Striations in molecular clouds: Streamers or MHD waves?

TL;DR

This study investigates the origin of striations in molecular clouds, using MHD simulations to compare different formation mechanisms and concluding that nonlinear MHD wave coupling best explains observed structures.

Contribution

The paper introduces a comprehensive simulation comparison of four models for striation formation, identifying nonlinear MHD wave coupling as the most plausible mechanism.

Findings

Models based on velocity gradients and Kelvin-Helmholtz instability do not match observed density contrasts.

Nonlinear MHD wave coupling reproduces the density contrast and power spectrum of observed striations.

The study supports MHD wave interactions as key to understanding molecular cloud structures.

Abstract

Dust continuum and molecular observations of the low column density parts of molecular clouds have revealed the presence of elongated structures which appear to be well aligned with the magnetic field. These so-called striations are usually assumed to be streams that flow towards or away from denser regions. We perform ideal magnetohydrodynamic (MHD) simulations adopting four models that could account for the formation of such structures. In the first two models striations are created by velocity gradients between ambient, parallel streamlines along magnetic field lines. In the third model striations are formed as a result of a Kelvin-Helmholtz instability perpendicular to field lines. Finally, in the fourth model striations are formed from the nonlinear coupling of MHD waves due to density inhomogeneities. We assess the validity of each scenario by comparing the results from our simulations with previous observational studies and results obtained from the analysis of CO (J = 1 - 0) observations from the Taurus molecular cloud. We find that the first three models cannot reproduce the density contrast and the properties of the spatial power spectrum of a perpendicular cut to the long axes of striations. We conclude that the nonlinear coupling of MHD waves is the most probable formation mechanism of striations.