The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). IX. Physical Properties and Spatial Distribution of Cores in IRDCs

Abstract

The initial conditions found in infrared dark clouds (IRDCs) provide insights on how high-mass stars and stellar clusters form. We have conducted high-angular resolution and high-sensitivity observations toward thirty-nine massive IRDC clumps, which have been mosaicked using the 12 and 7 m arrays from the Atacama Large Millimeter/submillimeter Array. The targets are 70 μm dark massive (220–4900 M ⊙), dense (>104 cm−3), and cold (∼10–20 K) clumps located at distances between 2 and 6 kpc. We identify an unprecedented number of 839 cores, with masses between 0.05 and 81 M ⊙ using 1.3 mm dust continuum emission. About 55% of the cores are low-mass (<1 M ⊙), whereas ≲1% (7/839) are high-mass (≳27 M ⊙). We detect no high-mass prestellar cores. The most massive cores (MMC) identified within individual clumps lack sufficient mass to form high-mass stars without additional mass feeding. We find that the mass of the MMCs is correlated with the clump surface density, implying denser clumps produce more massive cores. There is no significant mass segregation except for a few tentative detections. In contrast, most clumps show segregation once the clump density is considered instead of mass. Although the dust continuum emission resolves clumps in a network of filaments, some of which consist of hub-filament systems, the majority of the MMCs are not found in the hubs. Our analysis shows that high-mass cores and MMCs have no preferred location with respect to low-mass cores at the earliest stages of high-mass star formation.

Direct Observational Evidence of the Multi-scale, Dynamical Mass Accretion Toward a High-mass Star-forming Hub-filament System

“…HFSs have been the focus of several statistical studies (Kumar et al 2020;Zhou et al 2022;Liu et al 2023). In particular, Liu et al (2023) studied a sample of 17 HFSs composed of two distinct evolutionary stages using high-angular resolution (∼1″-2″) Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm (Li et al 2023;Morii et al 2023) and 3 mm continuum data (Liu et al 2020b;Sanhueza et al 2019). Liu et al (2023) discuss the observed trend on multi-scales (i.e., clumps and cores) of increasing mass and mass surface density with the evolution from IR-dark to IR-bright stage, the mass-segregated cluster of young stellar objects, and the potentially preferential escape directions of outflow feedback.…”

Clustering Properties of Intermediate and High-mass Young Stellar Objects*

“…Likewise, we find no argument from this analysis supporting a more isolated, monolithic-type formation for the more-massive YSOs. An explanation for these results could be that mass segregation happens at later stages of cluster lifetimes (Della Croce et al 2023;Farias & Tan 2023), and that cluster properties at early stages are determined by the initial cloud size and mass, and not the stellar content (Farias & Tan 2023;Morii et al 2023). It is also an option that the YSOs considered in this work are not the most-massive YSOs in several of the detected clusters, which might contain heavily embedded massive YSOs that escaped the Gaia survey.…”

The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles (ASSEMBLE): Core Growth, Cluster Contraction, and Primordial Mass Segregation

“…The larger core mass in the fragments demands either additional support from turbulence and magnetic fields (Wang et al 2012) or a continuous accretion onto the core (Vázquez-Semadeni et al 2023). On the other hand, observations also find that the mass of these cores does not contain sufficient material to form a massive star (Sanhueza et al 2017(Sanhueza et al , 2019Morii et al 2023), and the cores typically continue to fragment when observed at higher angular resolution (Wang et al 2011(Wang et al , 2014Zhang et al 2015;Olguin et al 2021Olguin et al , 2022, or at slightly later evolutionary stages (e.g., Palau et al 2015;Beuther et al 2018). Therefore, the idea of monolithic collapse (McKee & Tan 2003) for massive star formation does not match the observations.…”

“…Such a continuous mass accretion is observed directly (Dewangan et al 2022;Redaelli et al 2022;Xu et al 2023a) or indirectly (Contreras et al 2018). More recent observations of IRDCs with the Atacama Large Millimeter/submillimeter Array (ALMA) routinely reach a mass sensitivity far below the thermal Jeans mass and detect a large population of low-mass cores in the clumps that are compatible with the thermal Jeans mass (Sanhueza et al 2019;Svoboda et al 2019;Morii et al 2023). These cores may form low-mass stars in a cluster.…”