Stellar pathways, STFC “Armagh” 2021 – Robert Izzard – University of Surrey 1
Stellar pathways
Robert Izzard
STFC school “Armagh” 20201
Stellar pathways, STFC “Armagh” 2021 – Robert Izzard – University of Surrey 2
Stellar pathways: contents
● Stellar structure and evolution overview● The Sun: what is it and what will it become?
● Stars of other mass
● Binary stars● Mass transfer → unique channels
● Stellar population synthesis● How to model populations, predictions vs observations
● Try some stellar pathways yourself
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Stellar structure
● Stars are ~ spheres of gas → structure equations
● Mass conservation
● Energy conservation
● Hydrostatic equilibrium
● Energy
transport
● No explicit time evolution …
Recommended read:Dina Prialnik’s book (CUP 978-0521866040)
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Single stellar evolution● … why evolution? Where
● Depends on composition, temperature, density
● Nuclear fuel is limited. What happens when it runs out?
● Sun is born ~70% hydrogen, by mass
4p → He3He → 12C
12C + He → 16O16O + He → 20Ne
20Ne → 24Mgetc… → 56Fe
… then endothermic.
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Evolution of the Sun
binary_c models
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Evolution of the Sun
binary_c models
Now
Birth
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Evolution of the Sun
binary_c models
Xc=0.7
Xc=0.0
Main sequence
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Evolution of the Sun
Older → less fuel, hotter core →
Helium ignition line
Xc=0.7
Xc=0.0
Main
sequence
Main sequence: slow changes only
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Evolution of the SunMain sequence
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Evolution of the SunMain sequence
Core hydrogen exhaustion
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Evolution of the Sun
binary_c models
Core helium burning
Asymptotic Giant branch
(first) Giant branch
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Evolution of the Sun
binary_c models
Core helium burning
Asymptotic giant branch
(first) Giant branch
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Evolution of the Sun
Shell hydrogen burning
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Evolution of the Sun
binary_c models
Core helium burning
Asymptotic giant branch:Shell H & He burning
(first) Giant branch:Shell H burning
Fuel exhaustion → shell burning → bright, big, short-lived
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Evolution of the Sun
binary_c models
Whitedwarf
Mass loss on AGB→ strips to core
Sun is not hot enough to ignite past He burning → planetary nebula? → CO white dwarf
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Evolution of the Sun
binary_c models
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Evolution of the Sun
binary_c models
Hertzsprung-Russell diagram: L vs Teff
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Evolution of the Sun
binary_c models
Hertzsprung-Russell diagram: L vs Teff
PN
Post-AGB
Main sequence H-burning
Giant branch
AsymptoticGB
Coreheliumburning
H ignition
He ignition“He flash”
H shell ignition
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Evolution of the Sun
binary_c models
Time as each type → Number observed
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Mass: the main parameter
● Mass determines most of the evolution and fate
● Very low mass < 0.9M☉ → slow: only H-burning MS
● Low/intermediate mass < 8M☉ → up to He burning → CO core → white dwarf
● High mass > 8M☉ → up to Si burning → Fe core → supernova
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Very low-mass stars● < 0.8M☉ → never leave the main sequence in 15 Gyr
→ lock up mass and very numerous “red dwarfs”
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Low-mass stars M<2.5M☉
Evolve through H, He burning → planetary nebulaCompact “red clump” of core-helium burning stars
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Intermediate-mass stars: 2.5-8M☉
Bluer He-burning → planetary nebula
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Massive stars: 8<M/M☉<20
H, He, C, Ne, O, Si, Fe → red supergiant then supernova
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Massive stars: M>20M☉
H, He, C, Ne, O, Si, Fe → no RSG phase, then supernova
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Massive stars: M>20M☉
Strong mass loss → strips star to helium core →“Wolf-Rayet” stars
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Stellar remnants
White dwarfs are born hot → cool + dim with timeSame for neutron stars! (but much rarer)
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Pathways vs mass
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Pathways vs mass
He-burning“short”
Massive helium stars
ONe WDs
Red giants
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Pathways vs mass
Wind + Supernovae
Wind + Supernovae
Wind mass loss
Wind mass loss
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Uncertainties...
Wind + Supernovae
Wind + Supernovae
Wind mass loss
Wind mass loss
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Mass loss● Loss of mass in stellar winds
– Stripped stars: e.g. helium or “Wolf-Rayet” stars
– Stops evolution if T < Tignition
M < 0.8M☉(age > 15 Gyr) 0.8 < M/M ☉< 8
M > 8M☉
HH
HHe
He CO
He
CCNeOSi
C
Fe
Structure at end of evolution
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Metallicity: “the other parameter”
Hotter → shorter lives
Massboundaries→ lower
If metallicity, Z, is lower...
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Binary stars
● Gravitationally bound orbits → P, M1, M2, e
● Kepler’s laws → P2 ∝ a3
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Mass transfer● Stars get old and big
→ “Roche-lobe overflow”
● Strips the primary
→ Adds mass to secondary
→ Spins up secondary
● Stars may merge
● May leave compact binary after “common envelope”
→ such systems cannot form in single-star evolution
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Binary fraction → Period distributionDuquennoy & Mayor (1991)Raghavan+ (2010)
Sana+ (2012)
Massive stars
Low-mass stars
Massive stars→ mostly multiple→ hierarchical systems 3 = 1+2; 4 = 2+2
Moe & di Stefano 2017
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Binary-specific channels● COWD-COWD merger → RCrB stars and SNeIa
● Stripped red giants → sdB/O helium-stars, HeWDs
● Stripped massive stars → WR stars, SNeIb/c
● Massive close binaries: collapsars → long GRBs
● Blue and red straggler stars “youthful” old populations
● Thermonuclear and classical novae
● Merging NS/BH → gravitational wave sources
● … + many more!
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Pathways → population synthesis
● Quantitative link models ↔ observations
● Birth functions : single/binary, mass functions, period distribution, eccentricity distribution etc.
● Star formation and metallicity history
● Stellar evolution model: single and binary
Izzard & Halabi (arXiv 1808.06883)
● Predict number and properties of stars in channels● Compare to observed number
→ understand model parameters→ rule in/out models → quantitatively validate our understanding ←
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Number counts: probabilities
Numberof
stars
Selectionfunction
Starformation
rate
Time in
channel= ×
Birthprobability(M1,M2,P,...)
× ×
galaxydependent “initial mass
function”
Star you want? → 1 → Stellar Evolution ←
Summed over all stars and all time
Izzard & Halabi (arXiv 1808.06883)
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Number counts: probabilities
Numberof
stars
Selectionfunction
Starformation
rate
Time in
channel= ×
Birthprobability(M1,M2,P,...)
× ×
Simplest : Assume SFR = const. , metallicity = const.
We can calculate these with “only” → the input distributions and
→ a fast stellar evolution algorithm
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Predictions vs observations: ratesTy
pe
Ia S
N r
ate
Claeys et al. (2014)
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Predictions: chemical yieldsM
ass
loss
rat
e o
f 1
2C
binary_c models
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Even compare to Gaia...
Bin
ary
frac
tio
n
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Try things for yourself● http://personal.ph.surrey.ac.uk/~ri0005/cgi-bin/binary5.cgi
● http://personal.ph.surrey.ac.uk/~ri0005/window.html
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