Astronomy Quiz Feb 22
Why can debris disks not be remnants of the proto-planetary disks of pre-main sequence stars
1) The stars are old 2) The dust is small 3) Small grains have short lifetimes due to P-R drag 4) They also have short lifetimes due to collisions
Jeans Length
It is a critical radius of an interstellar cloud in space. It depends on the temperature, and density of the cloud, and the mass of the particles composing the cloud. A cloud that is smaller than its Jeans length will not have sufficient gravity to overcome the repulsive gas pressure forces and condense to form a star, whereas a cloud that is larger than its Jeans length will collapse.
What is the second step of the protostellar evolution?
Protostar shrinks and heats as gravitational potential energy is converted into thermal energy
Molecular Cloud
Sometimes called a stellar nursery, a molecular cloud is a type of interstellar cloud, the density, and size of which permit the formation of molecules, most commonly molecular hydrogen (H2). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
What is the third step of the protostellar evolution?
Surface temperature rises when radiation becomes the dominant mode of energy flow within the protostar
What happens after condensation has started?
The cloud enters its "protostar" phase
Daguerreotype
The first publicly available photograph process, and for nearly twenty years it was the most commonly used
What is the fourth step of the protostellar evolution?
The fusion rate increases until it balances the energy radiated from the stars surface
What is the physical evolution of protostars like?
It begins as a nebula, and with increasing pressure, it turns into a protostar
When is a new star formed?
When the core becomes hot and dense enough to maintain stable hydrogen fusion. Inside the remaining protoplanetary disk, planets can form. Finally the disk will disappear and a young stellar system will present with the central star and planets.
What is the first step of the protostellar evolution?
A protostar assembles from a collapsing cloud fragment. It is concealed beneath a shroud of dusty glass
What is T-Tauri
A variable star in the constellation Taurus, the prototype of the T Taurus stars. The protostar, at first, only has about 1% of its final mass. But the envelope of the star continues to grow as in falling material is accreted. After a few million years, thermonuclear fusion begins in its core, then a strong stellar wind is produced which stops the infall of new mass. The protostar is now considered a young star since its mass is fixed.
What are bipolar flows?
Along the axis of rotation of a protostellar disk, in falling material has only little angular momentum and the in-fall proceeds relatively unhindered. Therefore, the molecular cloud becomes thinner along the axis of rotation and two cone-shaped voids are formed at the poles by the jets and outflows. It allows the stellar light to escape and to illuminate these cones from the inside. Material migrates within the protostellar disk towards the protostar. The spinning up protostar and protostellar disk winds up magnetic fields along the polar axis and the formation of bipolar outflows or jets.
Where was the first debris disk discovered?
At 50 light years away, young star Beta Pictoris became important as satellite and ground based telescopic observations revealed the presence of a surrounding outer, dusty, debris disk, and an inner clear zone about the size of our solar system - strong evidence for the formation of planets. Infrared observations clearly detected a source in the clear zone that is most likely a giant planet orbiting Beta Pic. It is more than 1000 times fainter than the direct starlight that has been carefully subtracted from the image data. It is aligned with the disk at a projected distance that would place it near the orbit of Saturn if found in our solar system
Bok Globules
Bok globules are isolated small dark nebulae, containing dense dust and gas from which star formation may take place. Bok globules are found within H II regions, and typically have a mass of about a light year or so across. They contain molecular hydrogen, carbon oxides and helium, and around 1% of silicate dust. Bok globules most commonly result in the formation of double or multiple star systems.
What is dust?
Cosmic dust is dust which exists in space. Most cosmic dust particles are between a few molecules to 0.1 μm in size. A smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left in the stars.
What must occur around protostars?
Disks must occur around protostars and leftover stuff must swirl to "obey" angular momentum
How much of our star formation in our galaxy is simply due to the spontaneous gravitational collapse of a cold molecular cloud and how much requires the collapse to be "triggered" by external forces?
Massive stars have strong radiation pressure and stellar winds. Thus, within molecular clouds, it is widely accepted that star formation can be triggered by ionization or shock fronts from these massive stars. Triggered star formation does not require new, hot stars, to occur. A similar situation can arise following a supernovae explosion in a dense region.
What are Proto-planetary objects like?
Optically thick (she THICC) Dust from 0.1-100 AU Massive gas disk Accretion onto star <10Myr
What are debris disks like?
Optically thin Dust at one radius ~30 AU No gas No accretion >10Myr
Collapsing cloud
The usual tug of war between gravity and pressure
What is Jeans instability?
Thermal pressure cannot support the gas cloud against its self-gravity. The cloud collapses and fragments. It goes from Molecular cloud/dark nebula to cloud fragments to protostars.
Where are protostar debris (pre-planet) disks are common around what?
They are common around stars known to be young from other observations
