Transcript Massive stars MPalme..

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
• 312C 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 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  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
Woosley et al. 2002
• 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 20Ne16O + 24Mg +4.59MeV
• Abundances increased
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 sprocess) break down to Iron group by
photodisintegration
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