Section 12

Question 1

What does the blue wave in images of jets from young stars indicate?

Answer 1

Shock propagating from jet

Question 2

Why do jets appear as knots?

Answer 2

Due to episodic accretion

Question 3

What are Herbig-Halo objects?

Answer 3

Nebulous optical patches of gas and dust located at the end of jets and outflows

Question 4

Why do Herbig-Halo objects arise?

Answer 4

Due to interaction of jets and clumps of gas and dust or dense plugs of material which plough supersonically into more diffuse medium

Question 5

What shape do Herbig-Halo objects have and how fast are they?

Answer 5

Often bow shaped as they are supersonic

velocities = 300 km/s

(they have some evidence for episodic ejection)

Question 6

What are the properties of optical jets?

Answer 6

Shocked ionised gas

Low ionisation fraction

Highly collimated

Dense

Fast

Knots along the jet

Some evidence of precession (jet is changing angle)

Question 7

What does a negative (trace) in the infrared indicates about outflows?

Answer 7

There are line emissions of molecular hydrogen which indicates swept up material in the outflow cavities (only seen in mid-IR emission)

Question 8

What is the chemical composition of jets and outflows?

Answer 8

CO outflow closest to star

Dust and H_2 at cavity wall

H_2 is swept up material

Question 9

In what wavelength emission is the jet visible?

Answer 9

in optical (only the jet can be seen)

Question 10

How is most of the mass ejected?

Answer 10

via molecular outflows which occur to conserve momentum so the final formation of the star can occur

Question 11

What are molecular outflows?

Answer 11

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)

Question 12

What are the sizes of molecular outflows?

Answer 12

Extends to around an arcmin (1 -3 pc)

Masses: 0.1 - 100M_o

Question 13

What are radio jets and what do they indicate?

Answer 13

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

Question 14

What do the outflows interact with?

Answer 14

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

Question 15

What does every star produce?

Answer 15

An outflow for the first 10^5 years of its young stellar object phase

Question 16

What can energy from the the shocks cause?

Answer 16

Dissociate molecules, heat gas, sputter the dust -> triggering chemical reactions that do not occur in quiescent gas

Question 17

What could help end the infall stage?

Answer 17

The interaction between the outflow and circumstellar envelope

Question 18

What does the escape velocity of a 1 M_o star indicate?

Answer 18

That jets originate close to star

Question 19

Can radiation pressure drive the outflows of stars?

Answer 19

No as it is 1000 times weaker than the rate of change of momentum of the outflow (mv)

Question 20

Where can a magnetic field arise?

Answer 20

In a star or rotating disk and as it is rotating, it can accelerate gas

Question 21

What is the shape of the magnetic field?

Answer 21

Hour glass morphology of magnetic field as flux is frozen in field lines

Question 22

What causes the magnetic field lines to twist?

Answer 22

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

Question 23

What can magnetohydrodynamics (MHD) generate?

Answer 23

MHD disk wind (field lines are anchored in the disk)

centrifugal acceleration overcomes gravity and generates these disk winds

Question 24

What do rotating field lines cause?

Answer 24

The co-rotation of material in the atmosphere of the disk

Question 25

What can’t produce a highly collimated jet?

Answer 25

WIND which will expand more rapidly in direction of lowest density (bipolar flow)

Question 26

What does a jet look like on a young star?

Answer 26

Flattened distribution in the cloud surrounding

Question 27

What acts as a sustained mechanism which causes the collimation of jets?

Answer 27

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)

Question 28

Why does outflow not launch closer to star?

Answer 28

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)

Question 29

What causes knots?

Answer 29

When material falls on star from disk, pulses of ejection occur (knots)

Question 30

How are jets seen in the optical?

Answer 30

The gas is atomised and ionised, shock generated heats up the gas causing the gas to ionise and become visible in optical wavelengths