Will ALMA reveal the true core mass function of protoclusters?

Abstract

Characterizing pre-stellar cores in star-forming regions is an important step towards the validation of theoretical models of star formation. Due to their sub-arcsecond resolution, ALMA observations can potentially provide samples of pre-stellar cores up to distances of a few kpc, where regions of massive star formation can be targeted. However, the extraction of real cores from dust-continuum observations of turbulent star-forming clouds is affected by complex projection effects. In this work, we study the problem of core extraction both in the idealized case of column-density maps and in the more realistic case of synthetic 1.3 mm ALMA observations. The analysis is carried out on 12 regions of high column density from our 250 pc simulation. We find that derived core masses are highly unreliable, with only a weak correlation between the masses of cores selected in the synthetic ALMA maps and those of the corresponding three-dimensional cores. The fraction of real three-dimensional cores detected in the synthetic maps increases monotonically with mass and remains always below 50 per cent. Above ∼ 1 M, the core mass function derived from the column-density maps is steeper than that of the three-dimensional cores, while the core mass function from the synthetic ALMA maps has a slope closer to that of the real three-dimensional cores. Because of the mass uncertainties, proper guidance from realistic simulations is essential if ALMA observations of protoclusters at kpc distances are to be used to test star formation models.