Section 13 Flashcards
What happens when a star heads towards the main sequence (class II and class III phase) ?
Major accretion and outflows have ended
There is a remnant disk around the young star
What happens as star evolves?
Inner region of disk disappears (inside out clearing)
Disk loses dust and becomes fainter
Cool black body hence a weaker emitter
What causes SED of star to drastically change over time?
The envelope dissipating (it becomes fainter at infrared)
What is used to indicate life-time of disk?
Infrared excess measured using SEDs
What happens after 10 million years to the disks around stars?
Up to a few million years, all disks seen to have similar amount of dust mass as Jupiter, then it drastically disperses at 10 million years and disappears entirely after 100 million years
In the stellar frame of reference, what type of path does a photon follow?
It is orthogonal to the velocity of the dust grain
In the grain’s frame of reference, what type of path does the photon follow?
It is tilted by small angle, creating a retarding force
What kind of pressure is on the grain?
Tangential radiation pressure so grain slows down and its orbit shrinks
What does the disk lifetime imply?
There is an upper limit for the timescale for planet formation to occur
Can the forces on a dust grain due to radiation pressure (acting inwards) and gravity (acting outwards) explain inside out clearing?
No as it is too fast a process and has no dependence on radius
What does blow out due to radiation pressure affect?
All same-sized particles equally at all radii (it has no dependence on radii so cannot explain inside out clearing)
What is the Poynting-Robertson effect?
A relativistic effect whereby the dust grain experiences a retarding force due to tangential radiation pressure on the grain
What can explain the observed inside out clearing?
Poynting-Robertson effect (clears the disk from the inside out and the small grains are removed first) however it is still too fast to explain the observed disk lifetimes
What must happen to small dust grains which are lost to the star and what is is consistent with?
They must be replenished; best candidate mechanism is collisions of larger bodies (e.g asteroids, planetesimals)
Consistent with evidence from solar system (i.e late heavy bombardment)
Which observations is the Poynting-Roberts effect inconsistent with?
micron wavelength emission from micron-sized grains (otherwise need too large cold grains)
fits imply disk size of the order of 100 AU but few images show disk of this size
