Hydrogen Burning in Stars-II

Transkript

1 ydrogen Burning in Stars-II ydrogen in induced reaction have lowest oulomb barrier highest reaction rate Reaction chain with lowest Z elements are the -chains -chains limited by weak interaction based -reaction Take over of hydrogen burning at higher Z elements cycles governed by electromagnetic and strong interaction based reaction cross sections. burning stars are the stars on the uer art of the Main Sequence in the R Diagram!

2 ydrogen Burning in Massive Stars

3 The main cycle e

4 Reactions in the cycles -: 2 3 S 2 =3 0-3 MeV-barn 3 ν S 4 =2 0-3 MeV-barn 5 ν 5 5 α 2 S 5α = 0 2 MeV-barn -2: 5 6 S 5 =5 0-2 MeV-barn ν 7 7 α 4-3: ν 8 8 α 5-4

5 etwork for cycle α α d d d d d d

6 time [s] Reaction network of onversion of the initial to 4 e 2 6 to 4

7 etwork for higher order s [ ] [ ] α α α α α α α d d d d d d d d d leakage -II mod. -I -III -IV

8 mass fraction X etwork simulations at low temerature conditions of 0 7 K time [s]

9 etwork simulation for ydrogen burning at K mass fraction X time [s]

10 Resonance measurements Reaction rates in cycle reactions have considerable cross section contributions from low energy resonances. Resonance cross section is determined by Breit-Wigner function strong energy deendence of cross section!!! 2 2 tan 2 tan max M m M M m M d R R tot R ex in ε ω π ε ω π ε σ ω π σ = Γ Γ = = Γ Γ Γ = D if the energy deendence of ε Γ in/ex is negligible over wih Γ of resonance and if «Γ!!! if» Γ ield measurement gives resonance strength!

11 Resonant Reaction Rate σv =.540 3/ T9 ω [ MeV ] µ T 9 e R [ MeV ] ω = 2π M 2 M ε m max n exact measurement of the resonance energy R and the thick-target yield max rovides the necessary information for calculating the resonant reaction rate σν!

12 Thick target ield urve ield /2 R δ beam resolution

13 Limitation of Thick Target Technique 9 Be 0 B; R =.07 MeV; Γ=4keV wih of yield curve reresents energy loss of articles in target rule for thick target yield alication: 5 Γ

14 Problems in cycle? 4 5 is the slowest reaction in the cycle Therefore 4 enrichment during burning! Tyical question: are there missing Resonances near the threshold? These could enhance the reaction rate by orders of magnitude and turn 4 5 into a fast rocess.

15 The low energy range of 4 5 data oints? Gamow Range

16 ew Resonance? Infinite thick target measurement TUL 200 o confirming evidence in U data 2002

17 Gamma yield at cm =200 kev Q = 36 I = 400 µ t = day background σ in Gran Sasso Lab beam kev

18 Imact of seculated resonance! Reaction Rate [cm3/moles] new resonance? res. acceted rate Temerature [GK]

19 The hot cycle mode X σν X σν 7 8 e 7 7

20 ot cycle network [ ] = α α d d d d d d slowest reactions become decays of 4 5

21 Imact on energy generation ε 26 MeV/00s =260 kev/s

22 Summary hydrogen burning ydrogen burning oerates mainly through the -chains for low mass stars the cycles for massive stars hydrogen induced reactions on higher Z nuclei are unlikely due to oulomb-barrier high T required ucleosynthesis consequences are of only minor relevance for galactic abundance distribution but creates the basis for the stellar evolution and the subsequent roduction of massive nuclei!