Stellar Helium Burning. He-Burning in massive Stars

Transcription

1 Stellar lium Burig -Burig i massive Stars -burig is igited o the ad ashes of the precedig hydroge burig phase! Most importat reactio -triple alpha process 3 12 C 7.96 MeV Eergy productio i helium burig eutro productio i helium burig Ucertaities i the weak s-process

2 Stellar Evolutio of Massive Stars m/m log time util core collapse [y]

3 Critical Reactios i -burig O helium fuel produced i precedig hydroge burig Oxyge-16 Eergy source i stellar burig Eergy release determied by associated reactio rates

4 etwork for stellar lium burig C C O O O C C C e e O O O O C C dt d dt d dt d dt d = = = =

5 budace evolutio i stellar core Declie of time-scale icrease i 12 C 16 O equilibrium 12 C/ 16 O Rapid declie i. time [s]

6 The case of: 3- ad 12 C 16 O Reactio rates determied by strog resoaces ad E2 direct capture cotributios sigifyig strog alpha cluster state cofiguratios i the T=0 uclei alog the reactio path! θ 2 1 θ MeV MeV MeV MeV MeV MeV MeV MeV 2 θ θ θ Be 12 C 16 O

7 The Reactio as two step process r first step! = Q=0.092 MeV 8 Be 0 8 Be 12 C 8 Be T 1/2 8 Be = s Γ =6.8 ev pure cluster cofiguratio fast capture equilibrium betwee capture ad decay pplicatio of Saha Equatio For calculatig 8 Be equilibrium: π µ kt Be = h 3/ 2 e Q kt

8 Example for 8 Be equilibrium abudace: Case of typical -burig: T=0.1GK T 9 =0.1; =10 5 g/cm 3 8 Be = / 2 T T9 e Be.10 = i i 8 Be ~ oe 8 Be ucleus for 3 10 particles

9 Resoat capture o 8 Be secod step! The Hoyle resoace! E R =0.287MeV 7.65 MeV 0 Q=7.367 MeV 8 Be e e - σv = ω µ T 3/ 2 e E T R.39 MeV 2 ω = 2J 1 ω = Γi Γ Γ 9 tot out MeV ± 12% 12 C 0 Decay by sequetial E2 trasitios or iteral e e - pair coversio

10 12 C C 16 O the Holy Grail Level ad Iterferece Structure betwee 1 - levels E1 ad 2 states ad direct capture E2. Ucertaity i low eergy extrapolatio

11 R-matrix aalysis Complex resoace structure iterferig broad resoaces Parameters from probig 16 O compoud ucleus through elastic scatterig β-delayed -decay resoat capture -trasfer reactio photo-dissociatio 12 C 12 C Bochum D 16 β 12 C ale TRIUMF L 12 C 16 O Caltech Müster Quees Stuttgart Bochum 12 C 7 Lit 16 O UM TUL 16 O 12 C HIS

12 R-matrix fit examples E1-term E2-term S E1 80 kev bar S E2 85 kev-bar From Kuz et al. PRL

13 12 C 16 O reactio rate = T 2/3 9 S eff [ MeV b] e / 3 T 9 cm s 3 S eff 0.17 [ MeV b] T 2/3 9 e / 3 T 9 cm s 3 Oly very crude estimate! E-T depedecy eeds to be cosidered!

14 16 O 20 e rate ad Oly a few sigle resoaces o strog o-resoat term i the excitatio curve observed! o-resoat direct capture E2 compoet expected. o-resoat M1 compoet most likely too weak to be of relevace?

15 ew results E2 Direct Reactio Compoet 1e-12 1e06 1e-13 1e05 ormalized ield 1e- 1e-15 1e-16 1e-17 Resoaces S-Factor MeV-bar Ceter of Mass Eergy MeV 10 1e Ceter of Mass Eergy MeV 1e-13 ormalized ield 1e- 1e-15 Upper Limits S eff 2. 8 MeV bar 1e-16 1e-17 Resoaces ad E2 Direct Capture Ceter of Mass Eergy MeV ppreciable o resoat compoet which eeds to be implemeted as DC reactio rate cotributio.

16 Reactio Rates for 12 C 16 O & 16 O 20 e 1.0E00 reactio rate [ccm/s mol] 1.0E E E E E E-30 Gamow Rage 16 O 20 e 12 C 16 O temperature [GK] 12 C 16 O rate domiates over the 16 O 20 e rate at the typical -burig temperatures T~ GK.

17 Cosequeces of the 12 C 16 O ad 16 O 20 e reactio rates! O Late Stellar Evolutio determies Carbo ad/or Oxyge phase = / i i i C Type Ia Superova cetral carbo burig of C/O white dwarf M =13 M 1.0 CF CF e.1e.3e.5e Time s Type II Superova shock-frot ucleosythesis i C ad shells of pre-superova star

18 Stellar eutro Sources i massive Stars Fβ ν O e or e T-depedet icrease of -productio

19 Reactio etwork for -sources e e x O O e e e e F F O O O p F F F F F dt d dt d dt d dt d dt d β β β λ λ λ = = = = = eutro productio through e 25 Mg depeds o reactio rates of this sequece

20 Example: e 25 Mg e 25 Mg low eergy resoaces? The potetial existece of low eergy resoaces causes cosiderable ucertaity i reactio rate

21 Two chael problems! Extrapolatio to lower eergies Impact of the e brach The MeV state is a proouced cluster state i 26 Mg! re there uboud cluster states above the threshold? re there uboud cluster states below the -threshold?

22 eutro productio β β β β λ λ λ λ e e e x e e x e e e e F F O O O F F F dt d dt d dt d dt d dt d dt d = = = = = = = = = eutro flux depeds o seed abudace from H burig ad reactio rate ad brachig coditios i alpha capture sequece!

23 Measuremet with gas target Low eergy beam ito exteded e gas-target

24 eutro Detector rray Gas-cell target Combiatio with udergroud muo shieldig ecessary for reductio of muo iduced eutros

25 Complemetary -trasfer studies i 26 Mg Q = MeV? Q = MeV Observatioal evidece for cluster cofiguratio ear the threshold of 26 Mg at MeV! Recet studies with better resolutio by Ugalde et al cofirmed more resoaces.

26 Reactio Rate Limits ad Ucertaities

27 Weak s-process β β λ λ e e x Z Z dt d dt d ± ± = = Leads to ~100

28 budace evolutio i weak s-process

29 Weak-s-process abudaces CRE lower limit CRE upper limit Large ucertaities i the fial weak s-process abudace distributio due to the ucertaities i the e rates. This traslates ito ucertaity for: Weak s-process abudaces Seed material for p-process ad p-process abudaces LEPP or secod r-process

30 From s- to p-process FZ Karlsruhe LSCE/LL _ToF/CER OREL/ORL s-process build up of elemets ear =100 depedig o eutro flux ad -capture reactio rates TOMKI Demokritos/DTL Bochum FZ Karlsruhe otre Dame TU Darmstadt s-process build up of seed isotopes High photo flux required high temperature eviromets uclear ucertaities associated with the uclear structure i the Z=50 =50 closed shell regio.

31 Possible impact o seed coditios i ext burig sequece ucleosythesis for Z 12 eutro productio lower limit 20 M upper limit lower limit upper limit e for Carbo burig as secod s-process site -productio for s-process

32 The low eergy cross sectio of 13 C 16 O Direct measuremet Brue et al Drotleff et al Strog idicatio of subthreshold state! Idirect trasfer studies 13 C 6 Lid 17 O Kuboo et al Keeley et al Johso et al Hamache et al 2008 Ucertaities: -3 kev 1/2 subthreshold state -90 kev 3/2 subthreshold state First C results idicate a weaker resoace stregth as suggested by direct S-factor extrapolatio secod C study icreases value!

33 Low-eergy low-backgroud Experimet with π BaF 2 -array - beam eutro detectio through the coversio of eutros through Cd with -detectio i surroudig BaF 2 array! ctive shieldig techiques; Pulse shape aalysis; eutro backgroud 0.0 /s il et al. 2008

34 Fit predictios for 13 C S-factor Fits of all possible reactio chaels 13 C 16 O 16 O 13 C 13 C 13 C 16 O 16 O 13 C 13 C 16 O 16 O This work il et al LPH EERG [kev] Low eergy extrapolatio i good agreemet with the C predictios!