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Aa Distant Universe > 4. The First Stars > Flashcards

4. The First Stars Flashcards

1
Q

What are the two populations of stars in the local Universe?

A

Population I and II

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2
Q

Define Population I stars

A

Stars like the sun. 2% by mass of metals
- Z = 0.02
- Associated with MW disk

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3
Q

What is meant by “metals” in astronomy?

A

Elements heavier than He

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4
Q

Define population II stars

A

Older, associated with galactic halo and globular clusters. Metal poor
- Z = 0.1 Z_solar = 0.002

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5
Q

Explain which population of stars came first and why

A

Pop II came before Pop I but these cannot be primordial (original) due to the compositions of the Universe after the BB

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6
Q

What was the composition of the universe after the BB?

A

X ~ 75% (H)
Y ~ 25% (He)
Z ~ 10^-6 (Metals)

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7
Q

How are metals formed?

A

In the cores of massive stars or supernovae

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8
Q

If Pop II stars aren’t primordial, what population of stars came first and define them

A

Population III stars which were effectively metal free
- They have a chemical evolution defined by a lack of metals

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9
Q

Describe the cooling process

A

Gas Virialises as it falls into a DM halo
- GPE -> KE -> Heat
- To get enough dense material to trigger nuclear burning, we need to cool it

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10
Q

Describe what happens to the most massive halos

A

They are the hottest and collapse the earliest

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11
Q

What is the fundamental condition for stars to form?

A

The pressure needs to be less than gravity
- Pressure is a function of Temperature

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12
Q

What is the Jeans instability, and explain?

A

The pressure timescale is less than or equal to the dynamical timescale
- If the dynamical timescale is larger, then the gas can move out the way before the pressure gets to it. Pressure can no longer support it

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13
Q

Explain how the gas sound speed and Jeans inequality are related to form a star

A

To make a star, we need to cool the gas to reduce its sound speed
- This satisfies the Jeans inequality

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14
Q

How do we cool a gas (referring to energy), and where is most cooling observed?

A

Need to convert the KE (fn of T) to EM photons which can escape the Halo
- Most cooling occurs by Forbidden Line electron transitions

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15
Q

What is a forbidden line?

A

A photon released by an electron deexciting from a Q state with a low excitation potential through Q interactions

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16
Q

What happens to most photons released by the forbidden line transitions?

A

They are very unlikely to be absorbed before they escape the cloud if they have appropriate energy

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17
Q

Explain how electrons can be exicted into Q forbidden energy levels

A

By collisions
- KE excites the electron
- Once the electron is there it can de excite by emitting a photon

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18
Q

Which type of elements are most forbidden transitions found in

A

Multi electron atoms (metals)
- This cannot occur in Pop III stars

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19
Q

How are forbidden transitions represented notation wise?

A

In square brackets

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20
Q

Explain the replacement of forbidden line transition cooling for Pop III stars

A

Molecular hydrogen transitions

21
Q

What are Molecular hydrogen transitions?

A

Condensation of atomic H to molecular gas releases a photon which typically escapes
- Less efficient than metal line cooling

22
Q

Explains what happens if the gas is too dense when trying to form stars

A

The collisions between gas molecules become too efficient at redistributing energy
- Observe a BB spectrum
- Cooling is less efficient

23
Q

Explain at what temperatures metal rich and metal free clouds become BB like

A

Metal rich - 10K
Metal free - 200K

24
Q

Explain the properties of Pop III stars vs Pop II/I

A

They are typically much larger than “normal” stars
- A typical massive star has a lifetime of a few Myrs
- Pop III stars may reach a mass of 1000 solar masses which is much larger than those in our local U (100-200 solar masses)

25
Q

Explain what fragmentation is

A

The framenting of molecular clouds into stars
- These stars have a range of masses due to density variations, turbulence and gravitational instabilities

26
Q

Describe the graph of the inital mass function (IMF)

A

Axis: log(dN/DM) vs log(M)
Pop I/II start with grad of -1 until 0.5 M solar, then transition to -2.35 grad
Pop III start later and below Pop I/II but have a grad of -1 and continue much further into the logM axis

27
Q

What does the IMF graph shouw about Pop III stars?

A

They will fragment less and we have a top heavy IMF
- More high mass stars

28
Q

State the halo, temperature and cooling for Pop III

A

Pristine halo
T ~ 200K
Molecular hydrogen (H2) cooling

29
Q

State the composition, teemperature and cooling for Pop I/II

A

Metal enriched
T ~ 10K
Metal Line cooling

30
Q

State the halo, temperature and cooling for Pop III.2

A

Pristine Halo
Heated by neighbouring Pop III stars
T >= 10K
Deuterium cooling (H_2 + (D+) -> HD + (H+) + photon

31
Q

What is radiative support?

A

When atoms in the atmosphere of the star absorb photons and are kicked outwards
- Atoms then de excite, emitting a photon in a random direction

32
Q

What is thermal pressure?

A

When the atoms that acquire a change in velocity (and wavelength due to Doppler effect) interact with different photons, and the radiative support process occurs again
- Leads to radiatively driven wind

33
Q

Describe the radiatively driven wind

A

The wind escapes the surface
- Scales with stellar mass and metal concentration
- Carries away angular momentum, energy and mass
- Leads to less massive, slower rotating, longer lived stars than in absence of wind

34
Q

Describe the winds of Pop III stars

A

Very weak winds as they have no metals
- More massive, faster spinning at the ends of their very short lifetimes

35
Q

What temperature do Pop I/II stars collapse at, and undergo nuclear burning at?

A

Collapse at 10K
Nuclear burning at ~10,000K
- So to collapse, they need to cool to 10K but Primordial clouds only cool to ~200K

36
Q

What temperature do Pop III stars reach when undergoing nuclear burning, and what is their corresponding luminosity and an observable of this

A
  • Reach 100,000K
  • Will have 10^7-8 solar luminosities
  • Will have a very blue, hard spectrum peaking in the UV. Lots of photons above E > 13.6eV so we see ionised hydrogen
37
Q

At which energies do we observe ionised hydrogen and helium?

A

Hydrogen at > 13.6eV
He at > 24.6eV, 54.4eV

38
Q

Describe the emission spectrum obseved from ionised gas

A

Ionised gas produces strong and narrow emission lines from recombination
- Excited electrons lose energy by photon emission

39
Q

What is the Balmer series?

A

When electrons reach the first excited level of hydrogen
- In the optical spectrum
- Represented as H_alpha, beta, gamma etc.

40
Q

What is the Lyman series?

A

Transitions to the ground state of Hydrogen
- In the UV spectrum
- Represented at Ly_alpha, beta, etc.

41
Q

Which metal lines are seen in Pop III stars?

A

None

42
Q

State the CR7 case study by Sobral

A

A Pop III candidate at redshift z=6.6
- Turned out to contain less ionised He II, and some metals are ionised before He
- Very hard to do these observations

43
Q

Describe how Pop I/II stars die

A

Massive stars with 8-10 solar masses at the end of their lives
- Core collapse under gravity when burning stops = Core collapse supernovae (CCSN)
- Leaves a neutron star or black hole

44
Q

Describe how Pop III stars die

A

More massive and hotter core than Pop I/II
- Retain more angular momentum so die spinning rapidly

45
Q

Describe the Pair instability supernovae (PISNe) process

A

In the core of very massive stars, we see pair production
- Energy transfers to mass (photon -> elec + posi)
- Temperature and pressure fall
- Massive core collapse with no remnant
- Requires a core mass of 60-130 solar masses with an initial star mass between 140-260 solar masses

46
Q

Describe the pulsation pair instability supernovae (PPISNe) process

A

Slightly lower core mass than PISNe process of ~30-60 solar masses
- Collapse throws off outer layers before the star stabilises
- Happens repeatedly with candidates observed

47
Q

Describe the Superluminous supernovae (SLSNe) process

A

Over x100 brighter than a “normal” SNe like Type II etc.
- Observed rarely in low Z (metallicity), star forming G’s
- Theory: Core collapse to a rapidly spinning, magnetic, massive neutron star (magnetar) with M > 10 M solar

48
Q

Describe how gamma rays are produced

A

Core collapse into a spinning BH
- Accretion disk forms inside the massive star
- Converts GPE -> KE, get EM photons and shocks
- Launches relativistic jets along the polar axis

Aa Distant Universe (7 decks)

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