Hydrogen Burning in Stars

Introduction to stellar evolutionpp-chain driven energy production & nucleosynthesisNeutrino production in the sunCNO cycle driven nucleosynthesis in massive stars

Fundamentals in nucleosynthesis driven stellar evolution

Hydrogen induced reaction have lowest Coulomb barrier ⇒ highest reaction rate

Hydrogen burning provides energy production in “Main Sequence Stars” in the HR Diagram (sun) until hydrogen fuel is depleted ⇒ the life time of main sequence star depends on the reaction rates

The stellar evolution, or subsequent evolutionary stages depend on the subsequent nucleosynthesis mechanisms or their nuclear fuel processing!

Hertzsprung Russell Diagram

Main Sequence Stars are identified as stars in their hydrogen burning stage. As more massive the star is as larger is its size, its energy production (temperature), and its luminosity.

Temperature and Density Evolution in Stellar Core

log (ρc)

O-ignition

Ne-ignition

Si-ignition

log

(Tc)

H-ignition

He-ignition

C-ignition

7

8

10

9

0 42 8 104 6

Mass-loss

Graphical presentations of stellar evolution

Hydrogen Burning Stage of Stellar EvolutionStars with M>1.5Mo

Stars with M<1.5Mo

The pp-Chains

pp-1: 1H(p,e+ν)2H2H(p,γ)3He3He(3He,2p)4He 84.7%

pp-2: 3He(α,γ)7Be 13.8%7Be(e-,ν)7Li 13.78%7Li(p,α)4He

pp-3: 7Be(p,γ)8Be 0.02%8Be(β+ν)24He

fusion of 4 1H → 4He + 2e+ + 2νe + 26.7 MeV energy release

Neutrino production & neutrino energy

Neutrino production reaction

p + p ⇒ 2H + e+ + νe E= 0.0 to 0.4 MeVp + e- + p ⇒ 2H + νe E= 1.4 MeVe- + 7Be ⇒ 7Li + νe E= 0.86, 0.38 MeV 8B ⇒ 8Be + e+ + νe E= 0.0 to 15 MeVH

He

Li

Be

B

C

pp-1

pp-3

pp-2

Neutrino flux in different energies depends on pp-chain branchings determined by the associated reaction rates:

1H+1H versus 1H +e- determines neutrino flux at 1.4 MeV3He+3He versus 3He+4He determines neutrino flux above 0.4 MeV7Be+p versus 7Be+e- determines neutrino flux above 0.9 MeV

Reaction rates are equivalent to neutrino production rates!

Solar Neutrino Production

Neutrinos as signature for probing the solar coreWith neutrino detectors with sensitivity to specificneutrino energies: SNO, Borexino, Kamiokande …

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http://www.cococubed.com/talk_pages/jina.shtml

The p+p reaction1H(p,e+ν)2H is a reaction based on weak interaction mechanism

the S-factor is calculated: S=5·10-25 MeV-barn

What would be the life time of hydrogen with strong interaction S=5·10-5 MeV-barn?

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Impact on solar neutrino detectorsBorexino

SNO & Superkamiokande

pp-chains in the sun

Life Time Characteristics

Enormous differences in S-factors due to nuclear interaction

Sp+p = 5 10-25 MeV-barn weak interactionS7Be(p,γ) = 2 10-5 MeV-barn electromagnetic interactionS3He(α,γ) = 5 10-4 MeV-barn electromagnetic interactionS2H(p,γ) = 2 10-4 MeV-barn electromagnetic interactionS3He(3He,2p) = 5 MeV-barn strong interaction

Differences translate into differences in reaction rate and life times some nuclei will be processed extremely fast, others will be processed extremely slow.

Slowest process in the fusion sequencedetermines life time of burning phase

Results and Improvementsthrough underground measurements

(with refined detector technology)

3He(α,γ)7Be S-factor increase by 10% to 5.5·10-4 MeV-barn

Confortola et al. PRC 75, 065803 (2007)

3He(3He,2p)4He S-factor increase by 20% to 6.0±xxx MeV-barn?

Bonetti et al. PRL 82, 5205 (1999)

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Hydrogen Burning in Massive Stars

Requires pre-existing CNO abundances as catalyzing isotopes for the helium production through consecutive four proton capture and two beta-decay processes

CNO burning is necessary for massive star evolution to stabilize the stellar core against its internal gravitational contraction!

The main CNO cycle

N

C

O

F

Ne

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CNO--2

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Reactions in the CNO cyclesCNO-1: 12C(p,γ)13N S12C(p,γ)=3 10-3 MeV-barn

13N(β+ν)13C13C(p,γ)14N14N(p,γ)15O S14N(p,γ)=2 10-3 MeV-barn15O(β+ν)15N15N(p,α)12C S15N(p,α)=1 10+2 MeV-barn

CNO-2: 15N(p,γ)16O S15N(p,γ)=5 10-2 MeV-barn16O(p,γ)17F17F(β+ν)17O17O(p,α)15N

CNO-3: 17O(p,γ)18F18F(p,α)15O

BUT: recent results & new questions

14N(p,γ)15O

14N(p,γ)15O

Uncertainties in the 14N(p,γ)15O rate caused considerable uncertainties, in age determination for Globular Clusters, CNO energy generation, and in neutrino flux of massive main sequence low metallicity stars!

Relevance of higher energy data and channels in r-matrix analysis

6.18 3/2-

14N(p,γ)15O

Inconsistencies in 12C(p,γ)13N extrapolation

Energy (MeV)

12C(p,γ)13N

The first CNO branching

15N(p,γ)16O

Energy (MeV)

Non-resonant direct capture

Ground state transition is isotropic;deviations point towards deficiencies in set-up or efficiency corrections

Transition to 1st excited state shows sin2Θ distribution; similar deviationsas for gs transition!

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Conversion of the initial • 1H to 4He• 12C,16O to 14N

4He and 14N are the ashes of the CNO burning, the fuel and seed for the following He burning stage

time [s]

M=20M☉

Life Time of Main Sequence StarsThe life time of pp-burning stars is determined by the weak interaction based reaction rate of 1H(p,e+ν)2H.

From 109 years To 10-9 seconds

M5 - NGC 5904

∆t=14±0.5Gyrecorded 13.7±0.2Gy

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log (time until core collapse) [y]-46 -8-60 -24 2

m/M

Subsequent Burning Phases

Stellar helium Burning on helium ashes from pp/CNO