Face-on accretion onto a protoplanetary disc
  Globular clusters (GCs) are known to harbor multiple stellar populations. To
explain these observations Bastian et al. suggested a scenario in which a
second population is formed by the accretion of enriched material onto the
low-mass stars in the initial GC population. The idea is that the low-mass,
pre-main sequence stars sweep up gas expelled by the massive stars of the same
generation into their protoplanetary disc as they move through the GC core. We
perform simulations with 2 different smoothed particle hydrodynamics codes to
investigate if a low-mass star surrounded by a protoplanetary disc can accrete
the amount of enriched material required in this scenario. We focus on the gas
loading rate onto the disc and star as well as on the lifetime of the disc. We
find that the gas loading rate is a factor of 2 smaller than the geometric
rate, because the effective cross section of the disc is smaller than its
surface area. The loading rate is consistent for both codes, irrespective of
resolution. The disc gains mass in the high resolution runs, but loses angular
momentum on a time scale of 10^4 yrs. Two effects determine the loss of
(specific) angular momentum in our simulations: 1) continuous ram pressure
stripping and 2) accretion of material with no azimuthal angular momentum. Our
study and previous work suggest that the former, dominant process is mainly
caused by numerical rather than physical effects, while the latter is not. The
latter process causes the disc to become more compact, increasing the surface
density profile at smaller radii. The disc size is determined in the first
place by the ram pressure when the flow first hits the disc. Further evolution
is governed by the decrease in the specific angular momentum of the disc. We
conclude that the size and lifetime of the disc are probably not sufficient to
accrete the amount of mass required in Bastian et al.'s scenario.
