Massive Star Evolution overview Michael Palmer

Intro - Massive Stars Massive stars M > 8M o Many differences compared to low mass stars, ex: Lifetime Dominate energy production Initial temperature Convective core (?)

Reactions Below ~ 11M o, lose envelope and become ONe WD Above 11M o, star can complete all burning stages in hydrostatic equilibrium Until ~ 15M o off centre ignition may still occur

Hydrogen Burning Look at 25M o star Lifetime 6.38 x 10 6 years T = 3.81 x 10 7 K Dominated by the CNO cycle 12 C(p,  ) 13 N(e +,  ) 13 C(p,  ) 15 O(e +,  ) 15 N(p,  ) 12 C 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 25M o star Lifetime 6.30 x 10 5 years T = 1.96 x 10 8 K Two principal reactions 3  12 C and 12 C( ,  ) 16 O MeV MeV Secondary reaction 14 N( ,  ) 18 F(e+,  ) 18 O, before helium burning 18 O( ,  ) 22 Ne at high temperatures 12 C( ,  ) 16 O important for determining amount of carbon left after helium burning

Carbon Burning 25M o star Lifetime 9.07 x 10 2 years T = 8.41 x 10 8 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 Help explain deviations from p  T 3, loss of entropy Dominate reactions 12 C + 12 C  23 Mg + n MeV  20 Ne +  MeV  23 Na + p +2.24MeV Neutron excess begins to develop 20 Ne(p.  ) 21 Na(e +,  ) 21 Ne and 21 Ne(p.  ) 22 Na(e +,  ) 22 Ne

Neon Burning 25M o star Lifetime 74 days T = 1.57 x 10 9 K 16 O, 20 Ne, 24 Mg => main components 16 O has smallest coulomb barrier, but high energy photons make another reaction more favourable 20 Ne( ,  ) 16 O  particles reacts with 16 O to create 20 Ne, equillibrium  start to react with 20 Ne to create 24 Mg 2 20 Ne  16 O + 24 Mg +4.59MeV Abundances increased Woosley et al. 2002

Oxygen Burning 25M o star Lifetime 147 days T = 2.09 x 10 9 K 16 O, 24 Mg, 28 Si => main components Traces of other elements 25,26 Mg, 26,27 Al for ex Main reaction 16 O + 16 O  32 S*  31 S + n MeV  31 P + p MeV  30 P + d MeV  28 Si +  MeV Elements above Nickel (created by s- process) break down to Iron group by photodisintegration Neutron excess reactions 30 P(e +,  ) 30 S, 33 S(e -,  ) 33 P 35 Cl(e -,  ) 35 S, 37 Ar(e -,  ) 37 Cl

Silicon Burning 25M o star Lifetime 1 day T = 3.65 x 10 9 K Some 28 Si breaks down 28 Si( ,  ) 24 Mg( ,  ) 20 Ne( ,  ) 16 O( ,  ) 12 C( ,2  )  Equilibrium 28 Si( ,  ) 32 S( ,p) 31 P( ,p) 30 Si( ,n) 29 Si( ,n) 28 Si To add to Iron 28 Si( ,  ) 32 S( ,  ) 36 A( ,  ) 40 Ca( ,  ) 44 Ti( ,  ) 48 Cr( ,  ) 52 Fe( ,  ) 56 Ni

Meaning Neutrino loss helps explain deviations from T vs p diagram w.r.t. p  T 3 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 On ASTR 501 homepage