[Paper Review] Star-forming regions of the Aquila rift cloud complex. II. Turbulence in molecular cores probed by NH3 emission

S. A. Levshakov, C. Henkel+2 more|arXiv (Cornell University)|May 29, 2014

Astrophysics and Star Formation Studies54 references3 citations

TL;DR

This study investigates turbulence in low-mass star-forming molecular cores within the Aquila Rift cloud complex using high-resolution NH3 (1,1) and (2,2) emission line maps from the Effelsberg 100-m telescope. By analyzing spatial two-point autocorrelation functions (ACF) and structure functions of radial velocity fields, the authors identify oscillating ACFs with slow decay over 0.04–0.5 pc scales, suggesting damping of turbulent flows and a transition to gravitational collapse in dense cores, with kinetic temperatures between 8.8 K and 15.1 K and H2 densities of (1–6)×10⁴ cm⁻³.

ABSTRACT

(Abridged) Aims. We intend to derive statistical properties of stochastic gas motion inside the dense low mass star forming molecular cores 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. 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, 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.

Motivation & Objective

• To characterize the statistical properties of stochastic gas motions in dense, low-mass molecular cores using NH3 emission.
• To investigate the role of turbulence in regulating gravitational collapse in star-forming regions.
• To determine whether observed velocity field structures reflect a transition from turbulent support to gravitational dominance.
• To compare kinematic properties across cores with and without embedded protostars (YSOs).
• To assess the reliability of ACF-based diagnostics for identifying dynamic stages in core evolution.

Proposed method

• Acquired high-spectral-resolution NH3 (1,1) and (2,2) line maps using the Effelsberg 100-m telescope with channel widths of 0.015–0.077 km s⁻¹.
• Calculated spatial two-point autocorrelation functions (ACF) and structure functions from radial velocity field maps to quantify velocity fluctuations.
• Used hyperfine structure of NH3 lines to derive optical depth, excitation temperature, and column density.
• Derived kinetic temperature from NH3 (2,2)/(1,1) line ratios, assuming collisional excitation.
• Estimated H2 density using the relation between Tkin, Tex, and collisional excitation from Ho & Townes (1983).
• Computed non-thermal velocity dispersion σ_turb from linewidths and thermal widths to assess turbulent motions.

Experimental results

Research questions

• RQ1 Do the observed velocity field structures in Aquila's molecular cores exhibit signatures of turbulence damping and gravitational dominance?
• RQ2 How do the autocorrelation functions (ACF) of radial velocity fields differ between cores with and without embedded protostars?
• RQ3 What is the relationship between turbulent velocity dispersion and core size or density gradients?
• RQ4 To what extent do the observed power spectra of velocity fluctuations deviate from a single power-law cascade?
• RQ5 How do the physical properties (Tkin, nH2, abundance) correlate with the presence of YSOs in the cores?

Key findings

• Oscillating ACFs with slow decay over 0.04–0.5 pc scales were detected, indicating possible damping of turbulent flows in dense cores.
• Kinetic temperatures range from 8.8 K (SS2B, starless core) to 15.1 K (YSO-containing cores), with typical values around 10–12 K.
• H2 densities are estimated at (1–6)×10⁴ cm⁻³, with mean values of 1.7–3.6×10⁴ cm⁻³ across the four observed cores.
• Ammonia abundances X[NH3]/[H2] vary from 2×10⁻⁸ to 1.5×10⁻⁷, with higher values in cores hosting YSOs.
• The core L1251C exhibits a regular velocity gradient consistent with rigid-body rotation (˙ϕ ≈ 7×10⁻¹⁴ s⁻¹), though rotational energy is negligible compared to gravitational energy.
• The power spectrum of velocity fluctuations shows a 'knee' rather than a single power-law, suggesting a transient dynamic state where self-gravity competes with nonconservative forces.