Filaments and striations: anisotropies in observed, supersonic, highly-magnetised turbulent clouds (1911.13090v1)

Abstract: Stars form in highly-magnetised, supersonic turbulent molecular clouds. Many of the tools and models that we use to carry out star formation studies rely upon the assumption of cloud isotropy. However, structures like high-density filaments in the presence of magnetic fields, and magnetosonic striations introduce anisotropies into the cloud. In this study we use the two-dimensional (2D) power spectrum to perform a systematic analysis of the anisotropies in the column density for a range of Alfv\'en Mach numbers ($\mathcal{M}_A=0.1$--$10$) and turbulent Mach numbers ($\mathcal{M}=2$--$20$), with 20 high-resolution, three-dimensional (3D) turbulent magnetohydrodynamic simulations. We find that for cases with a strong magnetic guide field, corresponding to $\mathcal{M}_A<1$, and $\mathcal{M}\lesssim 4$, the anisotropy in the column density is dominated by thin striations aligned with the magnetic field, while for $\mathcal{M}\gtrsim 4$ the anisotropy is significantly changed by high-density filaments that form perpendicular to the magnetic guide field. Indeed, the strength of the magnetic field controls the degree of anisotropy and whether or not any anisotropy is present, but it is the turbulent motions controlled by $\mathcal{M}$ that determine which kind of anisotropy dominates the morphology of a cloud.

• The paper demonstrates that anisotropic patterns in molecular clouds are primarily driven by the interplay between magnetic fields and turbulent motions using 3D MHD simulations.
• It reveals that sub-Alfvénic conditions produce magnetosonic striations along magnetic field lines, while higher Mach numbers lead to the formation of perpendicular dense filaments.
• The novel 2D power spectral analysis quantifies scale-dependent anisotropies, offering critical insights for refining star formation models.

Analysis of Anisotropies in Supersonic, Magnetized Turbulent Molecular Clouds

The research paper titled "Filaments and striations: anisotropies in observed, supersonic, highly-magnetised turbulent clouds" presents a detailed investigation into the anisotropic behaviors observed in molecular clouds, specifically focusing on anisotropic structures like filaments and striations within supersonic, highly magnetized turbulent environments. This paper employs 20 high-resolution three-dimensional (3D) magnetohydrodynamic (MHD) simulations to systematically analyze the impact of varying Alfvén ($\Ma=0.1$--$10$) and turbulent Mach numbers ($\M=2$--$20$) on the column density distributions of such molecular clouds.

Key Findings and Methodologies

The research underlines the anisotroic nature of molecular clouds which is primarily influenced by the interplay between magnetic fields and turbulent motions. The assumption that molecular clouds are isotropic is challenged by the observation of filamentary structures and striations that introduce anisotropies. Through spectral analysis of the column density using two-dimensional (2D) power spectra, the paper elucidates the orientations and dominance patterns of these anisotropies over a range of Alfvénic and turbulent Mach numbers.

1. Anisotropy Analysis:
   • The paper distinguishes between two primary anisotropic modes based on the Alfvén Mach number. The elongated structures along the $\kperp$ axis observed in $\M \lesssim 4$ (sub-Alfvénic) scenarios are attributed to magnetosonic striations, whereas higher Mach numbers ($\M \gtrsim 4$) reveal perpendicular high-density filaments dominating the cloud structure, resulting in elongations along the $\kpar$ axis.
2. Influence of Magnetic Field:
   • The strength and alignment of the magnetic field serve as crucial determinants of cloud anisotropy. Strong magnetic fields align structures with field lines, mainly affecting lower Mach numbers' clouds, while increased turbulent motions introduce perpendicular dense filaments at higher Mach numbers.
3. 2D Power Spectra Methodology:
   • By transforming column density data into zero-mean fields and analyzing their power spectra, the study identifies the scale-dependent anisotropies that contribute significantly to cloud morphology. This novel approach effectively captures fluctuations in cloud anisotropy across different regimes.
4. Implications of Results:
   • The anisotropic patterns identified highlight the essential role that magnetic fields play, even in the presence of substantial turbulence, in guiding molecular cloud structure formation. Such findings could revise star formation models by incorporating anisotropic characteristics predicted by MHD simulations, suggesting that observers might employ these anisotropic signatures to deduce parameters like $\Ma$ and $\M$ from real cloud observations.

Conclusion and Future Directions

This study substantially advances our understanding of the anisotropic nature of molecular clouds under supersonic, magnetized conditions, underscoring that both $\Ma$ and $\M$ distinctly influence anisotropic expressions in these turbulent environments. The revelation that high-density filaments and low-density striations exhibit separate modes of anisotropy intertwined with the magnetic and turbulent dynamics signifies an important direction for future theoretical and observational research. Potential future investigations could explore how gravitational dynamics might further influence these structures, integrating insights from gravito-magnetohydrodynamics to holistically model cloud collapse and star formation.

In summary, this paper provides a comprehensive examination of anisotropic phenomena within magnetized molecular clouds, offering pivotal insights into the structural intricacies affected by magnetic fields and turbulence — an essential consideration for realistic modeling of star formation processes.