Nucleosynthesis in Massive Stars,

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z
Z
Zzzzoom
Nucleosynthesis in Massive Stars, at Low
Metallicity
Z
z
Z(z)
Z
S. E. Woosley and A. Heger
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T. Rauscher, R. Hoffman, F. Timmes
Z
z
z
Zzzzzz...
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Topics

• Characteristics of low metallicity massive stars
• - they are different
• A new survey of nucleosynthesis in massive
  stars - WW95 and TWW95 redone
• Mixing Fallback - it takes both
• Neutrino winds and jets - the
  r-process and rotation

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Effects of Low Metallicity
Low metallicity can have a variety of effects on
the evolution of an nucleosynthesis in massive
stars

• The initial mass function Low
  metallicity may favor the formation of more
  massive stars. (see talks by Abel, Heger,
  Bromm)
• Mass loss is greatly reduced in low metallicity
  stars The mass loss rate is thought to
  scale as Z1/2

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• Presupernova stars will be more compact. This
• may affect mixing as well as light curves
  Lower metallicity favors a bluer star
• The stars will rotate more rapidly. This may
  affect the r-process. Less mass loss and a
  more compact progenitor favors larger angular
  momentum at death

In general Stars will be more massive at
death and possibly more difficult to explode.
Fall back may be more important and black hole
formation, common. Rotation rates in the inner
core may be higher.
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Woosley, Heger, Weaver, RMP (2002)
Helium Core Mass
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Binding Energy External to Fe Core
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Iron Core Masses
1.65
1.9
Solar
Low Z
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Remnant Masses (1995)
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To Summarize

• Low metallicity stars will die with higher
  masses
• potentially greater nucleosynthesis in more
  massive stars
• But the heavier members will be more
  difficult to explode and will experience
  greater amounts of fall back
• Rotationally enhanced mixing may be increased
  and the effects of angular momentum more
  pronounced during the late stages
• More black holes will be made

It will be awhile before all these effects
are properly accounted for!
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Currently in progress ... (Heger, Woosley,
Rauscher, and Hoffman)

• A new survey of nucleosynthesis and stellar
  evolution using revised nuclear and stellar
  physics Z-dependant mass loss, new weak
  rates, 12C(a,g)16O, opacities, etc.
• Complete" adaptive network of typically 2000
  isotopes. Best current reaction rates
• Stars of Z 0, 10-4, 10-2, 10-1, 0.5, 1, and 2
  Z-solar
• Fine mass grid (e.g., 0.2 Msun binning for
  solar metallicity models). M 11 to 40 Msun.
  Coarse grid for lower metallicity stars up to
  300 Msun.

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15 Solar Mass Supernova
The figures at the right show the first results
of nucleosynthesis calculations in realistic
(albeit 1D) models for two supernovae modelled
from the main sequence through explosion carrying
a network of 2000 isotopes in each of 1000
zones. A (very sparse) matrix of 2000 x 2000
was inverted approximately 8 million times for
each star studied. The plots show the log of the
final abundances compared to their abundance in
the sun.
25 Solar Mass Supernova
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light curves without mixing - will be
recalculated
Fall back absorbs all the 56Ni
30 models
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w/r Fe Cr - excessive Ti -
a little deficient Sc, Mn, Co -
quite deficient
Abundances at Fe/H -4
O
Si
Ca
Cr
Fe
Zn
Ti
Ni
Al
Mn
Co
Sc
Timmes, Heger, Woosley (2002)
N
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Data as summarized by Norris, Ryan, Beers
ApJ, 561, 1034, (2001)
Approximate first results from Timmes, Heger,
Woosley (2002)
dashed line in right hand frames from Timmes et
al (1995)
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?
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??
s-process?
??
Cr is made as 52Fe
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Summary of Origins
Species Site
Species Site
H Big Bang
Ar Oxygen burning
He Big Bang stars
K Oxygen burning
s-process Li Big Bang, L nu
process Ca Oxygen burning Be
Cosmic rays
Sc s-process B
Nu-process Ti
Expl Si burningC Helium
burning, LM V Expl Si
burning N CNO cycle,
L VMS Cr Expl Si
burning O Helium burning
Mn Expl Si burning, Ia F
Nu-process
Fe Expl Si burning, Ia Ne
Carbon burning Co
alpha-rich freeze out Na
Carbon burning Ni
alpha-rich freeze outMg Carbon
burning Cu
alpha-rich freeze out s-process Al
Neon burning Zn
Nu-powered wind Si Oxygen
burning p-proc
Explosive neon burning, O-burning P
Neon Burning s-proc
Helium burning, L and M S
Oxygen burning r-proc
Nu wind, jets? Cl Oxygen burning
s-proc
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??
a-rich freeze out
also a-rich freeze out
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At 408 ms, KE 0.42 foe, stored dissociation
energy is 0.38 foe, and the total explosion
energy is still growing at 4.4 foe/s
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First three-dimensional calculation of a
core-collapse15 solar mass supernova.This
figure shows the iso-velocity contours (1000
km/s) 60 ms after core bounce in a collapsing
massive star. Calculated by Fryer and Warren at
LANL using SPH (300,000 particles). Resolution
is poor and the neutrinoswere treated
artificially (trapped or freely streaming, no
gray region), but such calculations will be used
to guide our further code development.
The box is 1000 km across.
300,000 particles 1.15 Msun remnant 2.9
foe1,000,000 1.15
2.8 foe 600,000 particles in convection
zone 3,000,000 in progress
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Mixing
As the expanding helium core runs into the
massive, but low density hydrogen envelope, the
shock at its boundary decelerates. The
deceleration is in opposition to the radially
decreasing density gradient of the supernova.
Rayleigh-Taylor instability occurs.
The calculation at the right (Herant and Woosley,
ApJ, 1995) shows a 60 degree wedge of a 15 solar
mass supernova modelled using SPH and 20,000
particles. At 9 hours and 36 hours, the growth
of the non-linear RT instability is
apparent. Red is hydrogen, yellow is helium,
green is oxygen, and blue is iron. Radius is
in solar radii.
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Aspiring to reality
Kifonidis et al. (2001), ApJL, 531, 123
Left - Cas-A SNR as seen by the Chandra
Observatory Aug. 19, 1999 The red material on
the left outer edge is enriched in iron. The
greenish-white region is enriched in silicon.
Why are elements made in the middle on the
outside? Right - 2D simulation of explosion
and mixing in a massive star - Kifonidis et al,
Max Planck Institut fuer Astrophysik
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As the Sedov solution shows, a shock wave moving
through a region of decreasing rho r3 will
accelerate and, conversely, one moving through a
region of increasing rho r3 will slow down.
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Fallback
S35B
Woosley and Weaver, (1995), ApJS, 101,
181

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Depagne et al. (2002) Z35C vs. CS22949-37
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Mix Z35C to 3.78 solar massesimplode 3.5 solar
masses. That is, make a black hole...
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The Lesson
One cannot reasonably approximate the yields
of massive stars by imposing artificial mass cuts
in one-dimensional models.
The Implication
Nuclei made deep in the star, e.g., 44Ti,
59Co, 58Ni, will often escape even in explosions
with major amounts of fall back. Actual yields
will be sensitive to mixing.
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r-Process Site 1 The Neutrino-powered Wind
Anti-neutrinos are "hotter" than the neutrinos,
thus weak equilibrium implies an appreciable
neutron excess, typically 60 neutrons, 40
protons
favored

sensitive to the density (entropy)
Nucleonic wind, 1 - 10 seconds
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Neutrino Powered Wind
In addition to being a possible site for the
r- process, the neutrino- powered wind also
produces 64Zn and 92,94Mo. These species are
thusprimary nucleosynthesisproducts and a
tracer of gravitational collapse.
Hoffman, Woosley, Fuller, Meyer, ApJ, 460,
478, (1996)
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So far the necessary highentropy and short time
scale for the r-process is not achieved in
realistic models for neutron stars (though small
radius helps). Takahashi, Witti, Janka
AA, (1994), 286, 857 Qian Woosley, ApJ,
(1996), 471, 331
For typical time scales need entropies gt 300.
blue lines show contraction from about 20 km then
evolution at constant R 10 km as the
luminosity declines.
Thompson, Burrows, and Meyer, (2001), ApJ, 562,
887
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Heger, Woosley, Spruit, in prep. for ApJ
note models b (with B-fields) and e (without)
Spruit, (2001), AA, 381, 923
Rotational kinetic energy is approximately 5 x
1050 (10 ms/P)2 erg
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Typical Neutrino wind conditions vwind
108 cm s-1 r 104 105 gm cm-3
r v2 1020 21 erg cm-3
Compare this to B2/8p with B 1011 gauss.Also
compare wind speed with wr for a 10 ms
rotation period at about 30 to 50 km 109 cm
s-1. Magneto-centrifugal wind? Extra energy
deposition greater than 1048 erg s-1?
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Complications

• Different mass stars will make different
  amounts of iron. E.g., a 10 solar mass star
  makes 20 times less iron than a 20 solar mass
  star.
• Different mass neutron stars will have a
  different
• sort of wind (higher M higher entropy).
• Magnetic fields and rotation rates will vary.
• Fall back will modulate the yield of both the
  r-process and iron

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r-Process Site 2 Accretion Disk Wind
Lorentzfactor
The disk responsible for rapidly feeding a black
hole, e.g., in a collapsed star, may dissipate
some of its angular momentum and energy in a
wind. Closer to the hole, the disk is a plasma
of nucleons with an increasing neutron excess.
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Radius
Nucleonic disk
0.50
Z N
ElectronMole Number
Neutron-rich
Radius
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Summary

• Metal deficient stars are a marvelous laboratory
  for studying nucleosynthesis in massive
  stars. Their nucleosynthesis is relatively
  uncontaminated by other sources.
• 2) Especially because of their reduced mass loss,
  low metallicity (very) massive stars have
  different properties when they die and possibly
  different nucleosynthesis. They are harder to
  explode, have more fall back, and rotate more
  rapidly.
• 3) Current surveys give good agreement with the
  abundancesin low metal stars for elements
  lighter than Sc. Nucleosynthesisof heavier
  elements is complicated because of the twin
  effects of mixing and fall back. Good overall
  agreement is possible in select cases.

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Summary
4) Making Zn, Sr, Y, and Zr is easy in the
neutrino-poweredwinds of young neutron stars
far too easy. These nucleimight have different
nucleosynthetic histories thanthe other
r-process nuclei. 5) One way or another,
r-process nucleosynthesis depends onstellar
rotation. Synthesis in either winds or jets
(ormerging neutron stars) are possibilities.
Rotation may have been greater in the past.