The role of magnetic field in molecular cloud formation and evolution

Abstract: We review the role that magnetic field may have on the formation and evolution of molecular clouds. After a brief presentation and main assumptions leading to ideal MHD equations, their most important correction, namely the ion-neutral drift is described. The nature of the multi-phase interstellar medium (ISM) and the thermal processes that allows this gas to become denser are presented. Then we discuss our current knowledge of compressible magnetized turbulence, thought to play a fundamental role in the ISM. We also describe what is known regarding the correlation between the magnetic and the density fields. Then the influence that magnetic field may have on the interstellar filaments and the molecular clouds is discussed, notably the role it may have on the prestellar dense cores as well as regarding the formation of stellar clusters. Finally we briefly review its possible effects on the formation of molecular clouds themselves. We argue that given the magnetic intensities that have been measured, it is likely that magnetic field is i) responsible of reducing the star formation rate in dense molecular cloud gas by a factor of a few, ii) strongly shaping the interstellar gas by generating a lot of filaments and reducing the numbers of clumps, cores and stars, although its exact influence remains to be better understood. % by a factor on the order of at least 2. Moreover at small scales, magnetic braking is likely a dominant process that strongly modifies the outcome of the star formation process. Finally, we stress that by inducing the formation of more massive stars, magnetic field could possibly enhance the impact of stellar feedback.

The Role of Magnetic Fields in Molecular Cloud Formation and Evolution

The formation and evolution of molecular clouds are fundamental processes in astrophysics, intricately connected to the lifecycle of stars and the dynamics of galaxies. This paper provides a comprehensive review of the influence of magnetic fields on molecular cloud formation and evolution. The authors approach the topic through multiple lenses, including magneto-hydrodynamics (MHD), the interstellar medium (ISM), magnetized turbulence, and molecular cloud dynamics, providing a detailed analysis of the current state of research and identifying areas where further study is needed.

Magneto-Hydrodynamics and the Interstellar Medium

The paper begins by laying the foundation with the equations of magneto-hydrodynamics (MHD), essential for modeling the evolution of molecular clouds. The ideal MHD framework assumes perfect fluid conductivity, described by the Lorentz force, and is complemented by the Maxwell equations. However, given the low ionization in the ISM, the paper highlights the necessity for non-ideal MHD scenarios, introducing significant corrections such as ion-neutral drift or ambipolar diffusion.

Multi-Phase Interstellar Medium

Molecular clouds form through a phase transition from warm atomic gas to cold dense gas. This process is crucially influenced by thermal instabilities. The paper discusses the complexity of the ISM's thermal structure, emphasizing how cooling and heating balance to guide phase transitions. Notably, the paper argues that the magnetic field plays a significant role in such transitions by affecting how gas compresses and instabilities develop, thereby facilitating or hindering molecular cloud origination.

Magnetized Turbulence

Molecular clouds are highly turbulent, and this turbulence is profoundly affected by magnetic fields. The paper discusses both incompressible and compressible magnetized turbulence, focusing on how turbulence contributes to molecular cloud structures and star formation within them. The influence of compressible magnetized turbulence and the role of strong shock waves in molecular cloud dynamics are particularly highlighted, illustrating their importance in the generation of dense filaments and sub-structures where star formation originates.

Magnetic Field and Density Correlation

The correlation between magnetic fields and density within molecular clouds offers insights into their dynamics. A key discussion is centered around the relation ( B \propto n^{\kappa} ), where the index (\kappa) depends on whether the collapse is aligned or perpendicular to the magnetic field lines. This relationship provides a mechanism by which magnetic fields influence star formation rates and efficiencies in galaxies, as demonstrated by simulations predicting different morphologies based on varying initial conditions and magnetic intensities.

Filament Formation and Characteristics

An unparalleled discovery in recent times is the existence of filamentary structures within molecular clouds, a topic given considerable attention. The authors propose that magnetic fields help in forming and stabilizing these filaments, further discussing the notion of a universal filament width of around 0.1 pc observed across different density scales. Various theories are considered, including the possible role of ion-neutral drift in setting this characteristic scale.

Implications for Star Formation

Magnetic fields are acknowledged for their ability to hinder and potentially lower star formation rates, offering a plausible explanation for the inefficiency of star formation in galaxies relative to their potential. Subcritical clouds, heavily influenced by magnetic pressure, are suggested to undergo significantly slower evolutionary processes due to ambipolar diffusion. This results in reduced star formation rates, impacting the broader galactic evolution.

Conclusions and Future Directions

The review concludes by positing that while magnetic fields do not wholly regulate star formation, they are likely responsible for systematic delays and lower efficiencies. This influential role extends to processes such as the formation of filaments and star-forming cores, each critically vital to galactic structure and dynamics. Despite significant advances, uncertainty remains regarding the precise mechanisms and scales at which magnetic fields exert their influence, signaling a clear need for ongoing numerical and observational research in this domain.

This paper stands as a key contribution to understanding the complexities of molecular cloud formation, emphasizing the interplay of magnetic fields, turbulence, and gravity in these fundamental astrophysical phenomena.