Jets and Outflows

Question 1

What do Herbig-Haro objects look like?

Answer 1

-patches at the end of the two outflow jets in young stars

Question 2

Herbig-Haro Object

Definition

Answer 2

• nebulous optical patches located at the end of jets and outflows
• arise due to the interaction of jets with clumps of gas and dust or dense plugs of material which plough supersonically into a more diffuse medium

Question 3

Herbig-Haro Object

Properties

Answer 3

• often bow shaped
• proper motions: velocities ~300km/s
• some evidence for episodic ejection

Question 4

Optical Jets

Properties

Answer 4

• shocked ionised gas
• low ionisation fraction ~10%
• highly collimated ~100:1
• dense ~10^9 cm^(-3)
• fast ~300km/s
• knots along the jets
• some evidence of precession

Question 5

Infrared Outflows

Answer 5

• in general, infra-red flows trace the bow-shock flanks (swept-up material)
• note that the outflow cavity wall is traced by the mid-IR emission
• the cavity walls are seen only in mid-IR, not the optical or near IR

Question 6

Molecular Ourflows

Answer 6

• low density molecular gas seen at high velocities 10-50km/s
• mainly CO J=1-0 line (2.6mm), collisional excited
• red and blue lobes spatially separated, bipolar outflow
• this is because the system is tilted so some material is coming towards the observer and some away
• usually poorly collimated ~2-1
• extent is ~arcmin (~1-3pc)
• masses ~0.1-100M☉

Question 7

Radio Jets

Answer 7

• dense ionised (since it is close to the star which emits UV) gas at the base of the jet seen at radio wavelengths
• free-free continuum emission
• usually less than ~1 arcsec long and aligned with the outflow axis

Question 8

When do stars produce outflows?

Answer 8

-every star produces an outflow for the first 10^5-10^6yrs of its young stellar object phase

Question 9

How do outflows effect the surrounding gas?

Answer 9

• outflows interact with their surrounding gas, injecting energy and momentum into the cloud, this is though to help drive turbulence
• energy from shocks can dissociate molecules, remove ices from dust grains and oxygen from silicates leading to water emission, heat gas, sputter the dust, thereby triggering chemical reactions that do not (and 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

Question 10

Summary of Jets and Ouflows

Answer 10

• optical jets are fast, shocked, dense and highly collimated: some have evidence of precession
• jets carve an outflow cavity, the wall of which is visible only at mid-IR wavelengths
• young stars also generate molecular outflows visible in CO emission: these are slow, bipolar can extend to ~parces scales and are poorly collimated
• the base of the jets are brights at radio wavelengths: this is dues to free-free emission form the ionised material very close to the young star

Question 11

What is the driving force / energy source for jets and outflows?

Answer 11

• radiation pressure

- magnetic fields (rotational and gravitational potential)

Question 12

How are the jets collimated ?

Answer 12

• pressure of the ambient cloud

- magnetic fields

Question 13

Where do jets originate?

Answer 13

• equate kinetic energy with gravitational potential energy to find the escape velocity
• jets originate close to the star

Question 14

Driving Mechanisms of Jets and Outflows

Radiation Pressure

Answer 14

• compare the rate of change of momentum of outflowing material with the rate of change of momentum of the stars radiation field
• radiation pressure is not strong enough by a factor of ~1000 to power the outflow
• this is a robust result seen for many objects

Question 15

Driving Mechanisms of Jets and Outflows

Magnetohydordynamics

Answer 15

• in certain geometric configurations, rotating magnetic fields can accelerate gas
• this magnetic field could arise in the star or in the disk
• generated dynamo (or fossil)
• the magnetic field lines are anchored in the disk and co-rotate with it
• so as the disk rotates, the field lines become twisted
• due to the keplarian velocity profile, the B field becomes more tightly twisted closer to the centre of the disk

Question 16

How do magnetohydrodynamics generate an MHD disk wind?

Answer 16

• field lines are anchored in the disk
• rotating field lines enforce co-rotation of the material in the atmosphere of the disk
• centrifugal acceleration overcomes gravity that then generates a ‘disk wind’
• these disk winds occur on very large scales

Question 17

Are jets collimated by wind?

Answer 17

• expect a flattened distribution in the cloud surrounding the young star
• a wind, which takes the path of least resistance, will expand most rapidly in the direction of lowest density leading to a bipolar outflow
• thus, a wind cannot produce the highly collimated jets observed from young stars

Question 18

Are jets magnetohydrodynamic in origin?

Answer 18

• yes, they are though to be so
• even if an initial toroidal component could collimate the outflow, it is hard to imagine that the B field would still be wound up at ~parsec distances from the star
• therefore we need another, more sustained mechanism

Question 19

How are jets collimated?

Answer 19

• a magnetic field will be induced by a current carrying wire
• the magnetic field exerts a force on another charge e.g. in a nearby wire
• if the currents are parallel, the force is attractive: a natural way to self collimate an ionised flow