Section 12 Flashcards
What does the blue wave in images of jets from young stars indicate?
Shock propagating from jet
Why do jets appear as knots?
Due to episodic accretion
What are Herbig-Halo objects?
Nebulous optical patches of gas and dust located at the end of jets and outflows
Why do Herbig-Halo objects arise?
Due to interaction of jets and clumps of gas and dust or dense plugs of material which plough supersonically into more diffuse medium
What shape do Herbig-Halo objects have and how fast are they?
Often bow shaped as they are supersonic
velocities = 300 km/s
(they have some evidence for episodic ejection)
What are the properties of optical jets?
Shocked ionised gas
Low ionisation fraction
Highly collimated
Dense
Fast
Knots along the jet
Some evidence of precession (jet is changing angle)
What does a negative (trace) in the infrared indicates about outflows?
There are line emissions of molecular hydrogen which indicates swept up material in the outflow cavities (only seen in mid-IR emission)
What is the chemical composition of jets and outflows?
CO outflow closest to star
Dust and H_2 at cavity wall
H_2 is swept up material
In what wavelength emission is the jet visible?
in optical (only the jet can be seen)
How is most of the mass ejected?
via molecular outflows which occur to conserve momentum so the final formation of the star can occur
What are molecular outflows?
Low-density molecular gas seen at high velocities (10-50 km/s)
Mainly CO (collisionally excited)
Appear as red and blue lobes, spatially separated -> bipolar outflows
(generated also by young stars)
What are the sizes of molecular outflows?
Extends to around an arcmin (1 -3 pc)
Masses: 0.1 - 100M_o
What are radio jets and what do they indicate?
Dense ionised gas at the base of the jet seen at radio wavelengths (due to free-free continuum emission from ionised material close to young star)
usually less than 1 arcsec long
indicate massive star
What do the outflows interact with?
The molecular cloud at different distances from the source (and modify the cloud’s structure)
they inject energy and momentum into the cloud, driving turbulence in clouds
What does every star produce?
An outflow for the first 10^5 years of its young stellar object phase
What can energy from the the shocks cause?
Dissociate molecules, heat gas, sputter the dust -> triggering chemical reactions that do not occur in quiescent gas
What could help end the infall stage?
The interaction between the outflow and circumstellar envelope
What does the escape velocity of a 1 M_o star indicate?
That jets originate close to star
Can radiation pressure drive the outflows of stars?
No as it is 1000 times weaker than the rate of change of momentum of the outflow (mv)
Where can a magnetic field arise?
In a star or rotating disk and as it is rotating, it can accelerate gas
What is the shape of the magnetic field?
Hour glass morphology of magnetic field as flux is frozen in field lines
What causes the magnetic field lines to twist?
Material is rotating as it collapsing causing the field lines to twist the closer to the star it gets in the Keplerian disk due to centrifugal force causing magneto-centrifugal accretion
What can magnetohydrodynamics (MHD) generate?
MHD disk wind (field lines are anchored in the disk)
centrifugal acceleration overcomes gravity and generates these disk winds
What do rotating field lines cause?
The co-rotation of material in the atmosphere of the disk
What can’t produce a highly collimated jet?
WIND which will expand more rapidly in direction of lowest density (bipolar flow)
What does a jet look like on a young star?
Flattened distribution in the cloud surrounding
What acts as a sustained mechanism which causes the collimation of jets?
Electromagnetism: fast moving charged particles can induce a magnetic field (the force is attractive) causing streamlines to be attracted to one another near the star causing self-collimation of an ionised outflow (leading to jets)
Why does outflow not launch closer to star?
Gravitational force is too large and further away from star and disk , centrifugal force assists material ejection (if Fc > Fg gas accelerates along field lines)
What causes knots?
When material falls on star from disk, pulses of ejection occur (knots)
How are jets seen in the optical?
The gas is atomised and ionised, shock generated heats up the gas causing the gas to ionise and become visible in optical wavelengths
