IX. Star and planet

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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.

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.

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.

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).

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.

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.

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.

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).

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.

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).

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).

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.

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.

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.

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.