massive stellar evolution

Massive stellar evolution Roni Waldman 1

Massive stellar evolution

Problems and challenges

November 2012


Problems in modeling massive star evolution Modeling is mostly done in the 1D

approximation. Considerable uncertainties: mass

loss, convection and mixing. New additions to models: rotation,

magnetic fields, 3D convection. Increasing wealth of observational

data enable better constraints on models.

November 2012


First: Overview of a massive star evolution sequenceCCSN progenitor M=15M

November 2012

M=15M Zero age main sequence

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M=15M Main sequence

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M=15M Main sequenceConvection starts receding

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M=15M End of main sequence

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M=15M Shell H burning

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M=15M Core He ignition

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M=15M Core He burning

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M=15M Core He burning – max C abundance

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M=15M Core He exhausted

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M=15M Shell He burning

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M=15M Core C ignition

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M=15M Core C exhausted

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M=15M Shell C burning

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M=15M Core Ne burning

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M=15M Shell Ne burning

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M=15M Core O burning

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M=15M Shell O burning

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M=15M Core Si ignites

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M=15M Core Si burning

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M=15M Si exhausted Fe core collapse

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M=15M Nicer view of burning shells

November 2012Woosley et al 2002

C O Si


Going to lower mass endAGB star 5 M

November 2012


M=5M AGB star

November 2012log10(t-tend/yr)


Close up on the double shell


He burning

H burning


A more detailed view

November 2012

2 Msun


M=9Msun TP-AGB

November 2012Siess 2010


AGB star

If no mass loss C will eventually ignite!

Growth of core is overcome by mass loss.

End in CO white dwarfs This is sensitive to:

Metallicity Uncertainty in mass loss

November 2012


Problems arise

Observation of luminosity function of C-stars show that stellar evolution calculations do not predict sufficiently large dredge-up at sufficiently low core mass.

Mixing length parameter, calibrated from solar data, is inadequate.

Or, mixing length theory is altogether inadequate.

3D modeling of convection is needed!November 2012


Intermediate regionSuper AGB star 8 M

November 2012


M=8Msun SAGB starCarbon ignites off-center



M=8MsunClose up on off-center C burning

November 2012


Massive star evolutionOutcomes

November 2012

Does carbon ignite?

No

AGB

CO WD

Yes

Does neon ignite?No

SAGB

Yes

Continue burning oxygen, silicon

CCSN

Does Ne core grow to Chandrasekhar mass?No Ye

sONe WD

ECSN


Final fate of stars:Different codes

November 2012Adapted from Poelarends et al. 2008

MESA

SAGBONe WD orECSNAGBCO WD

Iron core collapse SN

Max He core

before 2nd

dredge-up

Max He core

after 2nd

dredge-up

Ledoux + fast semiconvection

Ledoux + slow semiconvectionSchwartzschild

Ledoux + medium semiconvection


Final fate in the intermediate zone

November 2012

Langer 2012


Massive stellar evolutionWhat can we compare to?

November 2012


Observables

Characteristics of the SunWidth of the MS band in the HRDThe positions of red giants and

red supergiants (RSG) in the HRDRatio of WR to O starsSurface composition changesAveraged rotational surface

velocitiesNovember 2012


Comparison of HR diagram

November 2012Ekstrom et al 2012


Uncertainties

Mass loss Convection Reaction rates Opacities

November 2012


Mass loss

November 2012


What is mass loss

Hot stars – momentum transferred from radiation to matter through absorption by metal lines

Cool stars also have: Absorption by dust Pulsations

Examples: De Jager 88 empirical fit: Vink 2001 hot star models:

November 2012

1.7699 0.5

1.6767 10eff

LM ZT

2.2 10.85

1.51.3eff

esc

L TM const Z

M v v


How well do we know the mass loss rates?

Comparison of various mass loss rate prescriptions for RSG stars

(Mauron & Josselin 2011)

November 2012


Mass lossHow good can an empirical fit be?

Sample stars adjacent in HR diagram have more than order of magnitude difference in mass loss!

Fit formula accuracy: ~2 for hot luminous stars ~5 for cool luminous stars

Episodic mass loss Need for modeling! Currently available for

hot stars only.November 2012

5.26.9

de Jager et al 1988


Implications of uncertainty in mass loss Uncertainty in

mass loss has considerable effect on final masses and residual H

This determines which stars will become type IIP SNe 10 12 14 16 18 20 22 24

468

10121416

Final mass depen-dence on mass loss

nom-inal*3*5

Minitial/M

Mfin

al/M

November 2012

SN IIP


ConvectionUncertainties

November 2012


Uncertainties in Convection Convection is treated by 1D MLT

model, with single parameter – calibrated from solar model.

Is that universal? Modeling of SN IIP light curves

suggests radii too high mixing length parameter too low.

November 2012


Uncertainties in Convection

Semi-convection – mixing in zones stabilized by composition gradient (cold & light above hot & heavy). Important in post-main

sequence stages. Thermohaline mixing –

hot & heavy above cold & light. Important in off-center

burning stages. What are the efficiencies?

November 2012

M. Mocák et al. 2011 ApJ 743 55


Uncertainties in Convection Overshoot

calibrated from: Width of Main

Sequence Lately from astero-

seismology and binaries

Range still large: 0.1 – 0.6 Hp

November 2012

Ekstrom et al 2012


ConvectionMulti dimensional models

November 2012


Oxygen burning shell (2D)

November 2012

Meakin & Arnett 2007

Initial condition from 1D MLT

New steady state condition in 2D

Extensive entrainment of fuel above burning shell


Multiple shell burning in pre core collapse epoch (2D)

November 2012

Figure 2 from Toward Realistic Progenitors of Core-collapse SupernovaeW. David Arnett and Casey Meakin 2011 ApJ 733 78 doi:10.1088/0004-637X/733/2/78


Pre core collapse epoch (2D)

November 2012

Figure 3+4 from Toward Realistic Progenitors of Core-collapse SupernovaeW. David Arnett and Casey Meakin 2011 ApJ 733 78 doi:10.1088/0004-637X/733/2/78

Highly asymmetrical structure

Mixing of fuel layers


Reaction rates

November 2012


Uncertainties in reaction rates Most uncertainties affect

nucleosynthesis, but not evolution. Uncertainties in He burning rates:

3α , C12 (α, γ )O16

affect C/O ratios, and neutron star remnant masses.

November 2012


Sensitivity to reaction rate uncertainties – an example

November 2012

Tur et al 2007


Rotation

November 2012


What does rotation do?

Centrifugal force: Reduces the effective mass of the star L , ρc, Tc Enhances mass loss

Differential rotation: Creates magnetic fields Transport of angular momentum from

contracting core to expanding envelope Rotational mixing

November 2012

Maeder & Meynet 2012

Massive stellar evolution Roni Waldman 87Wenjin Huang et al. 2010 ApJ 722 605

Distribution of rotation velocity of young stars

November 2012

2 < M/M☉ < 4

4 < M/M☉ < 8

M/M☉ > 8


Spin down through evolution Old stars (dotted

line) are slower than young (solid line)

Spin down during evolution

Models in good agreement for higher masses, under- predicting for lower masses.

November 2012

Wenjin Huang et al. 2010 ApJ 722 605


Effect of rotation on HR diagram

Effect of rotation causes widening of MS strip

Comparable to effect of overshoot

November 2012

Ekstrom et al 2012


Effect of rotation on HR diagram

November 2012

Ekstrom et al 2012

Tip of RSG branch lower5.

75.3

Widening of MS at high masses

Widening absent – evolution to blue

Changes in surface abundances occur during MS for massive stars


Effect of rotation on final mass

Rotation increases mass loss for given L,R…

But also effects the evolution of the star.

So overall effect is non-trivial.

Large effect only for M>40M.

November 2012

Ekstrom et al 2012


Wolf Rayet stars

Above ~25M stars lose all their H envelope and become WR stars.

O-type log(Teff/K) > 4.5, XH > 0.3

WR log(Teff/K) > 4.0, XH < 0.3 WNL XH > 10−5

WNE XH < 10−5, XC < XN WC/WO XH < 10−5, XC > XN

November 2012


Lifetimes in various phases of evolution

November 2012

(Georgy et al 2012)


Can we fit number ratios?

Number ratios of different types of WR stars can be simultaneously fit to observations…

… but in a narrow range of φ.

November 2012

(Georgy et al 2012)φ = Fraction of close binary WRs

Region allowed by observations


RSG/WR ratios vs. metallicity

November 2012

Massey 2003

Still discrepant for low Z


Do we reproduce HR positions of WR stars? What are the low

luminosity stars? Are mass loss

rates too low? Close binaries?

November 2012

(Georgy et al 2012)


Summary

Although in general we have a fairly good understanding of massive stellar evolution…

There are many uncertainties and simplifications that need to be better treated theoretically: Mass loss Convection Rotation Input physics: reaction rates, opacitiesNovember 2012

Massive stellar evolution Roni Waldman 99November 2012

Thank you for your

attention!谢谢大家！

massive stellar evolution

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