Talk abstract

We present a global study of young stellar object (YSO) surface densities ($\Sigma$) in star forming regions based on a comprehensive collection of {\it Spitzer Space Telescope} surveys, which encompasses nearly all star formation in the solar neighbourhood. We show that the distribution of YSO surface densities is a smooth distribution, being adequately described by a lognormal function from a few to $10^3$ YSOs per $\textrm{pc}^{2}$, with a peak at $\sim22$~stars $\textrm{pc}^{-2}$ and a dispersion of $\sigma_{{\rm log}_{10} \Sigma}$ $\sim$ 0.85. We do not find evidence for multiple discrete modes of star-formation (e.g. clustered and distributed). Comparing the observed surface density distribution to previously reported surface density threshold definitions of clusters, we find that the fraction of stars in clusters is crucially dependent on the adopted definitions, ranging from $40$ to $90$\%. However, we find that only a low fraction ($< 20$\%) of stars are formed in dense environments where their formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours.