Massive Star Evolution overview Michael Palmer Intro Massive

Massive Star Evolution overview Michael Palmer

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

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

Hydrogen Burning • Look at 25 Mo star • Lifetime 6. 38 x 106 years • T = 3. 81 x 107 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. 97 Me. V 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 • 25 Mo star • Lifetime 6. 30 x 105 years • T = 1. 96 x 108 K • Two principal reactions • 3 12 C and 12 C( , )16 O • 7. 275 Me. V 7. 162 Me. V • 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 • 25 Mo 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 carbonoxygen 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 T 3, loss of entropy • Dominate reactions • 12 C +12 C 23 Mg + n - 2. 62 Me. V 20 Ne + + 4. 62 Me. V 23 Na + p +2. 24 Me. V • Neutron excess begins to develop • 20 Ne(p. )21 Na(e+, )21 Ne and 21 Ne(p. )22 Na(e+, )22 Ne

Neon Burning Woosley et al. 2002 • 25 Mo star • Lifetime 74 days • T = 1. 57 x 109 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. 59 Me. V • Abundances increased

Oxygen Burning • 25 Mo star • Lifetime 147 days • T = 2. 09 x 109 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 + 1. 45 Me. V 31 P + p + 7. 68 Me. V 30 P + d - 2. 41 Me. V 28 Si + + 9. 59 Me. V • Elements above Nickel (created by s-process) break down to Iron group by

Silicon Burning • 25 Mo star • Lifetime 1 day • T = 3. 65 x 109 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. 2002. On ASTR 501 homepage