10. Accretion Disks Flashcards
When does an accretion disk form?
When material falls onto a protostar with substantial angular momentum
What shape are accretion disks?
Geometrically thin (H«R)
What is the orbital velocity of an accretion disk?
Close to Keplerian
What is Keplerian velocity proportional to?
R^-1/2
What would the velocity vs radius graph look like for an accretion disk?
At radius = 0, velocity is very steep
As radius increases, velocity quickly decreases and eventually flattens
What is the trend for orbital velocity of planets in the solar system?
Follow Keplerian rotation i.e. velocity prop to 1/r^2
Why are they called accretion disks?
Viscosity in disk causes mass transfer inwards, and angular momentum transfer outwards
How do we know accretion disks are viscous?
Efficient mass and angular momentum transfer
Do we know the source of viscosity (viscosity mechanism) in the disk?
Unknown - turbulence or magnetic fields (or disk winds)
As time proceeds, how does the disk progress?
Ring spreads, and distributes more and more mass to smaller radii
(non-symmetric annulus)
How can accretion disks be observed (generally)?
Directly (imaging)
Indirectly (excess emission at IR wavelengths)
Direct evidence for disks?
Spatially resolved thermal emission from dust grains
Spatially and/or spectrally resolved molecular line emission
Reflected/scattered light
In silhouette against bright nebular background
Challenges of observing disks?
Tiny - only span a few arcsecs
Outer, cold regions only emit at long (~mm) wavelengths
What does spatially resolved thermal emission of a disk look like?
Can’t see star (doesn’t emit at mm wavelengths)
See dust around star
What did spatially resolved emission from about 10 years ago show?
Cavities in accretion disks (not smooth) - inner regions void of dust
In the ALMA images of spatially resolve thermal emission, there is a lot of structure in the disks. Why?
Protoplanetary disks
Each gap is a planet that is actively forming, orbiting here and sweeping up material from the disk
What do spiral arms in disks lead to?
Potential to be gravitational unstable, and mechanism to form gas giant planets
Can we use spectrally resolved molecular lines to infer disk presence?
Using kinematics of disk, and knowledge of keplerian rotation
How do orbiting molecular disks appear when using spectrally resolved molecular lines?
Double-peaked line profile
In the double peaked line profile of a disk, what is creating the emission?
Molecular line emission i.e., when electron is de-excited and emits a photon
How does a disks’ molecular lines compare to Larson’s?
Larson had a Gaussian with FWHM, disks had double peaks
Why do we see double peaks in the molecular line profile of a disk?
Disk is inclined towards the line of sight
One peak is from red-shifted side (moving away) and the other blue-shifted (moving towards)
Does does inclination of the disk affect line profile?
More inclined, so line is more rotationally broadened (greater velocities over which emission is occurring)
What have interferometers (e.g., ALMA) allowed in terms of spatial resolution?
Map not only thermal emission (and so dust distribution)
But also kinematics and distribution of molecular gas
When were the first spatially and spectrally resolved images of molecular line emission possible?
In 1990s, first interferometers available
What diagram can be made with spatially resolved molecular line emission?
Position-position-velocity
Why is there a different inclination fit for dust (40º) and gas (30º) for the molecular disk using spectrally resolved molecular line emission?
Disk is not geometrically thin - vertical extent
Warp in the disk - change in inclination from inner to outer region - giant planet orbiting in disk
In channel-by-channel images of spatially resolved molecular lines, why is the higher velocity emission more compact?
Only the inner material of the disk is orbiting at the projected high velocity
How do we see a disk from scattered light?
Do not see the disk directly, but see it indirectly because there is enough dust to obscure light from the central star
What do modern telescopes employ to see scattered starlight?
Coronagraphs
What wavelengths are we looking for scattered light from the disk?
Optical / NIR
Why might rings of emission not be concentric/spherical in a coronagraph?
Scattered light coming from elevated surface
Do coronagraphs show dust in the mid-plane?
No - its dust higher in the disk surface
We need longer wavelengths for planet formation in the mid-plane
How do we see disks via silhouette?
Disk obscures background light from nebula (mainly emitting at optical / UV which disk is good at blocking)
How can spectral energy distribution help to find dusty disks around young stars?
Broadband spectrum from UV to mm wavelengths - how emission from source varies as a fn of wavelength
Which cases do we consider generation and shape of SEDs?
Accretion disk spectrum
Emission from surrounding cloud
Evolution of IR spectra from young stars
How was it identified that SEDs can be used to find disks?
Some young stars were brighter than they should’ve been at IR wavelengths if there were emitting as black bodies = IR excess generated due to dust around young stars
What sets the SED of an object?
Its temperature structure
What is presumed in SEDs?
Material is emitting as a black body
What is the temperature structure within an accretion disk?
T prop to r^-3/4
(warmer in the centre)
Show the temperature dependence of the accretion disk is T ∝ r^-3/4
See notes
What does the SED for an accretion disk look like?
mm: Very little
IR: Spectrum peaks - accretion warms disk to emit at these wavelengths
Optical / UV: Very little as disk doesn’t get hot enough to emit
Why can we use SEDs to distinguish between a star and a disk?
Star does not emit strongly at IR, whereas disk does
What does can shape of SED tell us for disks?
Accretion rate
(Brighter disks with shallower spectra being most strongly accreting)
What does a shallower (and brighter) spectra on a SED mean for a disk?
Most strongly accreting
How can excess IR be generated?
Accretion
Passive absorption and remission of energy from star by disk
What can emission from the circumstellar dust help us find?
Deeply embedded protostars and massive star
Why is emission from circumstellar dust important to find stars?
When first formed, stars embedded in dusty envelope
Any UV/optical radiation from star (or accretion shock) is absorbed by dust and re-radiated in the IR
What wavelengths does a star emit?
UV / optical
What wavelengths does an accretion shock emit?
UV / optical
Why does circumstellar dust around a star become luminous?
It absorbs the UV from the star, heating the dust up so it emits in the IR
What do we assume for circumstellar dust emitting at IR?
They are black bodies
What wavelength does circumstellar dust emit at?
Mid IR
Where is the peak in the SED for a deeply embedded massive star?
Shifts to longer wavelengths compared to star, peaks in IR about 100µm
What value is SED peak for a deeply embedded star?
100 µm
Does emission from circumstellar dust give a different temperature profile than an actively accreting disk?
Yes
Show temperature dependence of dusty envelope is T ∝ r^-0.4
See notes
Assumptions when deriving temperature structure in dusty envelope?
Dust grain spherical
Star emitting light at UV
No dust along line of sight
Dust grain remits IR in all directions
How do the temperature profiles for the accretion disk and passive disk compare?
Accretion disk T ∝ r^-3/4
Passive disk T ∝ r^-2/5
Why are the temperature profiles for an accretion disk and a passive disk different?
Reason for the heating is different
(Energy gained equated to Accretion luminosity vs UV from star absorbed by dust grains remitted at IR)
What dominates heating (accretion vs passive disk) in an actual disk?
Heating dominated by accretion in centre
Outer regions, more optically thin to stellar light, more passive disk
How does the SED change as the process evolves?
Class 1 Early stages - dusty envelope optically thick (even at IR wavelengths) - peaks in far IR (cold)
Class 2 Envelope clears - peak shifts to shorter wavelengths until light from star and disk revealed (dusty disk)
Class 3 Disk disperses - leaves behind emission from star only
What is Class 0 objects for an SED?
Emit only at sub mm wavelengths, just a cool black body (envelope mass > protostar mass)
Core on brink of collapse / just started
What does viscous evolution lead to?
Mass transfer inwards, angular momentum transfer outwards
What happens as young stellar objects evolve from Class 0 to Class III?
Peak in spectral energy distribution shifts to shorter wavelengths, as the envelope, then the disk, disperse and the star is gradually revealed
