Star and planet formation Flashcards
Sketch and describe the HR diagram
Plots the temperature of stars against their luminosity (the theoretical HR diagram), or the colour of stars (or spectral type) against their absolute magnitude (the observational HR diagram, also known as a colour-magnitude diagram). Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves.
3 evolutionary stages of the HR diagram:
1. The main sequence (luminosity class V) stretching from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) dominates the HR diagram. It is here that stars spend about 90% of their lives burning hydrogen into helium in their cores.
2. red giant and supergiant stars (luminosity classes I through III) occupy the region above the main sequence. They have low surface temperatures and high luminosities which, according to the Stefan-Boltzmann law, means they also have large radii. Stars enter this evolutionary stage once they have exhausted the hydrogen fuel in their cores and have started to burn helium and other heavier elements.
3. white dwarf stars (luminosity class D) are the final evolutionary stage of low to intermediate mass stars, and are found in the bottom left of the HR diagram. These stars are very hot but have low luminosities due to their small size.
Other components–
The instability strip is a narrow, almost vertical region in the Hertzsprung-Russell diagram which contains many different types of variable stars (RR Lyrae, Cepheid variable, W Virginis and ZZ Ceti stars all reside in the instability strip). Most stars more massive than the Sun enter the instability strip and become variable at least once after they have left the main sequence. It is within this region that they suffer instabilities that cause them to pulsate in size and vary in luminosity (due to HeIII) .
How do stars form
Stars are born out of the gravitational collapse of cool, dense molecular clouds. As the cloud collapses, it fragments into smaller regions, which themselves contract to form stellar cores. These protostars rotate faster and increase in temperature as they condense, and are surrounded by a protoplanetary disk out of which planets may later form.
Describe and sketch the Hayashi track
The path on the Hertzsprung-Russell diagram that is followed by a fully convective pre-main sequence star to reach the zero-age main sequence. Hayashi tracks for low-mass stars are near vertical. At higher masses, stars become increasingly radiative as they contract and the Hayashi tracks are almost horizontal
Describe and sketch the Henyey track
A nearly horizontal path on the Hertzsprung-Russell diagram that a pre-main sequence star of small mass follows in an early stage of evolution after leaving the Hayashi track and before reaching the main sequence. During this stage, the pre-main sequence star remains almost wholly in radiative equilibrium.
Describe T Tauri and Herbig stars. How are they different?
T Tauri stars are variable stars showing periodic and random fluctuations in their brightnesses. They are newly-formed (< 10 million years old) low mass (< 2 solar masses) pre-main sequence stars.
Herbig stars are also variable, pre-main sequence stars. They are the intermediate mass equivalent of T-Tauri Stars (2-10 solar masses).
They are both characterized by accretion disks around them. The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir of water and simple organics in the inner regions and a lower mass of ices in the outer disk.
Describe a protoplanetary disk structure.
Thin structures
Vertical height much smaller than the radius
Mass much smaller than the central young star )0.001 to 0.3 solar masses)
several 10s to 1000 AU wide
gas: 99% by mass; the rest is dust
inside the disk, matter slowly moves inwards, and dust particles grow to centimeter-sized pebbles: the first steps toward the formation of kilometer-sized planetesimals.
Classical models assume a smooth disk structure, in which gas densities and temperatures decrease monotonically with distance from the host star. That would be expected from gravitational-collapse models, in which the primary heating mechanism is stellar irradiation.
Rings and gaps usually indicate planet formation
Describe the difference between a protoplanetary, transition, and debris disk.
Protoplanetary Disks– a rotating circumstellar disk of dense gas surrounding a young, newly formed star.
Transition Disks– Phase between a protoplanetary disk and a debris disk, but in this epoch, disks are still characterized by high gas-to-dust ratios and (often) continued gaseous accretion on the central star
Eventually, the transition disk dissipates through some combination of stellar and planetary mechanisms involving accretion, photoevaporation, winds, and agglomeration of large solid bodies. The timescale for the decrease in emission from such disks is wavelength dependent, with most disk emission gone by 3 Myr in the near-IR and 20 Myr in the submillimeter.
Debris Disks– Dust-dominated, gas-poor disks commonly observed around stars over a wide range of ages. They are sustained by the collision of planetesimals that produce a second generation of dust.
Describe the dust and gas structure in these disks.
PPD: a smooth disk structure, in which gas densities and temperatures decrease monotonically with distance from the host star. 99% gas, 1% dust. Gas typically found in the outer regions
TD: high gas-to-dust ratios and (often) continued gaseous accretion on the central star
Debris: Dust-dominated, gas-poor
Does our solar system have a disk?
Yes– the Kuiper belt and Zodiacal cloud (causes zodical light)
What is the snow line? What does the snowline depend on? Is the snowline the same for different elements?
The snow line marks the distance from the central protostar where it becomes cold enough for volatile compounds to condense into their respective ice grains. Different volatile compounds have different condensation temperatures at different partial pressures (thus different densities) in the protostar nebula, so their respective frost lines will differ.
What is the luminosity function for young exoplanets? How does this affect planet searches?
ChatGPT (lol): Young exoplanets are typically more luminous than older ones due to the residual heat from their formation process. They cool and dim over time, and this process can span billions of years. The luminosity of an exoplanet depends on several factors, including:
Its mass and radius: Larger and more massive planets have a greater gravitational potential energy that can be converted into heat and light.
Its age: Younger planets are generally hotter and therefore more luminous due to the residual heat from their formation. As they age, they cool and their luminosity decreases.
Its composition: The planet’s makeup (gas, rock, water, etc.) affects its heat capacity and hence its cooling rate, which in turn influences its luminosity.
Its atmospheric properties: The presence of clouds and the chemical composition of the atmosphere can affect how much heat is trapped by the planet, thus impacting its emitted radiation.
Because these factors vary widely among exoplanets, the luminosity function is likely to be quite complex and challenging to define accurately.
(Maria) Neglecting the influence of lithium burning, deuterium burning, and atmospheres, substellar objects with different masses cool in a similar monotonic fashion over time:
L_bol ∝ t^(-5/4)M(5/2)
What are various theories of planet formation? Describe the differences in “hot start” and “cold start” models.
Cold Start/Core Accretion: This involves accreting a solid core and once the solid core attains ~10 Earth masses, it becomes massive enough to accrete hydrogen and helium in a runaway process to form a giant planet. The solid core accretes gas through an accretion disk. Atmospheric mass increases quite rapidly, and at some point the whole gaseous envelope becomes self-gravitating. This process cools the gas, causing it to lose much of its initial entropy and forms a giant planet that has low initial entropy
Disk Instability/Hot Start: This involves a protoplanetary disk becoming gravitationally unstable and creating a clump which collapses directly to form a giant planet. The gas that collapses directly to form a giant planet retains most of it initial entropy, resulting in high initial entropy
What is the habitable zone? Compute it.
The distance from a star at which liquid water could exist on the surface of the orbiting planets.
Calculating: https://www.planetarybiology.com/calculating_habitable_zone.html
Describe the pre-main sequence classes
Class 0: Only a few thousand years old and have not yet started undergoing nuclear fusion at their cores. They are fueled solely by gravitational potential energy which is released as they accrete infalling material.
Class I: Still collect the dust and gas from the surrounding clouds and their luminosity is largely dependent on gravitational energy. However, unlike class 0 objects, they have begun to undergo nuclear fusion in their centres. These stars are invisible at optical wavelengths and can only be detected at infrared and radio wavelengths because they are still embedded in thick clouds of dust and gas.
Class II: Shrouded in disks of dust and gas, but the process of accumulating infalling material has mostly finished. These objects are also known as classical T Tauri stars.
Class III: Have lost their disks and roughly correspond to weak-line T Tauri stars.
