stellar pathways - eventsforce · stellar pathways, stfc “armagh” 2021 – robert izzard –...

Stellar pathways, STFC “Armagh” 2021 – Robert Izzard – University of Surrey 1

Stellar pathways

Robert Izzard

STFC school “Armagh” 20201


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


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)


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.


Evolution of the Sun

binary_c models



binary_c models

Now

Birth



binary_c models

Xc=0.7

Xc=0.0

Main sequence



Older → less fuel, hotter core →

Helium ignition line

Xc=0.7

Xc=0.0

Main

sequence

Main sequence: slow changes only


Evolution of the SunMain sequence


Evolution of the SunMain sequence

Core hydrogen exhaustion



binary_c models

Core helium burning

Asymptotic Giant branch

(first) Giant branch



binary_c models

Core helium burning

Asymptotic giant branch

(first) Giant branch



Shell hydrogen burning



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



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



binary_c models



binary_c models

Hertzsprung-Russell diagram: L vs Teff



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



binary_c models

Time as each type → Number observed


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


Very low-mass stars● < 0.8M☉ → never leave the main sequence in 15 Gyr

→ lock up mass and very numerous “red dwarfs”


Low-mass stars M<2.5M☉

Evolve through H, He burning → planetary nebulaCompact “red clump” of core-helium burning stars


Intermediate-mass stars: 2.5-8M☉

Bluer He-burning → planetary nebula


Massive stars: 8<M/M☉<20

H, He, C, Ne, O, Si, Fe → red supergiant then supernova


Massive stars: M>20M☉

H, He, C, Ne, O, Si, Fe → no RSG phase, then supernova


Massive stars: M>20M☉

Strong mass loss → strips star to helium core →“Wolf-Rayet” stars


Stellar remnants

White dwarfs are born hot → cool + dim with timeSame for neutron stars! (but much rarer)


Pathways vs mass


Pathways vs mass

He-burning“short”

Massive helium stars

ONe WDs

Red giants


Pathways vs mass

Wind + Supernovae

Wind + Supernovae

Wind mass loss

Wind mass loss


Uncertainties...

Wind + Supernovae

Wind + Supernovae

Wind mass loss

Wind mass loss


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


Metallicity: “the other parameter”

Hotter → shorter lives

Massboundaries→ lower

If metallicity, Z, is lower...


Binary stars

● Gravitationally bound orbits → P, M1, M2, e

● Kepler’s laws → P2 ∝ a3


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


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


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!


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 ←


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)


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


Predictions vs observations: ratesTy

pe

Ia S

N r

ate

Claeys et al. (2014)


Predictions: chemical yieldsM

ass

loss

rat

e o

f 1

2C

binary_c models


Even compare to Gaia...

Bin

ary

frac

tio

n


Try things for yourself● http://personal.ph.surrey.ac.uk/~ri0005/cgi-bin/binary5.cgi

● http://personal.ph.surrey.ac.uk/~ri0005/window.html

http://personal.ph.surrey.ac.uk/~ri0005/cgi-bin/binary5.cgi

http://personal.ph.surrey.ac.uk/~ri0005/window.html


You have gone too far!

stellar pathways - eventsforce · stellar pathways, stfc “armagh” 2021 – robert izzard –...

Documents