Not so different after all: properties and spatial structure of column density peaks in the pipe and Orion A clouds

Abstract

We present a comparative study of the physical properties and the spatial distribution of column density peaks in two Giant Molecular Clouds (GMC), the Pipe Nebula and Orion A, which exemplify opposite cases of star cluster formation stages. The density peaks were extracted from dust extinction maps constructed from Herschel/SPIRE farinfrared images. We compare the distribution functions for dust temperature, mass, equivalent radius and mean volume density of peaks in both clouds, and made a more fair comparison by isolating the less active Tail region in Orion A and by convolving the Pipe Nebula map to simulate placing it at a distance similar to that of the Orion Complex. The peak mass distributions for Orion A, the Tail, and the convolved Pipe, have similar ranges, sharing a maximum near 5 M , and a similar power law drop above 10 M . Despite the clearly distinct evolutive stage of the clouds, there are very important similarities in the physical and spatial distribution properties of the column density peaks, pointing to a scenario where they form as a result of uniform fragmentation of filamentary structures across the various scales of the cloud, with density being the parameter leading the fragmentation, and with clustering being a direct result of thermal fragmentation at different spatial scales. Our work strongly supports the idea that the formation of clusters in GMC could be the result of the primordial organization of pre-stellar material. 1 We avoid here to use the word "core", to be consistent with the prescription of Rathborne et al. (2009), that requires individ-

W51North: A protocluster emerging out of a thermally inhibited fragmenting cloud

“…3). This value is very consistent with the values obtained for the linear regime of the MSDC in the Pipe Nebula (-0.68) and the Lupus I cloud (-0.72) (Román-Zúñiga et al 2019), where this linear regime is found at similar or slightly larger spatial scales of 2000 -8000 AU. The Y-axis is the number of companions, and the X-axis is angular separations.…”

“…The clustering regime is associated with the fragmentation of the cloud, while the binary regime is affected by additional processes such as the dynamical interactions between the different members of a multiple system. The value of the slope found here for W51 North (SE1), -0.8±0.1 (Section 4.1), is fully consistent with the expected value for the clustering regime in other clouds of the Milky Way such as Taurus, Ophiucus, the Pipe Nebula, Lupus I, or OMC-1S (Gómez et al 1993;Simon 1997;Palau et al 2018;Román-Zúñiga et al 2019) and clearly shallower than the typical value of the binary regime. Although W51 North is very young and part of the fragments identified here could merge with time, the fact that the slope of the MSDC plot for W51 North precisely corresponds to the expected value for the clustering regime suggests that a protocluster is in the making in W51 North.…”

Fragmentation and kinematics in high-mass star formation

“…It is interesting to note that in a study of the integral shape filament in Orion, Kainulainen et al (2017) found that significant grouping of dense cores is found below scales of 17000 au (see also Román-Zúñiga et al 2019), similar to what we find Fig. 5.…”

Fragmentation and kinematics in high-mass star formation: CORE-extension targeting two very young high-mass star-forming regions

“…It is interesting to note that in a study of the integral shape filament in Orion, Kainulainen et al (2017) found that significant grouping of dense cores is found below scales of 17000 au (see also Román-Zúñiga et al 2019), similar to what we find in Fig. 5.…”