Digging into the Interior of Hot Cores with ALMA. VI. The Formation of Low-mass Multiple Systems in High-mass Cluster-forming Regions

Abstract

Most stars form in multiple systems, with profound implications in numerous astronomical phenomena intrinsically linked to multiplicity. However, our knowledge about the process on how multiple stellar systems form is incomplete and biased toward nearby molecular clouds forming only low-mass stars, which are unrepresentative of the stellar population in the Galaxy. Most stars form within dense cores in clusters alongside high-mass stars (>8 M$_{\odot}$), as likely the Sun did. Here we report deep ALMA 1.33 mm dust continuum observations at ~160 au spatial resolution, revealing 72 low-mass multiple systems embedded in 23 high-mass cluster-forming regions, as part of the Digging into the Interior of Hot Cores with ALMA (DIHCA) survey. We find that the companion separation distribution presents a distinct peak at ~1200 au, in contrast to the one at ~4000 au observed in nearby low-mass regions. The shorter fragmentation scale can be explained by considering the higher pressure exerted by the surrounding medium, which is higher than the one in low-mass regions, due to the larger turbulence and densities involved. Because the peak of the companion separation distribution occurs at much larger scales than the expected disk sizes, we argue that the observed fragmentation is produced by turbulent core fragmentation. Contrary as predicted, the multiplicity fraction remains constant as the stellar density increases. We propose that in the extremely dense environments where high-mass stars form, dynamical interactions play an important role in disrupting weakly bound systems.

Figures (7)

• Figure 1: The 1.33 mm continuum images of the G333.12-0.56 region. (a), Background image shows the emission from 1.33 mm dust continuum at lower resolution the synthesized beam is 033 $\times$028, corresponding to 1100 au at a distance of 3.3 kpc. The yellow contours outline the low-mass cores while the black contour highlights the high-mass core in this region as defined by Astrodendro. This image reveals 26 cores, each containing varying numbers of condensations. Condensations situated outside cores are classified as part of the orphan population. (b)-(e), High-angular-resolution images of at the same wavelength showing the multiple systems embedded within the cores (beam = 0064 $\times$0041), corrsponding to 210 $\times$ 135 au. One orphan example is indicated by the red arrow. (f)-(k) show the same images as (b)-(e), with Gaussian fitting results along with the names of the condensations and their associated cores. Black contour shows 5$\sigma$ and 10$\sigma$ levels, where $\sigma$ = 47.2 $\mu$Jy beam$^{-1}$ is the rms noise for config-8 data. Red ellipse and cross show the condensation size and locations as defined by PyBDSF.
• Figure 2: This histogram presents companion separations for all condensations in 23 HMCF regions. Panel (a) shows results obtained using leaf-defined core. Panel (b) presents the same results as panel (a) while also incorporating companion separations from nearby clouds, including Orion and Perseus.
• Figure 3: Image shows the cumulative distribution function (CDF) of core number density, with the blue line representing cores forming single systems and the orange line representing cores forming multiple systems.
• Figure 4: The figures illustrate the correlation between condensation surface density and multiplicity fraction in 23 HMCF regions. Colored dots represent the MF value points, while gray bars indicate error margins. The horizontal error bars show condensation surface density uncertainty is dominated by Poisson noise from counting objects. The first figure shows the combined multiplicity fraction results for all condensations, including both 'in core' and 'orphan' populations. The subsequent two figures present the same correlation separately for the in core and orphan populations. The gray filled circles indicate the region with MF = 0.
• Figure 5: Panel (a) shows the companion separation distribution for a fixed core size of 3000 au in 23 HMCF regions. Panel (b) displays the companion separation distribution without any core size constraints in 23 HMCF regions, with separations limited to a maximum of 6000 au. The gray histogram represents the core separation distribution in 23 regions from Ishihara24. To facilitate comparison, both distributions have been peak-normalized.