12. Jets and Outflows Flashcards
What is a bow shock?
Shock generated in surrounding ISM from jet propagation
Why do we see knotty outflows?
Episodic accretion means ejection mechanism is not constant
What are Herbig-Haro objects?
Nebulous optical patches located at the end of jets and outflows
Why do Herbig-Haro objects arise?
Interaction of jets with
clumps of gas and dust (or dense plugs of
material) which plough supersonically into a
more diffuse medium
What shape do Herbig-Haro objects often take?
Bow shaped
What is the proper motion / velocity of a Herbig-Haro object?
300 km/s
What are Herbig-Haro objects evidence for?
Episodic ejection
Are Herbig-Haro objects highly ionised?
Yes
Properties of optical jets?
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
What does highly collimated mean?
A lot longer than it is wide
Why do jets and HH objects travel at the same speed?
Jets create the HH objects
What produces molecular line emission in IR outflows?
Molecular H
(H2 doesn’t have pure rotational transitions but has some magnetic dipole transitions that can be excited at high temps)
Why can we see material in a jet in IR?
Fast jet penetrates ISM and sweeps up gas and heating it
Can we see a jet at optical wavelengths?
Yes, if the star is optically visible
How can cavity walls be seen in IR outflows?
Mid IR emission only
(not optical nor NIR)
What features do we discuss in the jets and outflows section?
Jets (optical)
Outflow cavity walls (MIR)
Swept up material / shocked gas (optical / NIR)
Molecular outflows (mm)
What are the properties of molecular outflows?
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⦿
Where do radio jets form?
Very close to the star where the gas is ionised
What are radio jets?
Dense ionised gas at the
base of the jet seen at radio wavelengths
Why does the base of the jet emit radio wavelengths?
Free-free emission
Why do we need high resolution to see radio jets?
Usually less than ~ 1 arcsec long
How are radio jets aligned?
With the outflow axis
If we can’t find the star at optical, or the outflow at IR wavelengths, what can we use?
Radio jets
Does every star produce an outflow?
Yes
During which phase of its life does a star produce an outflow?
YSO
How do outflows help drive turbulence in clouds?
They interact with their surrounding gas, injecting energy and momentum into the cloud
What are the effects of the energy released in shocks (from outflows)?
Dissociate molecules, heat gas, sputter the dust, thereby
triggering chemical reactions that do not (and cannot) occur in the quiescent gas
What do outflows create when they push gas around?
Cavities and shells
What effect can outflows have on parent cloud structure?
They can modify it, even at great distances from the source
What is the implication of the interaction between outflows and the circumstellar envelope?
May help end the infall stage
How do jets from young stars affect their environment?
Generation
of Herbig-Haro objects
Bow shocks
Injection of turbulence
Reshaping and carving the surrounding molecular cloud
What are optical jets?
Fast, shocked, dense, highly collimated gas
When are outflow cavity walls visible?
At mid IR wavelengths
When are molecular outflows visible?
In CO emission
What is the driving force / energy source of jets and outflows?
Magnetic fields (rotational, gravitational potential)
How are the jets collimated?
Pressure of the ambient cloud
Magnetic fields
How to show the origin of the jet is close to the star?
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
Where do jets originate?
Close to the star
Is radiation strong enough to power the outflow?
No
How to show that radiation pressure is not strong enough to power the outflow?
Photon momentum p = E/c
dp/dt = L/c (radiation pressure side)
Compare to rate of momentum of outflow L/c «_space;Ṁv
Plug in numbers to show
How can magnetic fields accelerate gas?
If the m field are rotating
Where can magnetic fields arise to fuel jets?
In the star or disk
What shape are magnetic fields at forming stars?
Hourglass shape
Why are magnetic fields around forming stars hourglass shaped?
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
How does magneto-centrifugal acceleration occur?
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
What is the driving mechanics for jets and outflows?
Magnetohydrodynamics (MHD)
How do highly collimated jets compared to what we expect to happen?
Expect a flattened distribution in the
cloud surrounding the young star (a hot gas expands into a cold one)
How does a jet likely begin? Does this explain its collimation?
Tight winding of magnetic field lines close to star
Does not explain high collimation up to 1000 AU
What is thought to be the origin of jets, along with outflows?
Magnetohydrodynamics
Even if jets are magnetohydrodynamic in origin, why can we still not explain why they are highly collimated?
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
What is the mechanism for collimation of a jet analogous to?
Magnetic field induced by current carrying wire
Multiple wires with parallel current, attractive force
Jet collimation process?
Jets launched and made up of streamlines of hot ionised gas (carrying current)
So streamlines attract to central stream of gas and self-collimate
Why are jets collimated?
Consist of fast moving charged particles
Induces a magnetic field that leads to sustained self collimation
How are disk winds produced?
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”
