Section 11 Flashcards
What is the stellar birth line?
The line in the HR diagram which young pre-main sequence stars have become visible
(the birth line for massive stars coincides with main sequence)
Before a protostar can join main sequence, what needs to happen?
It has to begin hydrogen burning and needs to contract further by radiating away the released grav energy
What is the timescale for contraction?
Kelvin-Helmholtz timescale: ratio of gravitational energy to luminosity
What is assumed if luminosity is taken to be proportional to M^4 ?
Constant radius
Constant gravity
What can be deduced when comparing Kelvin-Helmholtz timescale with free fall time?
t_k-h «_space; t_ff
freefall timescale has shallower relationship with mass compared to KH timescale
Massive stars arrive on main sequence while still embedded in their molecular cloud
How do the timescales of higher mass stars vary with smaller mass stars?
smaller ratio of timescales
shorter KH timescale compared to free fall timescale so massive stars can’t be seen in initial phases (contraction time is shorter than that for low mass stars)
How is a low mass star formed?
cloud -> collapse -> hydrostatic core -> contraction -> star
free fall time: cloud -> collapse
KH timescale: hydrostatic core -> contraction -> star
How is a high mass star formed?
cloud -> collapse -> collapse -> collapse… -> hydrostatic core -> contraction -> star
Why can’t high mass stars be seen until they are formed?
Pre-main sequence phase is deeply embedded in cloud
Still accreting when they reach main sequence
(upper right region of HR digram is empty)
How are massive stars identified?
Luminous IR sources
HII regions
What is the main characteristic of massive stars?
they have enough energy to ionise their surroundings (UV wavelengths), seen as HII regions
What do energetic photons do in the HII region?
Energetic photons dissociate H_2 and ionise atomic hydrogen
How is atomic hydrogen recreated?
Via process of RECOMBINATION with an electron
What is a star with a fixed output in UV radiation limited to?
Only being able to ionise limited regions in the surrounding cloud
What is the Stromgren sphere?
The surrounding medium of a massive star which is uniform and ionises the surrounding region, producing a HII region
What does the central massive star do?
Photo-ionises hydrogen atoms in the surroundings which results in a HII region
What happens in the ionised gas surrounding massive star?
Free electrons can recombine with protons to form neutral hydrogen
Which rates are balanced in the Stromgren sphere?
The photo-ionisation rate will be balanced by recombination rate
Why is it possible to calculate the size of the Stromgren sphere?
There is a finite number of ionising photons from the star
What determines the size of the HII region?
Stellar temperature and luminosity
density of medium
Why is the number density of electrons with HII region not equal to number density of H atoms externally?
Ionisation spreads quickly to the Stromgren radius and the original cloud density is not able to change appreciably
From Stromgren radius, what can be deduced about increasing the number density?
Stromgren radius decreases as increase in number density means faster recombination so HII region decreases in size)
Less material is ionised
What shows a sharp increase in the number of ionising photons?
Spectral type of stars
Which sizes of stars can ionise their surrounding gas?
M > 10 M_o
T > 30,000 K
Emits radio continuum (free-free radiation)
How can the HII region be observed?
At radio wavelengths via free-free emission of radiation from ionised gas (eventually visible in optical)
What happens at the Stromgren radius and why?
The ionisation fraction drops sharply
due to the PRESSURE DIFFERENCE between regions caused by:
ionised hydrogen has twice as many particles
Temperature drop (from 10,000 K to 10K)
What will the overpressure of the Stromgren radius cause?
The ionisation front to expand due to overpressure at the speed of sound (10km/s)
What is the HII region for an O6 star?
very sharp - edged
radius of 0.4 pc
T = 30,000 K
What holds the Stromgren sphere?
Ionisation balance
volumetric ionisation rate = recombination rate (p+ + e-)
What are the main phases of the molecular cloud period of stars?
Infrared dark cloud (IRDC)
hot core
massive young stellar object (MYSO)
hyper and ultra compact HII regions
compact then classical HII region
What are infrared dark clouds?
clouds that exhibit significant mid-infrared opacity (with extreme properties such as cold temp (<20K)
What wavelengths are MYSOS bright at?
mid and near infrared wavelengths
L > 10^4 L_o
(possess HII regions at radio wavelengths)
Why do MYSOs not have a HII region?
jets and outflows formed: they are still accreting so might be similar to low mass stars
In which wavelength are UCHII regions visible?
in the far-IR they are the most luminous objects in Galaxy
What needs to be considered to evaluate if massive stars are expected to form at all?
Mass accretion rate
Luminosity as a function of mass
What is the main problem with massive star formation?
Radiation pressure on dust considered a severe hinderance on accretion
(problem may be fixed by accretion through a disk)
What are the three theories on massive star formation?
isolated cores
strong clustering
stellar collisions and mergers, dense systems
(the formation of massive stars requires additional processes)
What shows that massive stars cannot form?
Comparing the rate of change of momentum of in-falling material with the force due to radiation
