Section 10

Question 1

Why is an accretion disk formed?

Answer 1

Due to material falling onto a protostar with substantial angular momentum

They are geometrically thin

Question 2

What is their orbital velocity close to?

Answer 2

Keplerian (in order to be in stable orbit around central mass)

Question 3

What objects have orbital velocities with Keplerian rotation?

Answer 3

Planets in the solar system (1/R^2 between orbital velocity and semi major axis))

Question 4

What does the viscosity in the disk cause?

Answer 4

Mass transfer inwards
angular momentum transfer outwards

(higher and lower angular momentum particles interacting leads to net mass transfer inwards and net angular momentum outwards)

Question 5

What is viscosity believed to be caused by?

Answer 5

Turbulence, magnetic fields or winds

Question 6

What happens to the accretion disk as time progressed?

Answer 6

The ring spreads and distributes more and more mass to smaller radii

Annulus time step changes (t) and annulus material viscosity spreads

Question 7

What leads to the shape of the amount of material in a column per unit area in the disk at t = 0.256?

Answer 7

More material towards centre of the disk than outwards

More angular momentum transport outwards leading to shape at t = 0.256

Question 8

How can these disks be observed?

Answer 8

Directly (imaging) or indirectly (excess emission)

They are warmer closer to the star so only inner disk emits at infrared

Beyond 10 AU only emits at mm wavelengths

Question 9

What is direct evidence for disks?

Answer 9

Spatially-resolved thermal emission from dust grains

Spatially-resolved molecular line emission

Reflected/scattered light (in near IR images)

In silhouette against bright nebular background

Question 10

What is needed to resolve the sub mm thermal emission from disks?

Answer 10

interferometry (of molecular gas)

(only the dust is visible from the disks surrounding the star)

The beam gives the spatial resolution of the observations: larger beam = less well resolved disk)

Question 11

What do the gaps in the images of disks indicate?

Answer 11

Places where planets are actively forming, orbiting at that location and sweeping up material in disk

Question 12

What indicates that the dust is in a ring from the interformetrry maps?

Answer 12

There is a peak in the dust emission that is not centralised at star location, it is offset from this

Question 13

How can spectrally resolved molecular lines reveal orbiting molecular disks?

Answer 13

Through use of Keplerian motion and using double-peaked line profile

Question 14

What does the flux density graph of a disk show?

Answer 14

The molecular transitions
There are two different disks

Question 15

What is the double peak structure an indication of?

Answer 15

That the disk orbiting the star is inclined

Incline to line of sight causes one peak to be blue shifted emission towards observer and the other peak has red shifted emission moving away from observer (giving evidence of molecular disk using kinematic signature of it)

Question 16

What does a cavity in disk mean?

Answer 16

There is a lack of emission as no gas orbiting close enough to star

Question 17

How is the position velocity spatial resolution graph created?

Answer 17

By taking a slice through a cube to get it

Question 18

What does spectrally-resolved molecular line emission give us?

Answer 18

Position-position velocity information

Question 19

What happens if a disk is flared?

Answer 19

There are asymmetries in channel map

Question 20

What do modern optical/near IR telescopes use?

Answer 20

Coronagraphs to see scattered starlight from disk (used to block light from star so that disk can be observed)

Question 21

How can dusty disks be found using indirect observation?

Answer 21

via measurements of spectral energy distributions (SEDs)

Question 22

What is emission from a disk at infrared wavelength most likely to be?

Answer 22

optically thick

Question 23

What is the accretion disk spectrum and what does it produce?

Answer 23

The sum of a number of black body spectra, each with a temperature corresponding to the temperature of a ring at a distance , R , from the star

Question 24

From the log Lv vs log frequency graph showing the shape of accretion spectrum, what can be inferred about the gradient of the line?

Answer 24

In the UV:
Shallower line shows higher accretion rate

Steeper line shows weaker accretion rate

Question 25

What happens to the UV/optical radiation from a star?

Answer 25

It is absorbed by dust in the disk (causing the dust to warm up and become luminous) and re-radiated in the IR causing EXCESS EMISSION

Question 26

What do emissions from circumstellar dust cause?

Answer 26

The peak in the SED to shift to longer wavelengths (100 um)

Question 27

What happens in the early stages of formation?

Answer 27

The dusty envelope is very optically thick, even at IR wavelengths (spectrum peaks in far-IR)

Question 28

What happens once the envelope clears?

Answer 28

The peak shifts to shorter wavelengths until light from star and disk is revealed

Question 29

What happens when finally the disk has dispersed?

Answer 29

Only emission from the star (showing a normal SED)

Question 30

Which stars only emit at sub mm wavelength?

Answer 30

Class 0 objects (protostellar core phase, only core collapsing)

Question 31

If energy gained through accretion is efficiently radiated away, what is the temperature profile?

Answer 31

T proportional r^ -3/4