12. Jets and Outflows

Question 1

What is a bow shock?

Answer 1

Shock generated in surrounding ISM from jet propagation

Question 2

Why do we see knotty outflows?

Answer 2

Episodic accretion means ejection mechanism is not constant

Question 3

What are Herbig-Haro objects?

Answer 3

Nebulous optical patches located at the end of jets and outflows

Question 4

Why do Herbig-Haro objects arise?

Answer 4

Interaction of jets with
clumps of gas and dust (or dense plugs of
material) which plough supersonically into a
more diffuse medium

Question 5

What shape do Herbig-Haro objects often take?

Answer 5

Bow shaped

Question 6

What is the proper motion / velocity of a Herbig-Haro object?

Answer 6

300 km/s

Question 7

What are Herbig-Haro objects evidence for?

Answer 7

Episodic ejection

Question 8

Are Herbig-Haro objects highly ionised?

Answer 8

Yes

Question 9

Properties of optical jets?

Answer 9

Shocked ionised gas (H-𝛼, [SII])

Low ionisation fraction (~ 10%)

Highly collimated (~ 100:1)

Dense (~ 109 cm-3)

Fast (~ 300 km/s)

Knots along the jet

Some evidence of precession

Question 10

What does highly collimated mean?

Answer 10

A lot longer than it is wide

Question 11

Why do jets and HH objects travel at the same speed?

Answer 11

Jets create the HH objects

Question 12

What produces molecular line emission in IR outflows?

Answer 12

Molecular H

(H2 doesn’t have pure rotational transitions but has some magnetic dipole transitions that can be excited at high temps)

Question 13

Why can we see material in a jet in IR?

Answer 13

Fast jet penetrates ISM and sweeps up gas and heating it

Question 14

Can we see a jet at optical wavelengths?

Answer 14

Yes, if the star is optically visible

Question 15

How can cavity walls be seen in IR outflows?

Answer 15

Mid IR emission only

(not optical nor NIR)

Question 16

What features do we discuss in the jets and outflows section?

Answer 16

Jets (optical)

Outflow cavity walls (MIR)

Swept up material / shocked gas (optical / NIR)

Molecular outflows (mm)

Question 17

What are the properties of molecular outflows?

Answer 17

Low-density molecular gas seen at high velocities (10-50 km/s)

Mainly CO J=1-0 line (2.6 mm) collisionally excited

Red and blue lobes, spatially separated -> bipolar outflow

Usually poorly collimated (~ 2-1)

Extent is ~ arcmin (~1-3 pc)

Masses ~ 0.1 - 100 M⦿

Question 18

Where do radio jets form?

Answer 18

Very close to the star where the gas is ionised

Question 19

What are radio jets?

Answer 19

Dense ionised gas at the
base of the jet seen at radio wavelengths

Question 20

Why does the base of the jet emit radio wavelengths?

Answer 20

Free-free emission

Question 21

Why do we need high resolution to see radio jets?

Answer 21

Usually less than ~ 1 arcsec long

Question 22

How are radio jets aligned?

Answer 22

With the outflow axis

Question 23

If we can’t find the star at optical, or the outflow at IR wavelengths, what can we use?

Answer 23

Radio jets

Question 24

Does every star produce an outflow?

Answer 24

Yes

Question 25

During which phase of its life does a star produce an outflow?

Answer 25

YSO

Question 26

How do outflows help drive turbulence in clouds?

Answer 26

They interact with their surrounding gas, injecting energy and momentum into the cloud

Question 27

What are the effects of the energy released in shocks (from outflows)?

Answer 27

Dissociate molecules, heat gas, sputter the dust, thereby triggering chemical reactions that do not (and cannot) occur in the quiescent gas

Question 28

What do outflows create when they push gas around?

Answer 28

Cavities and shells

Question 29

What effect can outflows have on parent cloud structure?

Answer 29

They can modify it, even at great distances from the source

Question 30

What is the implication of the interaction between outflows and the circumstellar envelope?

Answer 30

May help end the infall stage

Question 31

How do jets from young stars affect their environment?

Answer 31

Generation of Herbig-Haro objects Bow shocks Injection of turbulence Reshaping and carving the surrounding molecular cloud

Question 32

What are optical jets?

Answer 32

Fast, shocked, dense, highly collimated gas

Question 33

When are outflow cavity walls visible?

Answer 33

At mid IR wavelengths

Question 34

When are molecular outflows visible?

Answer 34

In CO emission

Question 35

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

Answer 35

Magnetic fields (rotational, gravitational potential)

Question 36

How are the jets collimated?

Answer 36

Pressure of the ambient cloud Magnetic fields

Question 37

How to show the origin of the jet is close to the star?

Answer 37

Compare to escape velocity of star Using M ~ 1 solar mass and R ~ 3 solar radii And compare to 300 km/s, the value we have been given

Question 38

Where do jets originate?

Answer 38

Close to the star

Question 39

Is radiation strong enough to power the outflow?

Answer 39

No

Question 40

How to show that radiation pressure is not strong enough to power the outflow?

Answer 40

Photon momentum p = E/c dp/dt = L/c (radiation pressure side) Compare to rate of momentum of outflow L/c << Ṁv Plug in numbers to show

Question 41

How can magnetic fields accelerate gas?

Answer 41

If the m field are rotating

Question 42

Where can magnetic fields arise to fuel jets?

Answer 42

In the star or disk

Question 43

What shape are magnetic fields at forming stars?

Answer 43

Hourglass shape

Question 44

Why are magnetic fields around forming stars hourglass shaped?

Answer 44

If you have a flux-frozen magnetic field threaded through a molecular cloud, as the material collapses it drags the field lines with it So 'pinched' and twisted up at the YSO and disk

Question 45

How does magneto-centrifugal acceleration occur?

Answer 45

Think beads on a wire As disk rotates, a component of the centrifugal force outwards that allows gas parcels to travel along the field lines

Question 46

What is the driving mechanics for jets and outflows?

Answer 46

Magnetohydrodynamics (MHD)

Question 47

How do highly collimated jets compared to what we expect to happen?

Answer 47

Expect a flattened distribution in the cloud surrounding the young star (a hot gas expands into a cold one)

Question 48

How does a jet likely begin? Does this explain its collimation?

Answer 48

Tight winding of magnetic field lines close to star Does not explain high collimation up to 1000 AU

Question 49

What is thought to be the origin of jets, along with outflows?

Answer 49

Magnetohydrodynamics

Question 50

Even if jets are magnetohydrodynamic in origin, why can we still not explain why they are highly collimated?

Answer 50

Even if an initial toroidal component could collimate the outflow, it is hard to imagine that this B-field would still be wound up at ~ parsec distances from the star

Question 51

What is the mechanism for collimation of a jet analogous to?

Answer 51

Magnetic field induced by current carrying wire Multiple wires with parallel current, attractive force

Question 52

Jet collimation process?

Answer 52

Jets launched and made up of streamlines of hot ionised gas (carrying current) So streamlines attract to central stream of gas and self-collimate

Question 53

Why are jets collimated?

Answer 53

Consist of fast moving charged particles Induces a magnetic field that leads to sustained self collimation

Question 54

How are disk winds produced?

Answer 54

MHD process Rotating m field (either in the star and the disk) can accelerate gas, if in the correct geometry Centrifugal acceleration overcomes gravity that then generates a “disk wind”