Spatial statistics in star forming regions: is star formation driven by column density alone?

Abstract

Star formation is known to occur more readily where more raw materials are available. This is often expressed by a `Kennicutt--Schmidt' relation where the surface density of Young Stellar Objects (YSOs) is proportional to column density to some power, μ. The aim of this work was to determine if column density alone is sufficient to explain the locations of Class~0/I YSOs within Serpens South, Serpens Core, Ophiuchus, NGC1333 and IC348, or if there is clumping or avoidance that would point to additional influences on the star formation. Using the O-ring test as a summary statistic, 95 per cent confidence envelopes were produced for different values of μ from probability models made using the Herschel column density maps. The YSOs were tested against four distribution models: the best-estimate of μ for the region, μ=0 above a minimum column density threshold and zero probability elsewhere, μ=1, and the power-law that best represents the five regions as a collective, μ=2.05 ± 0.20. Results showed that μ=2.05 model was consistent with the majority of regions and, for those regions, the spatial distribution of YSOs at a given column density is consistent with being random.Serpens South and NGC1333 rejected the μ = 2.05 model on small scales of ˜0.15 pc which implies that small-scale interactions may be necessary to improve the model. On scales above 0.15 pc, the positions of YSOs in all five regions can be well described using column density alone.