Massive Star

Evolution overview

Michael Palmer

Intro - Massive Stars• Massive stars M > 8Mo

• Many differences compared to low mass stars, ex:

• Lifetime• Dominate energy

production• Initial temperature• Convective core (?)

Reactions

• Below ~ 11Mo, lose envelope and become ONe WD

• Above 11Mo, star can complete all burning stages in hydrostatic equilibrium

• Until ~ 15Mo off centre ignition may still occur

Hydrogen Burning

• Look at 25Mo star• Lifetime 6.38 x 106 years• T = 3.81 x 107 K

• Dominated by the CNO cycle• 12C(p,)13N(e+,)13C(p,)15O(e+,)15N(p,)12C• End result: 1 particle, two ,e+

• For 70% H composition, ~24.97MeV per helium• Slightly less than energy in hydrogen burning in sun. This is

caused by the neutrinos being more energetic

• Other CNO cycles occur, CNO tricycle, but their contribution is not as great

• All CNO cycles produce same end products

Helium Burning

• 25Mo star• Lifetime 6.30 x 105 years• T = 1.96 x 108 K

• Two principal reactions• 312C and 12C(,)16O• 7.275 MeV 7.162 MeV

• Secondary reaction• 14N(,)18F(e+,)18O, before helium burning• 18O(,)22Ne at high temperatures

• 12C(,)16O important for determining amount of carbon left after helium burning

Carbon Burning• 25Mo star

• Lifetime 9.07 x 102 years• T = 8.41 x 108 K

• After helium burning, neutrino losses dominate energy budget• “neutrino-mediated Kelvin-Helmholtz contraction of a carbon-oxygen

core punctuated by occasional delays when the burning of a nuclear fuel provides enough energy to balance neutrinos” Woosley et al. 2002

• Help explain deviations from p T3, loss of entropy

• Dominate reactions• 12C +12C 23Mg + n - 2.62MeV 20Ne + + 4.62MeV

23Na + p +2.24MeV

• Neutron excess begins to develop • 20Ne(p.)21Na(e+,)21Ne and 21Ne(p.)22Na(e+,)22Ne

Neon Burning

• 25Mo star• Lifetime 74 days• T = 1.57 x 109 K

• 16O, 20Ne, 24Mg => main components• 16O has smallest coulomb barrier, but high energy photons

make another reaction more favourable

• 20Ne(,)16O particles reacts with 16O to create 20Ne, equillibrium start to react with 20Ne to create 24Mg

• 2 20Ne16O + 24Mg +4.59MeV• Abundances increased

Woosley et al. 2002

Oxygen Burning

• 25Mo star• Lifetime 147 days• T = 2.09 x 109 K

• 16O,24Mg,28Si => main components• Traces of other elements 25,26Mg,26,27Al for ex

• Main reaction• 16O + 16O 32S* 31S + n + 1.45MeV

31P + p + 7.68MeV 30P + d - 2.41MeV 28Si + + 9.59MeV

• Elements above Nickel (created by s-process) break down to Iron group by photodisintegration

• Neutron excess reactions• 30P(e+,)30S , 33S(e-,)33P • 35Cl(e-,)35S , 37Ar(e-,)37Cl

Silicon Burning

• 25Mo star• Lifetime 1 day• T = 3.65 x 109 K

• Some 28Si breaks down• 28Si(,)24Mg(,)20Ne(,)16O(,)12C(,2)

• Equilibrium • 28Si(,)32S(,p)31P(,p)30Si(,n)29Si(,n)28Si

• To add to Iron• 28Si(,)32S(,)36A(,)40Ca(,)44Ti(,)48Cr(,)52Fe(,

)56Ni

Meaning

• Neutrino loss helps explain deviations from T vs p diagram w.r.t. p T3 relation

Paxton et al. 2010

Meaning 2

• Neutron excess reactions result in excess of neutrons in core of star, resulting in electron fraction to decrease as seen

Paxton et al. 2010

What else?

• Between each core burning phase more and more shell burnings happening (resembles and onion by the end)

• Mass loss and rotation effects

• Processes to cause supernova

Questions?

I suggest reading Evolution and explosion of massive stars, Woosley et al. 2002. On ASTR 501 homepage