D if fe re n t e v o lu ti o n fo r d if fe re n t m a s s e s

7–1

7 . H o ri z o n ta l B ra n c h a n d b e y o n d

7–2 HorizontalBranchandbeyond:Lowmassstars1

D if fe re n t e v o lu ti o n fo r d if fe re n t m a s s e s

Evolutionofcentraltemperatureanddensityforstarsofdifferentmasses

7–3 HorizontalBranchandbeyond:Lowmassstars2

D if fe re n t e v o lu ti o n fo r d if fe re n t m a s s e s

initialmass

M /M

⊙

<

0

.

5verylowmassHeburningnotignited,nosignificant evolutionduringlifetimeofUniverse 0

.

5

.. .

2

.

3lowmassHeburningignitedindegeneratecore (heliumflash),noCburning 2

.

3

.. .

8intermediateHeburningignitedinnon-degenerate core,noCburning

>

8highmassHe,Cburning,ignitedinnon- degeneratecore→supernova 7–4 Low-massstars:0

.

5

.. .

2

.

3

M

⊙1

L o w -m a s s s ta rs : T h e h o ri z o n ta l b ra n c h

•⇒heatconductionheatsthecore. ConditionsfortheignitionofHeburningarereachedforacoremassof

M

c=0

.

48

M

⊙(independentofthetotalstellarmass)

7–5 Low-massstars:0

.

5

.. .

2

.

3

M

⊙2

H e liu m fl a s h

•HeburningstartsinthedegenerateHe core •energyproductionincreasestheenergy ofions(i.e.thetemperature)andelec- trons,butbecauseof

P

∼

ρ

⁵ 3noadia- baticcoolingbyexpansion •⇒temperatureincreases⇒energypro- ductionincreases⇒temperaturein- creases⇒... •thermalrunawayprocess •comesonlytoahalt,whentheelectron degeneracyislifted •quickevolutionfromRGBtothehorizon- talbranch 7–6 Low-massstars:0

.

5

.. .

2

.

3

M

⊙3

H e liu m fl a s h

•non-nuclearneutrinoproduction •⇒

T

maxcoolinglargestatcentre ⇒Temperaturemaximumoff-centre:

T

max •Energyproductionratesdepend stronglyontemperature •⇒He-burningignitesoff-centre

7–7 Low-massstars:0

.

5

.. .

2

.

3

M

⊙4

H e liu m fl a s h

Modelfor

M

=0

.

645

M

⊙(Sweigart1994) •He-burningignitesintheelectron- degenerateHe-coreof

M

= 0

.

48

M

⊙ •

T

maxat

M

r≈0

.

16:placeoffirst ignition •Degeneracyliftedalsointheouter layers •convectionzonedevelopesbut doesnotreachtheH-burningshell ⇒nomixing Aseriesofflashesoccursmovinginwardsuntilelectrondegeneracyislifted. ⇒stableburningestablished:HeinthecoreandHinashell 7–8 Low-massstars:0

.

5

.. .

2

.

3

M

⊙5

H e liu m fl a s h

Luminosityevolutionduringtheflashes •strongincreaseofenergyproductionintheHe-burningregion •Expansionofoutercorelayersduetoliftingofdegeneracy •⇒MaterialintheH-burningshellcools⇒

L

Hdecreases •Changesofthestellarluminosity

L

∗arealotsmallerthanthatof

L

He

7–9 Low-massstars:0

.

5

.. .

2

.

3

M

⊙6

H e liu m fl a s h

EvolutionintheHRD •Flashphaselastforabout1

.

5·106 years •totalHorizontalbranchlifetime1

.. .

2·108 years 7–10 Low-massstars:0

.

5

.. .

2

.

3

M

⊙7

T h e h o ri z o n ta l b ra n c h

•HorizontalbranchstarsarecoreHeburningwithaHburningshell •Theyhaveidenticalcoremasses,causedbytheHeflash •positiononthehorizontal branchdependontheen- velopemass:thelowerthe envelopemass,thebluerthe star •

q

=Mc M

>

0

.

95:noHburn- ingshell,ExtendedHorizontal Branch(EHB) •

q <

0

.

95:Hburningshell

7–11 Low-massstars:0

.

5

.. .

2

.

3

M

⊙8

B e y o n d th e h o ri z o n ta l b ra n c h : O v e rv ie w

Dorman,Rood&O’Connell(1993,ApJ419,596) 7–12 Intermediatemassstars1

In te rm e d ia te m a s s s ta rs : R G B a n d H B

Intermediatemassstars(M=2.3...8M⊙): Heliumignitionwhenelectrongasisstillnon-degenerate Evolutionofa5M⊙-star: NoHeflash,nohorizontalbranchbutblueloops

7–13 Intermediatemassstars2

In te rm e d ia te m a s s s ta rs : R G B a n d H B

A→C:mainsequencephase C→D:evolutiontotheHayashi-limit C→Drapidly:HertzsprunggapinHR-diagram D→E:redgiantphase(RGB): 1.dredge-upphase deepenvelopeconvectionbringsproductsofH-burningtothesurface E→G:atEheliumignitesinthecoreenergyproductionlimitedtotheinnermost 5%ofthemass.coreisconvective,envelopeisradiative. duration:20%ofmainsequencephase He-coreburningproduces6%(E),20%(F),48%(G)oftheenergy. G→H:coreHe-burningterminates(G)andtwoburningshellsexist. 7–14 Beyondthehorizontalbranch:LowandIntermediatemassstars1

B e y o n d th e h o ri z o n ta l b ra n c h : O v e rv ie w

Evolutionbeyondthehorizontalbranch:electron-degenerateC/O-core,Cdoes notignite.dependingonthestellarmass: AGB-Manqueevolution:lowestmasses:stardoesnotascendtheasymptotic giantbranchbutevolvesdirectlytothewhitedwarfgraveyard,validforHB starswithverytinyenvelopes(i.e.theextremeHBstars) Early-AGBevolution:AGBevolutionstartsbutterminatesbeforethermal pulsesoccur. TP-AGB-evolutionthermalpulses

7–15 Beyondthehorizontalbranch:LowandIntermediatemassstars2

B e y o n d th e h o ri z o n ta l b ra n c h : O v e rv ie w

TerminationofAGBevolutionandbeyond: •AGBevolutionterminatesbecausethestarexperiencesdrasticmassloss (so-calledsuperwind)leadingtothelossofalmosttheentireenvelope. •starevolvesatconstantluminositytoveryhightemperatures (

T

eff

>

100000K). •Evolutionarytimescaleisshort(103 ...105 years). •H-andHe-burningshellsarestillactivebutmoveoutwards •Hburningceasescausingtheluminositytodrop •Heburningceasescausingtheluminositytodrop •stardoesnothaveanythermonuclearenergysourceandstartstocool⇒ whitedwarfcoolingsequence 7–16 Beyondthehorizontalbranch:LowandIntermediatemassstars3

B e y o n d th e h o ri z o n ta l b ra n c h : O v e rv ie w

Evolutionofa3M⊙star

7–17 Beyondthehorizontalbranch:LowandIntermediatemassstars4

A G B e v o lu ti o n – th e rm a l p u ls e s

•⇒H-shellburningisignited •Thiscausestheconvectionzonetomoveoutwards(outerconvectiondriven byH-ionisationand–temperaturedependent–highopacity). •TheresultsisathermalinstabilityoftheHeshellthermalpulses 7–18 Beyondthehorizontalbranch:LowandIntermediatemassstars5

A G B e v o lu ti o n – th e rm a l p u ls e s

•TheHshellburnssteadilymostofthetimeuntilasufficientamountofHeis produced. •HeburningconditionsarereachedatthebaseoftheHerichlayer •Heburningisignited

7–19 Beyondthehorizontalbranch:LowandIntermediatemassstars6

A G B e v o lu ti o n – th e rm a l p u ls e s

•Thethermalinstabilitycausesrunawayburningwithveryhighluminositiesfor ashortamountoftime •ThehighluminosityintheHeburningshellcausestheformationofa convectionzone,whichmixesmostoftheintershellHeintotheburningregion 7–20 1

A G B e v o lu ti o n – th e rm a l p u ls e s

•Temperatureincreaseleadstoexpansionoftheintershellregion •DensityintheHburningshelldropsandHburningisextinguished •Heburningceasesdown,whenmostoftheHeisconsumedandHburning resumes •...andthecyclestartsagain

7–21 PlanetaryNebulae1

P la n e ta ry N e b u la e

PlanetaryNebulaNGC6853 7–22 PlanetaryNebulae2

P la n e ta ry N e b u la e

Abell39(WIYN,AURA,NOAO,NSF)

7–23 PlanetaryNebulae3

P la n e ta ry N e b u la e

RingNebula(HST/STScI/NASA) 7–24 PlanetaryNebulae4

P la n e ta ry N e b u la e

NGC6853/M27(“DumbbellNebula”;ESOVLT/FORS)

7–25 PlanetaryNebulae5

P la n e ta ry N e b u la e

IC4406(ESOVLT) 7–26 PlanetaryNebulae6

P la n e ta ry N e b u la e

HourglassNebula(HST/Sahai/Trauger)

7–27 PlanetaryNebulae7

P la n e ta ry N e b u la e

Kurucz(1979)

planetarynebulae: materialejected duringAGBphase, photoionizedonce thecentralstar’sTeff exceeds25000K. 7–28 PlanetaryNebulae8

P la n e ta ry N e b u la e

Ringnebula:MAISobservatory:http://mais-ccd-spectroscopy. com/images/wpe24.jpg

planetarynebulae: Emissionlinespectra: recombinationlines (hydrogen) andcollisionallyex- citedforbiddenlines. Temperatures: 10000K densities≈104 cm−3

7–29 PlanetaryNebulae9

P la n e ta ry N e b u la e

HIIregions:star formingregions:very similarspectra 7–30 Massloss,stellarwinds1

E v id e n c e fo r m a s s lo s s d u ri n g s te lla r e v o lu ti o n

•Uppermasslimitforwhitedwarfs:1

.

4

M

⊙ Doallstarswith

M >

1

.

4

M

⊙explodeassupernovae? •Inthiscasethesupernovaratesingalaxiesshouldbemuchhigherthan observed⇒masslimitforWD/SNevolution≈8

M

⊙. •⇒substantialmasslossduringtheevolution •Mainsequencestars: –sun:masslossrate:˙

M

=10−14

M

⊙,negligibleeverovertheentirelife timeofthesun. –earlytypestars:radiationdrivenwind.Radiationpressurehigh. •Empiricalformula(Reimers,1975),calibratedforredgiants: ˙

M

=4·10−13

η L g R g

:surfacegravity

η

:“efficiencyparameter”(1 3

< η <

1)

7–31 Massloss,stellarwinds2

E v id e n c e fo r m a s s lo s s d u ri n g s te lla r e v o lu ti o n

•Motivation: GM R=

g R

potentialenergy L gR=const:aconstantfractionoftheluminositypermassunitisusedto overcomegravitationalpotential(≈10−5

L

forRGBstars) 7–32 Massloss,stellarwinds3

E v id e n c e fo r m a s s lo s s d u ri n g s te lla r e v o lu ti o n

Initialmass–finalmassrelation: Semi-empiricalrelationbetweenthe initialstellarmass

M

ionthemain sequenceandthemassofthewhite dwarfremnant

M

f. Datepointsinthisplotwerederivedfromwhitedwarfsinopenclusters(known ageandturn-offmass).

7–33 Massloss,stellarwinds4

E v id e n c e fo r m a s s lo s s d u ri n g s te lla r e v o lu ti o n

Spectralenergydistribution.Fitofmeasuredflux valueswith3350Kblackbodyforthestarand 200Kblackbodyforthedust.Excessatthe longestwavelengthspointstoevencoolercir- cumstellardust.. •Directobservationsofmassloss:dustyenvelopesofsomeredgiants.These aremosteasilydetectedasstrongfluxexcessinthemid-IR. •Planetarynebulae 7–34 Massloss,stellarwinds5

A G B s ta rs : D u s t d ri v e n m a s s lo s s

IR-SpectrumofanAGB-star+modelfit

•manyAGB-starsshowinfrared(IR) excess,→evidenceforcircumstel- larDust(temperatures:afew100K ⇒fluxmaximuminIR) •strongdustextinction→manyAGB invisibleinopticallightbutde- tectableinIR •importantclasses:OH/IR-stars,de- tectedatIRonly,maseremissionof OH-lineat1612MHz(radioastron- omy) •dusthullformsthroughcondensa- tionofdustparticlee.g.silicatesin thewindofanAGBstar

7–35 Massloss,stellarwinds6

A G B s ta rs : D u s t d ri v e n m a s s lo s s

masslossontheAGBandbeyondforstarofvariousinitialmasses. Blöcker(1995) 7–36 Massloss,stellarwinds7

T h e e n d o f A G B e v o lu ti o n

Evolutionofa3M⊙star

•AGBevolutionendsaftermostoftheen- velopeislostduetointensivemassloss •whentheenvelopemassdropsbelowa few0.01M⊙thegiantdimensionscannot besustainedanymoreandthestarsstarts toshrink •thestarsmovesquickly(103 ...105 years) toveryhightemperatures.Shellburning stillactive⇒constantluminosity(

T

eff

>

100000K). •HandHeburningshellsmoveoutwards untilburningcannolongerbesustained •⇒evolutionalongthewhitedwarfcooling sequence

7–37 Whitedwarfcooling1

S tr u c tu re o f a w h it e d w a rf

nonuclearenergyproduction potentialsourcesofenergy:gravitational

E

G,internal

E

I C/Ocorestabilisedbypressureofdegenerateelectrongas →coolingoftheionicgas 7–38 Whitedwarfcooling2

C o o lin g o f W h it e D w a rf s

7–39 Whitedwarfcooling3

W h it e D w a rf lu m in o s it y fu n c ti o n a n d th e a g e o f th e G a la x y

Leggettetal.,1998coolestwhitedwarf=oldestwhitedwarf ageoftheGalaxy(solarneighbourhood):8-11Gyrs 7–40 Whitedwarfcooling4

M a s s e s o f w h it e d w a rf s

Kepleretal.,2007,MNRAS375,1315