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Star and Planet Formation > 11. Formation of Massive Stars > Flashcards

11. Formation of Massive Stars Flashcards

1
Q

What does HR in HR diagram stand for?

A

Hertzprung Russell

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2
Q

What is the stellar birth line?

A

The line in the HR diagram below which the young pre-main sequence stars have become visible

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3
Q

What does the birth line for massive stars on a HR diagram coincide with?

A

The main sequence

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4
Q

Can we see high mass stars in the pre main sequence phase?

A

No

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5
Q

What are the axes in a HR diagram?

A

Luminosity against temperature

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6
Q

What is the straight solid line on a HR diagram?

A

The main sequence

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7
Q

What is the main heating source once the star reaches main sequence?

A

Nuclear fusion

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8
Q

What are the dots in the HR diagram that are sitting above and right of the main sequence line?

A

Young low mass stars that have not yet reached main sequence

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9
Q

What is the source of heating in young low mass stars that have not yet reached main sequence?

A

Gravitational contraction

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10
Q

Why can’t we see massive stars in the pre-main sequence stage?

A

Different contraction timescales, in comparison to low mass, compared to tff

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11
Q

When do we see high mass stars?

A

When they reach main sequence

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12
Q

What has to happen between the core collapsing into a protostar and joining the main sequence?

A

It needs to contract further (second collapse)

By radiating away the released gravitational energy from second collapse

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13
Q

What is another word for protostar?

A

Hydrostatic core

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14
Q

What is the timescale for contraction of a pre-main sequence star onto the main sequence?

A

Kelvin-Helmholtz timescale

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15
Q

What is the Kelvin-Helmholtz timescale?

A

The timescale for contraction of a star

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16
Q

What can the timescale for contraction be derived from?

A

t_K-H ~ Gravitational energy / luminosity

[GM^2/RL]

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17
Q

What can the Kelvin-Helmholtz timescale be derived from?

A

t_K-H ~ Gravitational energy / luminosity

[GM^2/RL]

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18
Q

What are the most massive, hottest stars?

A

O type

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19
Q

What is the Mass-Luminosity Relation?

A

L ∝ M^4

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20
Q

Show the comparison between free fall time and Kelvin-Helmholtz time

A

tff ∝ n^-1/2

(assume constant density) = tff ∝ M^-1/2

so t_KH / tff = M^-3/2

Higher mass, smaller ratio i.e., shorter contraction vs freefall timescale

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21
Q

How does the Kelvin-Helmholtz timescale vary with mass? Why is this?

A

t ∝ M^-2

Since t ∝ GM^2/RL with L ∝ M^4

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22
Q

How does mass of star affect contraction onto the main sequence?

A

Higher mass means faster contraction onto the main sequence

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23
Q

For massive stars, how does contraction time and free fall time compare?

A

t_KH &laquo_space;tff

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24
Q

What implication does t_KH &laquo_space;tff have on massive stars?

A

Massive stars arrive on the main sequence while still embedded in their molecular clouds

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25
Q

How does Kelvin-Helmholtz time compare to free fall time for low mass stars?

A

Free fall time is faster - main accretion phase takes place long before low mass star joins main sequence

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26
Q

Why can we see low mass stars in their pre main sequence stage?

A

Free fall time is faster than K-H time

i.e., main accretion phase takes place long before low mass star joins main sequence so can see disks

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27
Q

Why do massive stars have an invisible pre-main-sequence phase?

A

They reach the main phase while still accreting material

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28
Q

How are massive stars identified, if they are invisible in the pre-main-sequence phase?

A

Luminous IR sources

HII regions

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29
Q

How do HII regions form around massive stars?

A

More luminous so generate enough energy to ionise surroundings

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30
Q

What radiation do we see in HII regions around massive stars?

A

Free-free emission from ionised gas

Emits at long, radio wavelengths which can pass through dusty envelope

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31
Q

What size are radio wavelengths?

A

cm wavelengths

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32
Q

What wavelength do massive stars emit a lot of their radiation at?

A

UV

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33
Q

What is effect does massive stars releasing a lot of UV have?

A

Energetic photons dissociate H2 and ionise atomic hydrogen (HI)

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34
Q

In H2 regions around a massive star, what processes is there a balance between?

A

Energetic photons dissociate H2 to ionise atomic hydrogen (HI)

Recombination with an electron to form atomic hydrogen

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35
Q

Why can a star with fixed UV output ionise a limited region?

A

Limited by number of photons emitted per unit time with correct energy for dissociation

(and recombination)

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36
Q

What is the Strömgren sphere?

A

The size of the HII region around a massive star

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37
Q

When does Strömgren sphere apply?

A

When surrounding medium of massive star is relatively uniform

38
Q

Generally, how to calculate size of Strömgren sphere?

A

Balance photo-ionisation rate with recombination rate

And use the fact there is a finite number of ionising (>13.6eV) photons from the star

39
Q

What is HI?

A

Neutral hydrogen

40
Q

What is HII?

A

Ionised hydrogen (+1)

41
Q

Derive the calculation for the Strömgren radius

A

See notes

(Integrate ionisation rate over a sphere for no. ionisation events per unit time,

From this, Volumetric rate of recombination

Integrate over sphere for total recombination rate

ne = nH0

Rearrange for Rs)

42
Q

What does each term mean in the derivation for the Strömgren sphere?

A

Curly R = Volumetric recombination rate
alpha_rec = Rate coefficient

43
Q

Assumptions when calculating Strömgren sphere?

A

Volumetric ionisation rate = recombination rate

Charge neutrality i.e., ne = np

Constant density

ne = nH0

44
Q

What determines the size of the HII region?

A

Stellar temperature and luminosity on one hand, density of medium on the other hand

45
Q

What assumption can we make since the ionisation spreads quickly to the Strömgren radius?

A

Original cloud density is not able to change much, so number density of e-s in HII region = to no. density of H atoms external to Rs

(ne = nH0)

46
Q

How does Rs (Radius of Strömgren sphere) vary with density?

A

Rs ∝ n_0H2 ^ -2/3

47
Q

How does lower density affect ionisation of material in a massive star?

A

More material is ionised

48
Q

To make a HII region, what energy photons do we need?

A

> 13.6 eV

49
Q

How does spectral type affect number of ionising photons?

A

As spectral type mass increases (cooler to hotter), number of ionising photons increases sharply

50
Q

Why do O type stars have a steep increase in ionising photons compared to B type?

A

Very hot, pushes peak in BB spectrum into extreme UV region

51
Q

Mass and temperature off stars that can ionise their surrounding gas?

A

M > 10 solar masses, T ? 30,000 K

52
Q

Can B type stars for HII regions?

A

Only really B0, at boundary between O type

53
Q

Why can we see HII regions?

A

Ionised, free electrons in presence of a proton can emit, emitting free-free radiation at long wavelengths

Radio radiation it bright and not absorbed by dust

54
Q

Why does the HII region around a massive star expand?

A

Overpressure - sharp boundary between neutral and ionised gas

55
Q

Why is there a pressure difference (P=nkT) at the boundary of the HII region?

A

Ionised H has twice as many particles

Temp is 10,000 K compared with 10K outside

56
Q

What is the expansion speed of the Strömgren radius?

A

~ sound speed ~ 10 km/s

57
Q

What is approximate sound speed?

A

10 km/s

58
Q

How can we approximate KH timescale?

A

Ratio of gravitational energy to luminosity

59
Q

What does it mean that massive stars have no pre-main sequence phase?

A

They arrive on the main sequence while still embedded in their surrounding envelopes

60
Q

How are HII regions produced?

A

Massive stars ionise their surrounding environment

61
Q

Why are HII regions observed at radio wavelengths?

A

Via free-free emission of radiation from the ionised gas

62
Q

What is the boundary of the HII region like?

A

Sharp edged due to pressure changes

63
Q

What is the radius of the HII region for an O6 star?

A

0.4 pc

64
Q

How much of a massive star’s life does it spend embedded in its molecular cloud?

A

15%

65
Q

What are the main phases while a massive star is embedded within its molecular cloud?

A

Infrared dark cloud (IRDC)

Hot core

Massive young stellar object

Hyper- then ultra-compact HII region

Compact then classic HII region

66
Q

How do IRDC compare to B68, for example?

A

Can’t see through cloud with IR (optically thick)

Whereas B68 could see reddened star embedded

67
Q

What are the sites of future massive star formation?

A

IRDCs

68
Q

What are IRDCs?

A

Clouds that exhibit significant IR opacity

69
Q

What are the properties of IRDCs?

A

Extreme properties:

Cold (<20 K)
Dense (>10^4 cm^-3)
Enormous column densities (> 10^23-25cm^-2)
Dark at 100 micron

70
Q

MYSO stands for?

A

Massive young stellar objects

71
Q

Properties of MYSOs?

A

Bright at mid- and near-
IR wavelengths

Luminous: > 10^4 L⦿

Radio quiet: UCHII has not yet formed

Bipolar molecular outflow

72
Q

What are the bipolar molecular outflows in MYSOs evidence for?

A

That accretion is still taking place

73
Q

UCHII stands for?

A

Ultra compact HII regions (ionised)

74
Q

How does the size of UCHII compare to classic HII regions?

A

Much smaller

Less luminous, more dense

75
Q

At FIR, what are the most luminous objects in the galaxy?

A

HII regions

76
Q

Show whether massive stars are expected to form at all

A

See notes

(Radiation pressure > rate of change of momentum of infall - to stop accretion

Macc and luminosity)

77
Q

What is the main question when asking whether massive stars are expected to form?

A

Can radiation pressure from accretion of infalling material?

78
Q

What forces are considered when answering whether massive stars are expected to form?

A

Mass accretion rate

Luminosity as a function of mass

79
Q

Can the radiation pressure from a ~ solar mass star stop the accretion of
infalling material?

A

A ~ solar mass star is too faint by a factor of ~ 10^6 to stop accretion

80
Q

Show the upper limit of massive stars to form is about 30 solar masses

A

See notes

(i.e., at what luminosity does radiation pressure matter?)

81
Q

Typical infall velocity?

A

100 km/s

82
Q

Typical mass accretion rate for a forming massive star?

A

10^19 kg/s

83
Q

Why are stars more massive than 30 solar masses not expected to form?

A

They would impede the accrediting material (via radiation pressure)

84
Q

What is the main problem in forming massive stars?

A

Radiation pressure on dust considered severe hindrance to accretion

(But accretion previously assumed to occur spherically)

Problem may be fixed by accretion disk

85
Q

Why might cavities in massive stars form?

A

Carved by radiation / stellar wind in the polar directions

86
Q

What are the 3 competing theories for the formation of massive stars?

A

Monolithic collapse and disk accretion: isolated cores

Competitive accretion and runaway growth: strong clustering (gravitational well)

Coalescence: stellar collisions and mergers, dense systems

87
Q

Main observable characteristics of MYSOs?

A

Luminous at IR

Posses HII regions which are bright at radio wavelengths

88
Q

What does a simple consideration of comparing rate of change of momentum of infalling material with force due to radiation suggest?

A

That massive stars cannot form

89
Q

How can problems with formation of massive stars be overcome?

A

Via same mechanism of low mass i.e., via an accretion disk

90
Q

As well as accretion disk formation, what does the formation of most massive stars require?

A

Additional process e.g., coalescence in a dense cluster

Star and Planet Formation (14 decks)

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