Different modes of star formation: gravitational collapse of magnetically subcritical cloud

Abstract

Star formation in magnetically subcritical clouds is investigated using a threedimensional non-ideal magneto-hydrodynamics simulation. Since rapid cloud collapse is suppressed until the magnetic flux is sufficiently removed from the initially magnetically subcritical cloud by ambipolar diffusion, it takes ∼ > 5-10 t ff to form a protostar, where t ff is the freefall timescale of the initial cloud. The angular momentum of the star forming cloud is efficiently transferred to the interstellar medium before the rapid collapse begins, and the collapsing cloud has a very low angular momentum. Unlike the magnetically supercritical case, no large-scale low-velocity outflow appears in such a collapsing cloud due to the short lifetime of the first core. Following protostar formation, a very weak highvelocity jet, which has a small momentum and might disappear at a later time, is driven near the protostar, while the circumstellar disc does not grow during the early mass accretion phase. The results show that the star formation process in magnetically subcritical clouds is qualitatively different from that in magnetically supercritical clouds.

Investigating the role of magnetic fields in star formation using molecular line profiles

“…Other effects not considered here, such as the cosmic ray ionization rate (Wurster et al 2018) and the initial magnetic field strength (Bate, Tricco & Price 2014), can also change the collapse time-scale. However, these effects do not change the collapse time-scale to the extent of the difference between supercritical and subcritical models (Machida, Higuchi & Okuzumi 2018). As lower initial densities result in more strongly delayed collapse (the ratio of ionized to neutral species, and thus the ambipolar diffusion time-scales, is larger at lower density), the difference in intensity ratio between models discussed here should be even more extreme in this case, while higher initial densities than the 10 4 cm −3 we assume here seem to be in tension with observed molecular abundances (Priestley et al 2019).…”

The First Two Thousand Years of Star Formation

“…In addition, we impose the gravity (self-gravity plus gravity of the protostar after it forms) only inside the initial cloud radius r < R cl . Thus, the interstellar medium only exists to suppress boundary effects (for details, see Machida & Hosokawa 2013;Machida 2014;Machida et al 2018).…”

“…Therefore, when we adopt a different cloud parameter (different magnetic field strength or different angular velocity), we may see a different evolutionary path of the star formation. For example, when the magnetic field is considerably strong and the initial cloud has a subcritical mass-to-flux ratio, the ambipolar diffusion will play a role in the early gas collapsing phase (Basu & Mouschovias 1994, 1995a and the rotationally supported disk may not appear immediately after protostar formation (Machida et al 2018). In the future, we need to calculate the cloud evolution while varying the cloud parameters in order to fully clarify the star formation process.…”

Star formation from dense shocked regions in supersonic isothermal magnetoturbulence

“…Here we are particularly interested to address the internal structure and time evolution of the seeds of collapsed regions. Detailed smaller scale simulations of star formation, which explore, for example, the effects of ambipolar diffusion (Machida, Higuchi & Okuzumi 2018) or radiation (Rosen et al 2016), assume a collapsing core imposed by hand. These initial conditions typically fall into a class of critical Bonnor-Ebert spheres, may have laminar flow with angular momentum or a weakly turbulent velocity field imposed, and be threaded by uniform magnetic fields.…”