Section 10 Flashcards
Why is an accretion disk formed?
Due to material falling onto a protostar with substantial angular momentum
They are geometrically thin
What is their orbital velocity close to?
Keplerian (in order to be in stable orbit around central mass)
What objects have orbital velocities with Keplerian rotation?
Planets in the solar system (1/R^2 between orbital velocity and semi major axis))
What does the viscosity in the disk cause?
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)
What is viscosity believed to be caused by?
Turbulence, magnetic fields or winds
What happens to the accretion disk as time progressed?
The ring spreads and distributes more and more mass to smaller radii
Annulus time step changes (t) and annulus material viscosity spreads
What leads to the shape of the amount of material in a column per unit area in the disk at t = 0.256?
More material towards centre of the disk than outwards
More angular momentum transport outwards leading to shape at t = 0.256
How can these disks be observed?
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
What is direct evidence for disks?
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
What is needed to resolve the sub mm thermal emission from disks?
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)
What do the gaps in the images of disks indicate?
Places where planets are actively forming, orbiting at that location and sweeping up material in disk
What indicates that the dust is in a ring from the interformetrry maps?
There is a peak in the dust emission that is not centralised at star location, it is offset from this
How can spectrally resolved molecular lines reveal orbiting molecular disks?
Through use of Keplerian motion and using double-peaked line profile
What does the flux density graph of a disk show?
The molecular transitions
There are two different disks
What is the double peak structure an indication of?
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)
What does a cavity in disk mean?
There is a lack of emission as no gas orbiting close enough to star
How is the position velocity spatial resolution graph created?
By taking a slice through a cube to get it
What does spectrally-resolved molecular line emission give us?
Position-position velocity information
What happens if a disk is flared?
There are asymmetries in channel map
What do modern optical/near IR telescopes use?
Coronagraphs to see scattered starlight from disk (used to block light from star so that disk can be observed)
How can dusty disks be found using indirect observation?
via measurements of spectral energy distributions (SEDs)
What is emission from a disk at infrared wavelength most likely to be?
optically thick
What is the accretion disk spectrum and what does it produce?
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
From the log Lv vs log frequency graph showing the shape of accretion spectrum, what can be inferred about the gradient of the line?
In the UV:
Shallower line shows higher accretion rate
Steeper line shows weaker accretion rate
What happens to the UV/optical radiation from a star?
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
What do emissions from circumstellar dust cause?
The peak in the SED to shift to longer wavelengths (100 um)
What happens in the early stages of formation?
The dusty envelope is very optically thick, even at IR wavelengths (spectrum peaks in far-IR)
What happens once the envelope clears?
The peak shifts to shorter wavelengths until light from star and disk is revealed
What happens when finally the disk has dispersed?
Only emission from the star (showing a normal SED)
Which stars only emit at sub mm wavelength?
Class 0 objects (protostellar core phase, only core collapsing)
If energy gained through accretion is efficiently radiated away, what is the temperature profile?
T proportional r^ -3/4
