S12 - Jets and outflows Flashcards
Why do Herbig-Haro objects have lots of emission compared to the rest of the ISM?
Due to their high temperature. The very high temperatures of shocked gas allow many different atoms/molecules to become ionised/excited and emit atomic and ionic lines.
What are Herbig-Haro objects and how do they arise?
Nebulous optical patches (often bow-shaped) located at the end of jets and outflows. They arise due to the interaction of jets with clumps of gas and dust or dense plugs of material which plough supersonically (v ∼ 300km/s) into a more diffuse medium.
What are optical jets emitted by stars and their properties?
Shocked ionised gas (H-α atomic lines, [SII] ionic lines).
Low ionisation fraction (∼10%)(not many ions by percentage).
Highly collimated (L:W ∼ 100:1).
Very dense gas (∼10^9 cm^-3 (about 5 orders of magnitude denser than molecular clouds).
Fast (∼300km/s).
Contain knots along the jet.
What are molecular outflows?
Material swept-up by propagating supersonic jets in the surrounding molecular cloud. Can be seen at mid-IR wavelengths.
What are the properties of molecular outflows?
Low-density gas.
Velocity, v ∼ 10-50km/s.
Mainly emit CO J=1-0 line emission (2.6mm) due to high abundance of CO.
Poorly collimated (L:W ∼ 2:1).
Quite large (can extend ∼1-3pc from star).
Mass ∼ 0.1-100 solar masses.
Why can jets be seen through embedded gas?
Because jets emit radio waves (bremsstrahlung from dense ionised gas at the base of jets).
What is the importance of outflows?
Every star produces an outflow (and jets) for the first 100,000-1,000,000 years of young stellar object phase (before sufficient accretion by disk and star stops).
Outflows interact with their surrounding gas, injecting energy and momentum into the cloud driving turbulence.
Energy from shocks can dissociate molecules, heat gas, sputter the dust, therby triggering chemical reactions that cannot occur in the quiescent gas.
Outflows can push gas around, carving cavities and shells.
They can modify their parent cloud structure, even at great distances from the source.
The interaction between the outflow and the circumstellar envelope may help end the infall stage.
What is the driving mechanism of jets and outflows?
Radiation pressure is not sufficient. Magnetohydrodynamics is their driving mechanism. In certain geometric configurations, rotating magnetic fields can accelerate gas. Rotating disks twist (rotate) frozen magnetic field lines anchored in the disk forming an hour-glass-shaped magnetic field structure. The magneto-centrifugal acceleration caused by rotation of the disk (and frozen magnetic field lines) causes material to accelerate outwards as in the correct geometry, it is strong enough to overcome gravity.
How do jets self-collimate?
A magnetic field will be induced by an ionic jet of gas (like a current-carrying wire ). The magnetic field exerts a force on another charge (e.g. in a nearby wire (or jet of ionised gas)). If the currents are parallel, the magnetic force is attractive. This is a natural way for an ionised outflow to self-collimate into a more collimated jet.
