Protoplanetary Disks and Jets
Because collapsing gas carries angular momentum, a young star is surrounded by a rotating disk that feeds its growth and seeds planets, while magnetized jets shoot perpendicular to the disk and carry angular momentum away.
Definition
Protoplanetary disks are the rotating disks of gas and dust surrounding young stars from which planets form, and jets are the collimated, fast outflows launched along the disk axis that remove angular momentum during accretion.
Scope
The topic covers the structure, temperature, and lifetimes of circumstellar disks, the accretion of disk material onto the star and the problem of angular momentum transport, the launching of bipolar jets and molecular outflows by magnetic forces, and the role of disks as the birthplaces of planetary systems.
Core questions
- Why do young stars have disks?
- How does material in a disk lose angular momentum and accrete onto the star?
- How are the bipolar jets and outflows of young stars launched?
- How do disks give rise to planetary systems?
Key concepts
- circumstellar disk
- accretion
- angular momentum transport
- magnetorotational instability
- bipolar jet
- Herbig-Haro object
- disk wind
Key theories
- Disk accretion and angular momentum transport
- Conservation of angular momentum forces infalling gas into a rotating disk; for material to accrete onto the star, angular momentum must be redistributed outward, by turbulent viscosity thought to arise from magnetorotational instability or by magnetized disk winds.
- Magnetocentrifugal launching of jets
- Magnetic field lines threading the rotating disk and star fling ionized gas outward along the rotation axis, collimating it into fast bipolar jets that shock the surrounding medium as Herbig-Haro objects and carry off the angular momentum that accretion must shed.
Mechanisms
Rotating infall settles into a disk in which friction or magnetic torques transport angular momentum outward, allowing gas to spiral inward and accrete onto the star. Magnetic fields anchored in the rotating disk and star accelerate and collimate some of this material into jets along the poles, which plow into surrounding gas and remove angular momentum from the system.
Clinical relevance
Protoplanetary disks are the sites of planet formation, so their structure, masses, and lifetimes set the conditions for building planetary systems; jets and outflows regulate accretion, inject energy and momentum into clouds, and are striking signposts of ongoing star formation.
History
Disks around young stars were inferred from infrared excesses and later imaged directly; the magnetorotational instability was identified by Balbus and Hawley in 1991 as the likely source of disk turbulence, and the magnetocentrifugal mechanism developed by Blandford, Payne, and others explained the launching of jets.
Debates
- The dominant driver of disk accretion
- Whether accretion is governed mainly by turbulence from the magnetorotational instability or by magnetized disk winds that remove angular momentum vertically remains contested, especially in the poorly ionized regions where the instability may be suppressed.
Key figures
- Frank Shu
- Steven Balbus
- John Hawley
- Roger Blandford
Related topics
Seminal works
- frank2014
- shu1987
Frequently asked questions
- Why does a young star form a disk instead of a sphere?
- The collapsing gas has some rotation, and as it falls inward conservation of angular momentum spins it up; gas can fall freely along the rotation axis but is held up by rotation in the perpendicular plane, so it settles into a flattened, rotating disk.
- What are Herbig-Haro objects?
- They are bright knots and shock fronts created where the fast jets from a young star slam into the surrounding interstellar gas, heating it so that it glows; they trace the outflows that accompany the earliest stages of star formation.