Molecular clouds

Unveiling the early-stage anatomy of a protocluster hub

The energy and momentum output from high-mass stars sculpts the visible structure of the interstellar medium in galaxies. Because of the rarity of these stars we still do not understand how they form. One possibility is that they form at the junction of filamentary networks, referred to as 'hubs'. In this publication we used the Atacama Large Millimeter Array (ALMA) to study the anatomy of a dense hub of molecular gas with a mass roughly equivalent to 100x the mass of the Sun. This hub sits at the junction of a series of dense filaments within the Infrared Dark Cloud G035.39-00.33 (see below). We studied the gas distribution down to scales of a few thousand au. Rather than the hub appearing as a singular clump connected by a series of filaments, we find that that the filaments retain their structure within the clump itself. The gas distribution is less like a singular bead on a string (or several strings) and more like tangled spaghetti. These filaments are very narrow, with widths of the order 6000au, >3x narrower than the proposed Universal filament width of 20000au (about 0.1pc). Each of these narrow filaments is associated with its own core population with masses ranging from a few tenths of the mass of the Sun up to about 10x the mass of the Sun.

J. D. Henshaw et al., 2017, MNRAS, 464, L31

You can obtain the arxiv version of the text here.

Investigating the structure and fragmentation of a filamentary IRDC

Following on from our kinematic analysis of G035.39-00.33 (see below), in this publication we focused on the structure and fragmentation of the cloud. We studied dust continuum emission at high-angular resolution obtained with the Plateau de Bure Interferometer. We found that this emission is segmented into a series semi-regularly spaced cores, that follow the major axis of the cloud. We compared the distribution of these dense cores to theoretical work describing the Jeans-like fragmentation of cylinders. We found that the spacing between the cores is much smaller than that predicted by models. We concluded from this analysis that the small separation between the cores is likely a projection effect. Instead, consistent with our kinematic analysis (see below) we suggest that these cores are located within independent sub-filaments. The fact that these sub-filaments overlap along the line of sight leads to the illusion that the cores are situated close together. Many of the cores appear to be sub-structured. However, two of the cores, dark in the mid-infrared, centrally-concentrated, monolithic (with no traceable substructure at our PdBI resolution), and with estimated masses of the order ~20-25x the mass of the Sun, are good candidates for the progenitors of intermediate-to-high-mass stars. Our results imply a multilayered fragmentation process, which incorporates the formation of sub-filaments, embedded cores, and the possibility of further fragmentation.

J. D. Henshaw et al., 2016, MNRAS, 463, 146

You can obtain the arxiv version of the text here.

High-resolution mapping of dark cloud filaments

In this publication, we expanded on our study of G035.39-00.33 (see below), this time observing the cloud at high-angular resolution using the Plateau de Bure Interferometer. We confirmed the presence of the dense sub-filaments that make up the cloud. We found evidence for small-scale velocity fluctuations correlating with the locations of dense cores. These fluctuations were interpreted as evidence for accretion, signifying the growth of the cores and the supply of material for star formation.

J. D. Henshaw et al., 2014, MNRAS, 440, 2860

You can obtain the arxiv version of the text here.

Filament merging in dark clouds

An important step in developing a holistic understanding of the star formation process is identifying and categorising the elusive initial phases of high-mass star formation. Ultimately this requires obtaining detailed knowledge of their birth environments. Identifying massive and relatively quiescent molecular clouds is therefore a crucial step in understanding the initial conditions for high-mass star formation.

Discovered as silhouettes against the bright Galactic mid-infrared background, so-called infrared dark clouds (IRDCs) have received significant attention in the field of high-mass star formation. In this publication, we used different molecules to trace different density regimes in one such cloud, G035.39-00.33: C18O (a rare isotope of carbon monoxide) to trace the lower density cloud envelope, and N2H+, which traces the cold and dense interior of the cloud. We found that the appearance of the cloud as a singular object is deceptive. Instead, the gas is distributed throughout three independent sub-filaments. The kinematics of these filaments, as well as the presence of widespread emission from silicon monoxide (which traces shocked gas), lead us to believe that these filaments may be merging. We argue that this merger may be triggering star formation within the cloud.

J. D. Henshaw et al., 2013, MNRAS, 428, 3425

You can obtain the arxiv version of the text here.