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Star and Planet Formation > 12. Jets and Outflows > Flashcards

12. Jets and Outflows Flashcards

1
Q

What is a bow shock?

A

Shock generated in surrounding ISM from jet propagation

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2
Q

Why do we see knotty outflows?

A

Episodic accretion means ejection mechanism is not constant

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3
Q

What are Herbig-Haro objects?

A

Nebulous optical patches located at the end of jets and outflows

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4
Q

Why do Herbig-Haro objects arise?

A

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

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5
Q

What shape do Herbig-Haro objects often take?

A

Bow shaped

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6
Q

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

A

300 km/s

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7
Q

What are Herbig-Haro objects evidence for?

A

Episodic ejection

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8
Q

Are Herbig-Haro objects highly ionised?

A

Yes

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9
Q

Properties of optical jets?

A

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

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10
Q

What does highly collimated mean?

A

A lot longer than it is wide

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11
Q

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

A

Jets create the HH objects

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12
Q

What produces molecular line emission in IR outflows?

A

Molecular H

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

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13
Q

Why can we see material in a jet in IR?

A

Fast jet penetrates ISM and sweeps up gas and heating it

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14
Q

Can we see a jet at optical wavelengths?

A

Yes, if the star is optically visible

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15
Q

How can cavity walls be seen in IR outflows?

A

Mid IR emission only

(not optical nor NIR)

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16
Q

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

A

Jets (optical)

Outflow cavity walls (MIR)

Swept up material / shocked gas (optical / NIR)

Molecular outflows (mm)

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17
Q

What are the properties of molecular outflows?

A

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⦿

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18
Q

Where do radio jets form?

A

Very close to the star where the gas is ionised

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19
Q

What are radio jets?

A

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

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20
Q

Why does the base of the jet emit radio wavelengths?

A

Free-free emission

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21
Q

Why do we need high resolution to see radio jets?

A

Usually less than ~ 1 arcsec long

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22
Q

How are radio jets aligned?

A

With the outflow axis

23
Q

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

A

Radio jets

24
Q

Does every star produce an outflow?

A

Yes

25
Q

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

A

YSO

26
Q

How do outflows help drive turbulence in clouds?

A

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

27
Q

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

A

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

28
Q

What do outflows create when they push gas around?

A

Cavities and shells

29
Q

What effect can outflows have on parent cloud structure?

A

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

30
Q

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

A

May help end the infall stage

31
Q

How do jets from young stars affect their environment?

A

Generation
of Herbig-Haro objects

Bow shocks

Injection of turbulence

Reshaping and carving the surrounding molecular cloud

32
Q

What are optical jets?

A

Fast, shocked, dense, highly collimated gas

33
Q

When are outflow cavity walls visible?

A

At mid IR wavelengths

34
Q

When are molecular outflows visible?

A

In CO emission

35
Q

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

A

Magnetic fields (rotational, gravitational potential)

36
Q

How are the jets collimated?

A

Pressure of the ambient cloud

Magnetic fields

37
Q

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

A

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

38
Q

Where do jets originate?

A

Close to the star

39
Q

Is radiation strong enough to power the outflow?

A

No

40
Q

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

A

Photon momentum p = E/c

dp/dt = L/c (radiation pressure side)

Compare to rate of momentum of outflow L/c &laquo_space;Ṁv

Plug in numbers to show

41
Q

How can magnetic fields accelerate gas?

A

If the m field are rotating

42
Q

Where can magnetic fields arise to fuel jets?

A

In the star or disk

43
Q

What shape are magnetic fields at forming stars?

A

Hourglass shape

44
Q

Why are magnetic fields around forming stars hourglass shaped?

A

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

45
Q

How does magneto-centrifugal acceleration occur?

A

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

46
Q

What is the driving mechanics for jets and outflows?

A

Magnetohydrodynamics (MHD)

47
Q

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

A

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

48
Q

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

A

Tight winding of magnetic field lines close to star

Does not explain high collimation up to 1000 AU

49
Q

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

A

Magnetohydrodynamics

50
Q

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

A

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

51
Q

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

A

Magnetic field induced by current carrying wire

Multiple wires with parallel current, attractive force

52
Q

Jet collimation process?

A

Jets launched and made up of streamlines of hot ionised gas (carrying current)

So streamlines attract to central stream of gas and self-collimate

53
Q

Why are jets collimated?

A

Consist of fast moving charged particles

Induces a magnetic field that leads to sustained self collimation

54
Q

How are disk winds produced?

A

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”

Star and Planet Formation (14 decks)

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