additionally taking into account the role of eccentric
encounters that play a role in hot clusters with a 1D velocity dispersion
$\sigma_v > 2$ km/s. Our treatment is then applied statistically to varying
local environments to establish a canonical threshold for the local stellar
density ($n_{c} > 10^4$ pc$^{-3}$) for which encounters can play a significant
role in shaping the distribution of PPD radii over a timescale $\sim 3$ Myr. By
combining theoretical mass loss rates due to FUV flux with viscous spreading in
a PPD we establish a similar threshold for which a massive disc is completely
destroyed by external photoevaporation. Comparing these thresholds in local
clusters we find that if either mechanism has a significant impact on the PPD
population then photoevaporation is always the dominating influence.
