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.
Highlights
The interstellar cycle, which takes place within galaxies, is fundamental for our universe as it controls the formation of stars and the evolution of galaxies
It is likely the case that together with gravity, turbulence is playing a significant role in the evolution of molecular clouds for example by creating strong density fluctuations, owing to its supersonic nature, that may serve as seed for the mass reservoir of future stars
If we ignore the effect of magnetic field and choose the dynamical compression rate of interstellar medium (ISM) as the value of the cloud formation timescale Tf=1Myr, the powerlaw exponent of the mass function of molecular cloud would be too small (α ∼ 1), which is in stark contrast to the observed values
Summary
The interstellar cycle, which takes place within galaxies, is fundamental for our universe as it controls the formation of stars and the evolution of galaxies. Deciphering the various roles that magnetic field is playing is not obvious, (i) because measuring it remains a challenge, (ii) because magnetic field is not a mere pressure and is highly non-isotropic in nature, (iii) because observations do not allow us to vary the parameters as it is possible to do in experiments This can be done in numerical simulations where the influence of a specific parameter, like the magnetic intensity, can be modified and studied. The question of their formation, the physical origin of the possible characteristic width that has been recently inferred and their fragmentation in star forming cores are discussed.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
