The Nature of the Velocity Field in Molecular Clouds. I. The Non-Magnetic Case

TL;DR

This study uses numerical simulations to explore the velocity field and density structures in non-magnetic molecular clouds, revealing the significant role of self-gravity in formation processes and challenging some existing theoretical assumptions.

Contribution

It provides new insights into the role of self-gravity in molecular cloud turbulence and the formation of dense structures, highlighting deviations from lognormal density PDFs and the importance of inward motions.

Findings

Self-gravity influences the formation of dense structures.

Density PDFs deviate from lognormal with self-gravity.

Velocity dispersion includes inward motions from external flows.

Abstract

We present three numerical simulations of randomly driven, isothermal, non-magnetic, self-gravitating turbulence with different rms Mach numbers Ms and physical sizes L, but approximately the same value of the virial parameter, alpha approx 1.2. We obtain the following results: a) We test the hypothesis that the collapsing centers originate from locally Jeans-unstable ("super-Jeans"), subsonic fragments; we find no such structures. b) We find that the fraction of small-scale super-Jeans structures is larger in the presence of self-gravity. c) The velocity divergence of subregions of the simulations exhibits a negative correlation with their mean density. d) The density probability density function (PDF) deviates from a lognormal in the presence of self-gravity. e) Turbulence alone in the large-scale simulation does not produce regions with the same size and mean density as those of the small-scale simulation. Items (b)-(e) suggest that self-gravity is not only involved in causing the collapse of Jeans-unstable density fluctuations produced by the turbulence, but also in their {it formation}. We also measure the star formation rate per free-fall time, as a function of Ms for the three runs, and compare with the predictions of recent semi-analytical models. We find marginal agreement to within the uncertainties of the measurements. However, the hypotheses of those models neglect the net negative divergence of dense regions we find in our simulations. We conclude that a) part of the observed velocity dispersion in clumps must arise from clump-scale inwards motions, and b) analytical models of clump and star formation need to take into account this dynamical connection with the external flow and the fact that, in the presence of self-gravity, the density PDF may deviate from a lognormal.