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Z
Nucleosynthesis in Massive Stars,
at Low Metallicity
z
Z
Z(z)
Z
S. E. Woosley and A. Heger
Z
T. Rauscher, R. Hoffman, F. Timmes
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z
z
Zzzzzz...
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
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
•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.
Woosley, Heger, & Weaver, RMP (2002)
Helium Core Mass
Binding Energy External to Fe Core
Iron Core Masses
1.65
1.9
Solar
Low Z
Remnant Masses (~1995)
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!
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.
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
light curves without mixing will be recalculated
Fall back absorbs all the 56Ni
30 models
Abundances at [Fe/H] ~ -4
w/r Fe
Cr - excessive
Ti - a little deficient
Sc, Mn, Co - quite deficient
O
Si
Ca
Cr
Fe
Zn
Ti
Ni
Al
Mn
Co
Sc
Timmes, Heger, & Woosley (2002)
N
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)
?
??
s-process?
??
Cr is made as 52Fe
Summary of Origins
Species
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Site
Species
Big Bang
Ar
Big Bang + stars
K
Big Bang, L* + nu process
Ca
Cosmic rays
Sc
Nu-process
Ti
Helium burning, L*+M*
V
CNO cycle, L*+ VMS
Cr
Helium burning
Mn
Nu-process
Fe
Carbon burning
Co
Carbon burning
Ni
Carbon burning
Cu
Neon burning
Zn
Oxygen burning
p-proc
Neon Burning
s-proc
Oxygen burning
r-proc
Oxygen burning + s-proc
Site
Oxygen burning
Oxygen burning + s-process
Oxygen burning
s-process
Expl Si burning
Expl Si burning
Expl Si burning
Expl Si burning, Ia
Expl Si burning, Ia
alpha-rich freeze out
alpha-rich freeze out
alpha-rich freeze out + s-process
Nu-powered wind
Explosive neon burning, O-burning
Helium burning, L* and M*
Nu wind, jets?
??
a-rich freeze out
also a-rich freeze out
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
First three-dimensional
calculation of a core-collapse
15 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).
The box is 1000 km across.
Resolution is poor and the neutrinos
were treated artificially (trapped or
freely streaming, no gray region), but
such calculations will be used to
guide our further code development.
300,000 particles 1.15 Msun remnant 2.9 foe
1,000,000 “
1.15
“
2.8 foe – 600,000 particles in convection zone
3,000,000 “
in progress
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.
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
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.
Fallback
S35B
Woosley and Weaver, (1995),
ApJS, 101, 181
*
Depagne et al. (2002) Z35C vs. CS22949-37
Mix Z35C to 3.78 solar masses;
implode 3.5 solar masses. That is,
make a black hole...
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.
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
Neutrino Powered Wind
In addition to being a
possible site for the rprocess, the neutrinopowered wind also
produces 64Zn and
92,94Mo.
Hoffman, Woosley, Fuller, & Meyer,
ApJ, 460, 478, (1996)
These species are thus
primary nucleosynthesis
products and a tracer of
gravitational collapse.
So far the necessary high
entropy and short time scale
for the r-process is not
achieved in realistic models
for neutron stars (though
small radius helps).
Takahashi, Witti, & Janka
A&A, (1994), 286, 857
Qian & Woosley,
ApJ, (1996), 471, 331
For typical time scales need
entropies > 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
note models “b” (with
B-fields) and “e” (without)
Heger, Woosley, & Spruit,
in prep. for ApJ
Spruit, (2001), A&A,
381, 923
Rotational kinetic energy is approximately 5 x 1050 (10 ms/P)2 erg
Typical Neutrino wind conditions:
vwind ~ 108 cm s-1
r v2 ~ 1020 – 21 erg cm-3
r ~ 104 – 105 gm 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?
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
r-Process Site #2: Accretion Disk Wind
Lorentz
factor
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.
1
Radius
Closer to the hole,
the disk is a plasma of
nucleons with an
increasing neutron
excess.
Nucleonic disk
0.50
Electron
Mole
Number
Z=N
Neutron-rich
Radius
Summary:
1) 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 abundances
in low metal stars for elements lighter than Sc. Nucleosynthesis
of heavier elements is complicated because of the twin
effects of mixing and fall back. Good overall agreement is
possible in select cases.
Summary:
4) Making Zn, Sr, Y, and Zr is easy in the neutrino-powered
winds of young neutron stars – far too easy. These nuclei
might have different nucleosynthetic histories than
the other r-process nuclei.
5) One way or another, r-process nucleosynthesis depends on
stellar rotation. Synthesis in either winds or jets (or
merging neutron stars) are possibilities. Rotation may
have been greater in the past.