Close stellar flybys common in low-mass clusters
  Numerous protoplanetary discs show distinct spiral arms features. While
possibly caused by a range of processes, detailed pattern analysis points at
close stellar flybys as cause for some of them. Surprisingly, these discs
reside in young low-mass clusters, where close stellar flybys are expected to
be rare. This fact motivated us to take a fresh look at the frequency of close
flybys in low-mass clusters. In the solar neighbourhood, low-mass clusters have
smaller half-mass radii than their more massive counterparts. We show that this
observational fact results in the mean and central stellar density of low-mass
clusters being approximately the same as in high-mass clusters, which is rarely
reflected in theoretical studies. We perform N-body simulations of the stellar
dynamics in young clusters obeying the observed mass-radius relation. Taking
the mean disc truncation radius as a proxy for the degree of influence of the
environment, we find that the influence of the environment on discs is more or
less the same in low- and high-mass clusters. Even the fraction of small
discs($<$ 10 au) is nearly identical. Our main conclusion is that the frequency
of close flybys seems to have been severely underestimated for low-mass
clusters. A testable prediction of this hypothesis is that low-mass clusters
should contain 10%-15% of discs smaller than 30 au truncated by flybys. These
truncated discs should be distinguishable from primordially small discs by
their steep outer edge.
