No preferential spatial distribution for massive stars expected from their formation

Abstract

We analyse N-body and Smoothed Particle Hydrodynamic (SPH) simulations of young star-forming regions to search for differences in the spatial distributions of massive stars compared to lower-mass stars. The competitive accretion theory of massive star formation posits that the most massive stars should sit in deeper potential wells than lower-mass stars. This may be observable in the relative surface density or spatial concentration of the most massive stars compared to other, lower-mass stars. Massive stars in cool-collapse N-body models do end up in significantly deeper potentials, and are mass segregated. However, in models of warm (expanding) star-forming regions, whilst the massive stars do come to be in deeper potentials than average stars, they are not mass segregated. In the purely hydrodynamical SPH simulations, the massive stars do come to reside in deeper potentials, which is due to their runaway growth. However, when photoionisation and stellar winds are implemented in the simulations, these feedback mechanisms regulate the mass of the stars and disrupt the inflow of gas into the clouds' potential wells. This generally makes the potential wells shallower than in the control runs, and prevents the massive stars from occupying deeper potentials. This in turn results in the most massive stars having a very similar spatial concentration and surface density distribution to lower-mass stars. Whilst massive stars do form via competitive accretion in our simulations, this rarely translates to a different spatial distribution and so any lack of primordial mass segregation in an observed star-forming region does not preclude competitive accretion as a viable formation mechanism for massive stars.

On the spatial distributions of dense cores in Orion B

“…However, recent analyses of simulations in which massive stars do form from competitive accretion show that this process can occur without the massive stars becoming mass segregated, or residing in areas of higher than average surface density ;P a r k e r& Dale 2017). Parker & Dale (2017) find that massive stars are preferentially located in deeper potential wells than average stars only if the effects of feedback from the massive stars are switched off in the simulation. When photoionizing feedback is switched on, the massive stars do not assume a different spatial distribution to lower mass stars as they form.…”

“…We determine the amount of substructure using the Cartwright & Whitworth (2004) Q-parameter, the amount of mass segregation using the Allison et al (2009) MSR ratio, the relative surface density of the most massive cores using the Maschberger & Clarke (2011) LDR technique, and the relative depth of the gravitational potential around the most massive cores, PDR (Parker & Dale 2017). Our conclusions are the following: (i) In contrast to Kirk et al (2016b), who calculated Q-parameters consistent with smooth or centrally concentrated distributions, we find Q < 0.8 for all three regions, which suggests a substructured or hierarchical distribution.…”

“…Initially, mass segregation was thought to be a natural outcome of the competitive accretion theory for star formation (Zinnecker 1982;Bonnell, Bate & Zinnecker 1998;Bonnell et al 2001), where the most massive stars would from in the most gas-rich regions of the cluster, which, in turn, would likely be the more central regions. However, extensive analysis of several hydrodynamic simulations of star formation (Parker, Dale & Ercolano 2015;Parker & Dale 2017) suggests that competitive accretion does not necessarily lead to mass segregation, ostensibly because the star-forming region is substructured and the dense cores/stars cannot fully interact with one another during the formation process.…”

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

“…Primordial mass segregation appeared to be ubiquitous in early simulations advocating competitive accretion as the dominant channel of massive star formation (Bonnell et al 1997(Bonnell et al , 2001). However, in recent simulations that include more diverse physical processes such as feedback, primordial mass segregation is not present Parker & Dale 2017).…”

A tale of two clusters: dynamical history determines disc survival in Tr14 and Tr16 in the Carina Nebula

“…Under competitive accretion, high-mass stars form in the deepest part of the gravitational potential well, aided by high gas densities that enhance accretion rates (e.g., Bonnell et al 2001). However, feedback from these same massive stars may erase any observable difference in the spatial distribution (Parker & Dale 2017). For example, we wouldn't necessarily expect competitive accretion to give primordial mass segregation (Bonnell & Davies 1998).…”