Gravity drives the evolution of infrared dark hubs: JVLA observations of SDC13

Abstract

Context. Converging networks of interstellar filaments, i.e. hubs, have been recently linked to the formation of stellar clusters and massive stars. Understanding the relationship between the evolution of these systems and the formation of cores/stars inside them is at the heart of current star formation research. Aims. The goal is to study the kinematic and density structure of the SDC13 prototypical hub at high angular resolution to determine what drives its evolution and fragmentation. Methods. We have mapped SDC13, a ∼1000 M infrared dark hub, in NH 3 (1,1) and NH 3 (2,2) emission lines, with both the Jansky Very Large Array and Green Bank Telescope. The high angular resolution achieved in the combined dataset allowed us to probe scales down to 0.07pc. After fitting the ammonia lines, we computed the integrated intensities, centroid velocities and line widths, along with gas temperatures and H 2 column densities. Results. The mass-per-unit-lengths of all four hub filaments are thermally super-critical, consistent with the presence of tens of gravitationally bound cores identified along them. These cores exhibit a regular separation of ∼ 0.37 ± 0.16 pc suggesting gravitational instabilities running along these super-critical filaments are responsible for their fragmentation. The observed local increase of the dense gas velocity dispersion towards starless cores is believed to be a consequence of such fragmentation process. Using energy conservation arguments, we estimate that the gravitational to kinetic energy conversion efficiency in the SDC13 cores is ∼ 35%. We see velocity gradient peaks towards ∼ 63% of the cores as expected during the early stages of filament fragmentation. Another clear observational signature is the presence of the most massive cores at the filaments' junction, where the velocity dispersion is the largest. We interpret this as the result of the hub morphology generating the largest acceleration gradients near the hub centre. Conclusions. We propose a scenario for the evolution of the SDC13 hub in which filaments first form as post-shock structures in a supersonic turbulent flow. As a result of the turbulent energy dissipation in the shock, the dense gas within the filaments is initially mostly sub-sonic. Then gravity takes over and starts shaping the evolution of the hub, both fragmenting filaments and pulling the gas towards the centre of the gravitational well. By doing so, gravitational energy is converted into kinetic energy in both local (cores) and global (hub centre) potential well minima. Furthermore, the generation of larger gravitational acceleration gradients at the filament junctions promotes the formation of more massive cores.

From clump to disc scales in W3 IRS4

“…The high mass-infall rate from the large-scale, one-sided flow may explain both of the steeper temperature profile and the more disrupted nature of the material around W3 IRS4, to the point where there is not indication of any disc-like structure down to the spatial resolution of the observations (that is 600 AU). Flow along filamentary structures has been seen in other star formation regions (for example the hub-filament system SDC13, Peretto et al 2014;Williams et al 2018) or the spiral flows towards W33A (Maud et al 2017), suggesting that W3 IRS4 is unlikely to be unique, though the one-sided nature of the flow may make it more extreme.…”

Multiscale dynamics in star-forming regions: the interplay between gravity and turbulence

“…large-scale infall motions (Kirk et al 2013) or gravitational collapse along the filament (Peretto et al 2014). They may be due to large-scale motions not related with the internal dynamics of the gas, such as rotation (Kirk et al 2013) or compression from an external source (Williams et al 2018), and, for the most elongated objects, they may also originate from Galactic scale motions such as Galactic rotation, shear or compression (e.g. Duarte-Cabral & Dobbs 2016).…”

Dynamics of cluster-forming hub-filament systems

“…In the last years, an increasing number of works have focused on the study of the dynamics and fragmentation of filamentary structures from both, observational and theoretical points of view (see e. g., André et al 2010; Schneider et al 2010, 2012; Hennemann et al 2012; Busquet et al 2013; Galvan-Madrid et al 2013; Hacar et al 2013, 2018; Peretto et al 2013; Louvet et al 2014; Tafalla & Hacar 2015; Smith et al 2014; Henshaw et al 2014; Tackenbergt et al 2014; Seifried & Walch 2016; Kainulainen et al 2017; Seifried et al 2017; Arzoumanian et al 2019; Williams et al 2018; Clarke et al 2019). However, few of these works are focus on massive star forming regions within hub-filament system, and little is still known about the dynamics of filamentary networks (e. g., cluster-forming hub filament systems) and their role in the accretion processes that regulate the formation of high-mass star-forming clusters.…”