11. Formation of Massive Stars Flashcards
What does HR in HR diagram stand for?
Hertzprung Russell
What is the stellar birth line?
The line in the HR diagram below which the young pre-main sequence stars have become visible
What does the birth line for massive stars on a HR diagram coincide with?
The main sequence
Can we see high mass stars in the pre main sequence phase?
No
What are the axes in a HR diagram?
Luminosity against temperature
What is the straight solid line on a HR diagram?
The main sequence
What is the main heating source once the star reaches main sequence?
Nuclear fusion
What are the dots in the HR diagram that are sitting above and right of the main sequence line?
Young low mass stars that have not yet reached main sequence
What is the source of heating in young low mass stars that have not yet reached main sequence?
Gravitational contraction
Why can’t we see massive stars in the pre-main sequence stage?
Different contraction timescales, in comparison to low mass, compared to tff
When do we see high mass stars?
When they reach main sequence
What has to happen between the core collapsing into a protostar and joining the main sequence?
It needs to contract further (second collapse)
By radiating away the released gravitational energy from second collapse
What is another word for protostar?
Hydrostatic core
What is the timescale for contraction of a pre-main sequence star onto the main sequence?
Kelvin-Helmholtz timescale
What is the Kelvin-Helmholtz timescale?
The timescale for contraction of a star
What can the timescale for contraction be derived from?
t_K-H ~ Gravitational energy / luminosity
[GM^2/RL]
What can the Kelvin-Helmholtz timescale be derived from?
t_K-H ~ Gravitational energy / luminosity
[GM^2/RL]
What are the most massive, hottest stars?
O type
What is the Mass-Luminosity Relation?
L ∝ M^4
Show the comparison between free fall time and Kelvin-Helmholtz time
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
How does the Kelvin-Helmholtz timescale vary with mass? Why is this?
t ∝ M^-2
Since t ∝ GM^2/RL with L ∝ M^4
How does mass of star affect contraction onto the main sequence?
Higher mass means faster contraction onto the main sequence
For massive stars, how does contraction time and free fall time compare?
t_KH «_space;tff
What implication does t_KH «_space;tff have on massive stars?
Massive stars arrive on the main sequence while still embedded in their molecular clouds
How does Kelvin-Helmholtz time compare to free fall time for low mass stars?
Free fall time is faster - main accretion phase takes place long before low mass star joins main sequence
Why can we see low mass stars in their pre main sequence stage?
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
Why do massive stars have an invisible pre-main-sequence phase?
They reach the main phase while still accreting material
How are massive stars identified, if they are invisible in the pre-main-sequence phase?
Luminous IR sources
HII regions
How do HII regions form around massive stars?
More luminous so generate enough energy to ionise surroundings
What radiation do we see in HII regions around massive stars?
Free-free emission from ionised gas
Emits at long, radio wavelengths which can pass through dusty envelope
What size are radio wavelengths?
cm wavelengths
What wavelength do massive stars emit a lot of their radiation at?
UV
What is effect does massive stars releasing a lot of UV have?
Energetic photons dissociate H2 and ionise atomic hydrogen (HI)
In H2 regions around a massive star, what processes is there a balance between?
Energetic photons dissociate H2 to ionise atomic hydrogen (HI)
Recombination with an electron to form atomic hydrogen
Why can a star with fixed UV output ionise a limited region?
Limited by number of photons emitted per unit time with correct energy for dissociation
(and recombination)
What is the Strömgren sphere?
The size of the HII region around a massive star
When does Strömgren sphere apply?
When surrounding medium of massive star is relatively uniform
Generally, how to calculate size of Strömgren sphere?
Balance photo-ionisation rate with recombination rate
And use the fact there is a finite number of ionising (>13.6eV) photons from the star
What is HI?
Neutral hydrogen
What is HII?
Ionised hydrogen (+1)
Derive the calculation for the Strömgren radius
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)
What does each term mean in the derivation for the Strömgren sphere?
Curly R = Volumetric recombination rate
alpha_rec = Rate coefficient
Assumptions when calculating Strömgren sphere?
Volumetric ionisation rate = recombination rate
Charge neutrality i.e., ne = np
Constant density
ne = nH0
What determines the size of the HII region?
Stellar temperature and luminosity on one hand, density of medium on the other hand
What assumption can we make since the ionisation spreads quickly to the Strömgren radius?
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)
How does Rs (Radius of Strömgren sphere) vary with density?
Rs ∝ n_0H2 ^ -2/3
How does lower density affect ionisation of material in a massive star?
More material is ionised
To make a HII region, what energy photons do we need?
> 13.6 eV
How does spectral type affect number of ionising photons?
As spectral type mass increases (cooler to hotter), number of ionising photons increases sharply
Why do O type stars have a steep increase in ionising photons compared to B type?
Very hot, pushes peak in BB spectrum into extreme UV region
Mass and temperature off stars that can ionise their surrounding gas?
M > 10 solar masses, T ? 30,000 K
Can B type stars for HII regions?
Only really B0, at boundary between O type
Why can we see HII regions?
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
Why does the HII region around a massive star expand?
Overpressure - sharp boundary between neutral and ionised gas
Why is there a pressure difference (P=nkT) at the boundary of the HII region?
Ionised H has twice as many particles
Temp is 10,000 K compared with 10K outside
What is the expansion speed of the Strömgren radius?
~ sound speed ~ 10 km/s
What is approximate sound speed?
10 km/s
How can we approximate KH timescale?
Ratio of gravitational energy to luminosity
What does it mean that massive stars have no pre-main sequence phase?
They arrive on the main sequence while still embedded in their surrounding envelopes
How are HII regions produced?
Massive stars ionise their surrounding environment
Why are HII regions observed at radio wavelengths?
Via free-free emission of radiation from the ionised gas
What is the boundary of the HII region like?
Sharp edged due to pressure changes
What is the radius of the HII region for an O6 star?
0.4 pc
How much of a massive star’s life does it spend embedded in its molecular cloud?
15%
What are the main phases while a massive star is embedded within its molecular cloud?
Infrared dark cloud (IRDC)
Hot core
Massive young stellar object
Hyper- then ultra-compact HII region
Compact then classic HII region
How do IRDC compare to B68, for example?
Can’t see through cloud with IR (optically thick)
Whereas B68 could see reddened star embedded
What are the sites of future massive star formation?
IRDCs
What are IRDCs?
Clouds that exhibit significant IR opacity
What are the properties of IRDCs?
Extreme properties:
Cold (<20 K)
Dense (>10^4 cm^-3)
Enormous column densities (> 10^23-25cm^-2)
Dark at 100 micron
MYSO stands for?
Massive young stellar objects
Properties of MYSOs?
Bright at mid- and near-
IR wavelengths
Luminous: > 10^4 L⦿
Radio quiet: UCHII has not yet formed
Bipolar molecular outflow
What are the bipolar molecular outflows in MYSOs evidence for?
That accretion is still taking place
UCHII stands for?
Ultra compact HII regions (ionised)
How does the size of UCHII compare to classic HII regions?
Much smaller
Less luminous, more dense
At FIR, what are the most luminous objects in the galaxy?
HII regions
Show whether massive stars are expected to form at all
See notes
(Radiation pressure > rate of change of momentum of infall - to stop accretion
Macc and luminosity)
What is the main question when asking whether massive stars are expected to form?
Can radiation pressure from accretion of infalling material?
What forces are considered when answering whether massive stars are expected to form?
Mass accretion rate
Luminosity as a function of mass
Can the radiation pressure from a ~ solar mass star stop the accretion of
infalling material?
A ~ solar mass star is too faint by a factor of ~ 10^6 to stop accretion
Show the upper limit of massive stars to form is about 30 solar masses
See notes
(i.e., at what luminosity does radiation pressure matter?)
Typical infall velocity?
100 km/s
Typical mass accretion rate for a forming massive star?
10^19 kg/s
Why are stars more massive than 30 solar masses not expected to form?
They would impede the accrediting material (via radiation pressure)
What is the main problem in forming massive stars?
Radiation pressure on dust considered severe hindrance to accretion
(But accretion previously assumed to occur spherically)
Problem may be fixed by accretion disk
Why might cavities in massive stars form?
Carved by radiation / stellar wind in the polar directions
What are the 3 competing theories for the formation of massive stars?
Monolithic collapse and disk accretion: isolated cores
Competitive accretion and runaway growth: strong clustering (gravitational well)
Coalescence: stellar collisions and mergers, dense systems
Main observable characteristics of MYSOs?
Luminous at IR
Posses HII regions which are bright at radio wavelengths
What does a simple consideration of comparing rate of change of momentum of infalling material with force due to radiation suggest?
That massive stars cannot form
How can problems with formation of massive stars be overcome?
Via same mechanism of low mass i.e., via an accretion disk
As well as accretion disk formation, what does the formation of most massive stars require?
Additional process e.g., coalescence in a dense cluster
