Star-forming regions of the Aquila rift cloud complex

II. Turbulence in molecular cores probed by NH₃ emission^⋆

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: lev@astro.ioffe.rssi.ru
² Ioffe Physical-Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia
³ St. Petersburg Electrotechnical University “LETI”, Prof. Popov Str. 5, 197376 St. Petersburg, Russia
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁶ Purple Mountain Observatory, Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China

Received: 28 February 2014
Accepted: 16 May 2014

Abstract

Aims. We intend to derive statistical properties of stochastic gas motion inside the dense, low-mass star-forming molecular cores that are traced by NH₃(1, 1) and (2, 2) emission lines.

Methods. We use the spatial two-point autocorrelation (ACF) and structure functions calculated from maps of the radial velocity fields.

Results. The observed ammonia cores are characterized by complex intrinsic motions of stochastic nature. The measured kinetic temperature ranges between 8.8 K and 15.1 K. From NH₃ excitation temperatures of 3.5–7.3 K, we determine H₂ densities with typical values of n_H2~ (1−6) × 10⁴ cm^-3. The ammonia abundance, X = [NH₃]/[H₂], varies from 2 × 10^-8 to 1.5 × 10^-7. We find oscillating ACFs, which eventually decay to zero with increasing lags on scales of 0.04 ≲ ℓ ≲ 0.5 pc. The current paradigm supposes that the star-formation process is controlled by the interplay between gravitation and turbulence with the latter preventing molecular cores from a rapid collapse due to their own gravity. Thus, oscillating ACFs may indicate a damping of the developed turbulent flows surrounding the dense but less turbulent core, a transition to dominating gravitational forces and, hence, to gravitational collapse.