Subsonic islands within a high-mass star-forming infrared dark cloud

Abstract

High-mass star forming regions are typically thought to be dominated by supersonic motions. We present combined Very Large Array and Green Bank Telescope (VLA+GBT) observations of NH 3 (1,1) and (2,2) in the infrared dark cloud (IRDC) G035.39-00.33, tracing cold and dense gas down to scales of 0.07 pc. We find that, in contrast to previous, similar studies of IRDCs, more than a third of the fitted ammonia spectra show subsonic non-thermal motions (mean line width of 0.71 km s −1 ), and the sonic Mach number distribution peaks around M = 1. As possible observational and instrumental biases would only broaden the line profiles, our results provide strong upper limits to the actual value of M, further strengthening our findings of narrow line widths. This finding calls for a reevaluation of the role of turbulent dissipation and subsonic regions in massive-star and cluster formation. Based on our findings in G035.39, we further speculate that the coarser spectral resolution used in the previous VLA NH 3 studies may have inhibited the detection of subsonic turbulence in IRDCs. The reduced turbulent support suggests that dynamically important magnetic fields of the 1 mG order would be required to support against possible gravitational collapse. Our results offer valuable input into the theories and simulations that aim to recreate the initial conditions of high-mass star and cluster formation.

Fragmentation and kinematics in high-mass star formation

“…Therefore, it may well be the case that the individual cores do not have turbulent properties but it is, rather, only the superposition of many of these individual cores and spectral components mimicking the broader lines that are typically seen with single-dish telescopes at lower spatial resolution (e.g., Smith et al 2013;Hacar et al 2017b). At a somewhat coarser resolution of >10 000 au, similar results indicating sub-sonic non-thermal motions were also found by Ragan et al (2015) and Sokolov et al (2018) in lowerdensity lines of N 2 H + and NH 3 , respectively.…”

Identification of pre-stellar cores in high-mass star forming clumps via H2D+ observations with ALMA

“…However, we do highlight the fact that future observations recovering the zerospacing flux are needed (see e.g. Henshaw et al 2014;Sokolov et al 2018) if we want to use the ALMA o-H 2 D + (1 1,0 −1 1,1 ) data to discuss the large-scale kinematics of the clumps.…”

The JCMT BISTRO Survey: Revealing the Diverse Magnetic Field Morphologies in Taurus Dense Cores with Sensitive Submillimeter Polarimetry

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