1207.2059 (Mark Gieles et al.)

Do all stars in the solar neighbourhood form in clusters? A cautionary note on the use of the distribution of surface densities    [PDF]

Mark Gieles, Nick Moeckel, Cathie J. Clarke

Bressert et al. recently showed that the surface density distribution of low-mass, young stellar objects in the solar neighbourhood is approximately lognormal. 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 is also what follows 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 Myrs) 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 Myrs. We conclude that the degeneracy of the YSO surface density distribution complicates its use as a diagnostic of the stellar formation environment.

View original: http://arxiv.org/abs/1207.2059