The Earliest Stages of Star and Planet Formation: Core Collapse, and the Formation of Disks and Outflows

Abstract

The formation of stars and planets are connected through disks. Our theoretical understanding of disk formation has undergone drastic changes in recent years, and we are on the brink of a revolution in disk observation enabled by ALMA. Large rotationally supported circumstellar disks, although common around more evolved young stellar objects, are rarely detected during the earliest, "Class 0" phase; a few excellent candidates have been discovered recently around both low-and high-mass protostars though. In this early phase, prominent outflows are ubiquitously observed; they are expected to be associated with at least small magnetized disks. Whether the paucity of large Keplerian disks is due to observational challenges or intrinsically different properties of the youngest disks is unclear. In this review we focus on the observations and theory of the formation of early disks and outflows, and their connections with the first phases of planet formation. Disk formation -once thought to be a simple consequence of the conservation of angular momentum during hydrodynamic core collapse -is far more subtle in magnetized gas. In this case, the rotation can be strongly magnetically braked. Indeed, both analytic arguments and numerical simulations have shown that disk formation is suppressed in the strict ideal magnetohydrodynamic (MHD) limit for the observed level of core magnetization. We review what is known about this "magnetic braking catastrophe", possible ways to resolve it, and the current status of early disk observations. Possible resolutions include non-ideal MHD effects (ambipolar diffusion, Ohmic dissipation and Hall effect), magnetic interchange instability in the inner part of protostellar accretion flow, turbulence, misalignment between the magnetic field and rotation axis, and depletion of the slowly rotating envelope by outflow stripping or accretion. Outflows are also intimately linked to disk formation; they are a natural product of magnetic fields and rotation and are important signposts of star formation. We review new developments on early outflow generation since PPV. The properties of early disks and outflows are a key component of planet formation in its early stages and we review these major connections.

What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

“…This mechanism would amplify the field up to some saturation level, and would naturally explain why the field strength does not depend on the largescale field. Second, the field could be dominated by flux advected into the region around the young stars, but this could be modulated by the escape of flux via magnetic interchange instability, whereby field lines that are drained of mass become buoyant and rise away from accreting protostars (Zhao et al 2011;Krasnopolsky et al 2012;Li et al 2014). This would explain the lack of growth of field with stellar mass.…”

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…Viscosity depletes matter interior to the gap, leading to inner holes. Adams et al (2004) concluded that angular momentum support against gravity leads to the launching of flows at smaller radii (∼ 0.1 − 0.2r g , also Begelman et al 1983, Liffman 2003, Font et al 2004. Alexander et al (2006) recognized that the creation of a hole leads to the direct irradiation of the inner rim and results in a rapid dispersal of the outer disk.…”

“…Gorti et al further treat the flow dynamics using simple analytical estimates drawn from previous work on thermal winds (Begelman et al 1983, Liffman 2003, Adams et al 2004, Waters & Proga 2014, but conduct detailed thermo-chemical modeling. Owen et al claim that the flow structure is unimportant for soft X-rays, and adopt the opposite approach to solve forṀ pe using full radiation hydrodynamics models with simpler thermal physics.…”

“…Theories of jet acceleration all invoke the presence of relatively strong magnetic fields, whether for protostellar X-winds or for disk-anchored disk winds (Pudritz et al 2007, Li et al 2014). The nomenclature "wind" describes the idea that the flows are initially poorly collimated.…”

“…Gravitational instabilities in the initially massive disk lead to strong accretion onto the central star (e.g., Laughlin & Bodenheimer 1994). At early stages, magnetic fields drive powerful jets and winds that carry away angular momentum from close to the star (e.g., Königl 1991, see reviews by Li et al 2014, Turner et al 2014. As star formation proceeds, infall decreases and the disk becomes gravitationally stable.…”