Early Stages of Cluster Formation: Fragmentation of Massive Dense Cores Down to ≲ 1000 Au

Abstract

In order to study the fragmentation of massive dense cores, which constitute the cluster cradles, we observed with the PdBI in the most extended configuration the continuum at 1.3 mm and the CO (2-1) emission of four massive cores. We detect dust condensations down to ∼ 0.3 M ⊙ and separate millimeter sources down to 0.4 ′′ or 1000 AU, comparable to the sensitivities and separations reached in optical/infrared studies of clusters. The CO (2-1) high angular resolution images reveal high-velocity knots usually aligned with previously known outflow directions. This, in combination with additional cores from the literature observed at similar mass sensitivity and spatial resolution, allowed us to build a sample of 18 protoclusters with luminosities spanning 3 orders of magnitude. Among the 18 regions, ∼ 30% show no signs of fragmentation, while 50% split up into 4 millimeter sources. We compiled a list of properties for the 18 massive dense cores, such as bolometric luminosity, total mass, and mean density, and found no correlation of any of these parameters with the fragmentation level. In order to investigate the combined effects of magnetic field, radiative feedback and turbulence in the fragmentation process, we compared our observations to radiation magneto-hydrodynamic simulations, and obtained that the low-fragmented regions are well reproduced in the magnetized core case, while the highly-fragmented regions are consistent with cores where turbulence dominates over the magnetic field. Overall, our study suggests that the fragmentation in massive dense cores could be determined by the initial magnetic field/turbulence balance in each particular core.

ALMA survey of massive cluster progenitors from ATLASGAL

“…4). The CFE suggests an increasing concentration of mass in cores with the average clump volume density (n cl ); this trend has been seen, although inferred from smaller scales, towards high-mass infrared quiet MDCs in Cygnus-X , low-mass cores in ρ Oph (Motte et al 1998), and in a sample of infrared bright MDCs (Palau et al 2013). Although the CFE shows variations at high densities withn cl > 10 5 cm −3 , exceptionally high CFE of over 50% can only be reached towards the highest average clump densities.…”

“…Massive clumps at an early evolutionary phase, thus, prior to the emergence of luminous massive young stellar objects and UC-H II regions, are excellent candidates to host high-mass protostars in their earliest stages (e.g. Zhang et al 2009;Bontemps et al 2010;Csengeri et al 2011a,b;Palau et al 2013;Sánchez-Monge et al 2013). Large samples have only recently been identified based on large area surveys (e.g.…”

Ongoing star formation in the protocluster IRAS 22134+5834

“…VLA2 is not detected at millimeter wavelengths, probably due to the low sensitivity of our mm observations. Figure 3 shows the continuum cores detected by Palau et al (2013), and the southwest one is associated with MM1. We will discuss the detailed comparison in Sect.…”

Magnetically regulated fragmentation of a massive, dense, and turbulent clump

“…In general, the few studies performed so far with sub-arcsecond angular resolution, or close to ∼1 , reveal either low fragmentation (e.g. Palau et al 2013;Longmore et al 2011), or many fragments that are too massive, however, to be consistent with the gravoturbulent scenario (Bontemps et al 2010;Zhang et al 2015). Furthermore, comparisons with models that assume the actual physical conditions (temperature, turbulence) of the collapsing parent clump have not been published yet.…”

“…Krumholz 2006;Hennebelle et al 2011). Feedback from nascent protostellar objects through outflows, winds, and/or expansion of ionised regions (especially from newly born massive objects) can be important in relatively evolved stages (Bate 2009), but even then seems to be of only secondary importance (Palau et al 2013).…”