Magnetohydrodynamics (MHD) refers to the study of the behavior of electrically conducting fluids, such as plasmas, in the presence of magnetic and electric fields. MHD processes play a vital role in various astrophysical phenomena, including the star-forming process.
In the context of star formation, MHD processes are critical for understanding the dynamics, structure, and evolution of molecular clouds, which are the birthplaces of stars. Here are some key roles that MHD processes play in star formation:
Support Against Gravitational Collapse: Magnetic fields can provide support against gravitational collapse in a molecular cloud. In regions where the magnetic field is strong, it can resist or delay the gravitational pull that wants to compress the cloud and form a star.
Angular Momentum Transport: As molecular clouds collapse, they can start to rotate, or any initial rotation can be amplified. This rotation can hinder further collapse because of centrifugal forces. Magnetic fields can transport angular momentum out of the collapsing region, allowing the collapse to continue.
Formation of Protostellar Jets and Outflows: Observations have shown that young stars often eject material in narrow, highly-collimated jets. This is thought to be a result of MHD processes near the protostar. The rotation of the collapsing cloud, combined with the presence of magnetic fields, can launch and collimate these outflows.
Influence on the Initial Mass Function (IMF): The IMF describes the distribution of stellar masses in a newly formed star cluster. MHD processes, through their impact on cloud dynamics, fragmentation, and collapse, can influence the distribution of masses of the newborn stars.
Magnetic Braking: As mentioned above, magnetic fields can transport angular momentum. This process can "brake" the rotation of the collapsing cloud, impacting the final rotation rate of the forming star and possibly influencing the formation of planets around the star.
Regulating the Star Formation Rate: The interplay between gravity, turbulence, and magnetic fields in molecular clouds can regulate the rate at which stars form. For instance, stronger magnetic fields might support a cloud against collapse and thereby lower the star formation rate.
Fragmentation of Molecular Clouds: Magnetic fields, in conjunction with other physical processes like turbulence and thermal effects, can influence how and where a molecular cloud breaks apart, or fragments, leading to the formation of multiple stars.