ix. star and planet formationsgoodwin.staff.shef.ac.uk/phy111-lecture09.pdf · stars form in cores...
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IX. Star and planet
formation h"p://sgoodwin.staff.shef.ac.uk/phy111.html
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1. The ISM Most of the volume of space around us contains the diffuse ISM – at 104-106K with densities of only a few atoms per cc.
But in some regions the gas collects and we find giant molecular clouds (GMCs) with T=10-100K and densities of 103-106 atoms per cc.
The high densities allow the gas to cool, and for molecules to form (mainly H2, but also CO, H2O, and NH3) – hence ‘molecular’ clouds.
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1. GMCs The density of GMCs obscures background visible light causing them to appear as ‘dark clouds’ (extinction). But if we observe in the IR the background stars become visible.
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1. GMCs The temperatures of GMCs means the dust radiates in the sub-mm at 100-1000 microns (dust being solid is a BB).
We can also look for emission lines from molecules like CO which are most often in the radio. Annoyingly H2 doesn’t get excited at <200K so we can’t see it directly in GMCs.
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2. Dense cores GMCs contain dense cores with masses about 1 M! and sizes of about 0.1pc. These are gravitationally bound and will collapse to form new stars (this is an ammonia map).
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As stars grow inside their birth cores they go through a number of stages which are divided into ‘classes’.
These stars are not burning hydrogen and are not on the MS – they are pre-MS stars.
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3. Class I (~0.2Myr) Class I: Initially the pre-MS star is deeply embedded in the core and is invisible – so all we see is a cold BB from the envelope. As the pre-MS star grows in mass the envelope becomes less massive, and the star starts to appear but we still see lots of light from the cold gas surrounding it.
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3. Class II (~1 Myr) Class II: when the envelope has all been accreted we are left with a pre-MS star with a disc of gas and dust (a T Tauri star). We see a BB from the star, but also some IR from the colder disc (an IR excess). These discs are the sites of planet formation.
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3. Class III (~10 Myr) Class III: the disc turns into planets, accretes onto the pre-MS star, or is blown-away in about 10 Myr. The pre-MS star is slowly contracting and heating-up. Eventually it will become hot enough to start H-fusion and become a MS star (about 20 Myr for a solar-mass star, maybe 100 Myr for a red dwarf).
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4. Planet formation Planets form during the class II phase when the star has a massive gas disc of about 10% of the star’s mass.
Planets form by core accretion – micron-sized dust particles collect to form larger-and-larger solid objects.
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4. Planet formation Planet formation has four main stages.
Coagulation: dust particles randomly collide and stick together.
Runaway growth: once the larger particles reach a few cm they can sweep-up other particles. This depends on their cross-section (r2).
Oligarchic growth: once they reach ~10km in size gravity becomes important and they can attract nearby objects. This goes as mass2 (or roughly r6).
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4. Planet formation Oligarchic growth will cause a single object to eventually accrete everything in its part of the disc.
In the inner disc this leads to planets of about an Earth-mass(ish) made of the solid material that made the dust.
At a sweet-spot of 5-10 AU the planet is solid dust, but also solid ice and can grow to >10 Earth masses.
Gas giant formation: a >10 Earth mass planet is large enough to attract H and He and keep it making big, gassy planets.
(See PHY106 for more details).
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5. Multiple stars Most cores do not just produce a single star, but normally make a binary, triple or quadruple system.
Many of these decay or are destroyed, but about 50% of older Solar-type stars are in multiple systems.
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Summary Stars form in cores in giant molecular clouds.
Star formation has three main stages: Class I: a pre-MS star deeply embedded in the core. Class II: a pre-MS star surrounded by a gas/dust disc. Class III: a pre-MS star surrounded by planets and some left-over gas/dust.
Each stage lasts about 10x longer than the previous one.
Planets form during the class II phase by core accretion from dust.
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Key points To describe the three phases of star formation and what we observe at each stage.
Describe how planets form from dust in core accretion.
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Example short questions What is a class II pre-main sequence star?
During what phase of star formation do planets form?
Why does a class II pre-main sequence star not have a standard blackbody spectrum?
Would a young pre-main sequence Solar-mass star be bluer, redder, or the same colour as the Sun? Briefly explain your answer.