Do all stars in the solar neighbourhood form in clusters? A cautionary
  note on the use of the distribution of surface densities
  Bressert et al. recently showed that the surface density distribution of
low-mass, young stellar objects (YSOs) in the solar neighbourhood is
approximately log-normal. The authors conclude that the star formation process
is hierarchical and that only a small fraction of stars form in dense star
clusters. Here, we show that the peak and the width of the density distribution
are also what follow if all stars form in bound clusters which are not
significantly affected by the presence of gas and expand by two-body
relaxation. The peak of the surface density distribution is simply obtained
from the typical ages (few Myr) and cluster membership number (few hundred)
typifying nearby star-forming regions. This result depends weakly on initial
cluster sizes, provided that they are sufficiently dense (initial half mass
radius of <0.3 pc) for dynamical evolution to be important at an age of a few
Myr. We conclude that the degeneracy of the YSO surface density distribution
complicates its use as a diagnostic of the stellar formation environment.
