Abstract
Context. Massive stars are generally believed to form in environments characterized by supersonic turbulence. However, recent observations challenge this traditional view. High-spatial- and spectral-resolution observations of the Orion Molecular Cloud (OMC, the closest massive star formation region) and an infrared-dark cloud (IRDC) G35.39 (a typical distant massive star formation region) show a resolution-dependent turbulence, and that high-mass stars are forming exclusively in subsonic to transonic cores in those clouds. These studies demand a re-evaluation of the role of turbulence in massive star formation. Aims. We aim to study the turbulence in a typical massive-star-forming region G35.20-0.74 N (G35.20 in short) with sufficient spatial resolution to resolve the thermal Jeans length, and sufficient spectral resolution to resolve the thermal line width. Methods. We use the Atacama Large Millimeter/submillimeter Array (ALMA) dust continuum emission to resolve fragmentation, the Karl G. Jansky Very Large Array (JVLA) 1.2 cm continuum to trace ionized gas, and JVLA NH3 (1,1) to (7,7) inversion transition lines to trace line width, temperature, and dynamics. We fit those lines and remove line broadening due to channel width, thermal pressure, and velocity gradient to obtain a clean map of intrinsic turbulence. Results. We find that (1) the turbulence in G35.20 is overall supersonic, with mean and median Mach numbers 3.7 and 2.8, respectively. (2) Mach number decreases from 6–7 at a 0.1 pc scale to less than 3 toward the central cores at a 0.01 pc scale. (3) The central ALMA cores appear to be decoupled from the host filament, which is made evident by an opposite velocity gradient and significantly reduced turbulence. Because of intense star-formation activity in G35.20 (as compared to the relatively young and quiescent IRDC G35.39), the supersonic turbulence is likely replenished by protostellar outflows. G35.20 is therefore representative of an evolved form of IRDC G35.39. More observations of a sample of IRDCs are highly demanded in order to further investigate the role of turbulence in the initial conditions required for massive-star formation.
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