The First Stars

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The Late Evolution and Explosion of Massive
Stars With Low Metallicity
Stan Woosley (UCSC) Alex Heger (LANL)
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• Evolution and Explosion
• of Z 0 Stars, 12 100

• Evolution and Explosion of Z 0 stars, 140
  300

• Gamma-Ray Bursts, Supernovae, and Rotation

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The evolution of Z 0 massive stars has been
studied for many years, e.g. Ezer Cameron
(ApSS, 1971) pointed out that such stars would
burn on the CNO cycle Some generalities
k 0 affects star formation, the IMF, the
stars pulsational stability, binary evolution,
and the formation of red giants Hotter, bluer
stars on MS than modern stars. Greatly decreased
mass loss Different sort of final evolution
bigger stars, more tightly bound, more rapidly
rotating(?)
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With considerable uncertainty about the critical
masses, one can delineate four kinds of deaths
(neglecting rotation). He Core Main
Seq. Mass Supernova Mechanism
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Survey 1
(Heger Woosley, in preparation)
Big Bang initial composition, Fields (2002), 75
H, 25 He
Evolved from main sequence to presupernova and
then exploded with pistons near the edge of the
iron core (S/NAk 4.0)
Each model exploded with a variety of energies
from 0.3 to 10 x 1051 erg.
126 Models at least 500 supernovae
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• Use Kepler implicit hydrodynamics code
• Arbitrary equation of state (electrons, pairs,
  degeneracy, etc.)
• Adaptivenuclear reaction network. Nuclei
  included where flows indicate they are
  needed. Typically 900 isotopes
• Explosions simulated using pistons and models
  mixed artificially
• No mass loss
• Approximate light curves calculated using single
  temperature radiative diffusion. Radioactive
  decay included.

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He
Fe
H
Si
O
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Overall, good agreement with solar abundances,
but an appreciable odd-even effect. E.g. Na/Mg,
Al/Mg, and P/Si and no A gt 64.
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Some general features

• Large odd-even effect
• No synthesis above A about 64 (does not
  include neutrino powered wind or proton-rich
  bubble, which greatly affect, e.g., Sc, Zn
  Pruet et al (2004), astroph 0409446)
• Primary B and F from neutrino process
• Above M 50, primary N production
• Nucleosynthesis sensitive to mixing and fall back

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Integrating the yields of these models over a
Salpeter IMF for various explosion energies, one
obtains an approximation to the nucleosynthesis
from the first generation of stars.
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Si
Zn
Co
Heger Woosley (2004)
Ca
Ti
Mg
K
Fe
Sc
Ni
Al
Cr
1052 erg 1.2 x1051 erg
Mn
for Sc and Zn see also Pruet et al (astroph
0409446)
Na
Data from Cayrel et al, AA, 416, 1117, (2004)
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In most cases, up to about 50 solar masses, the
stars are blue supergiants when they die and
their light curves are not exceptionally brilliant
much like SN 1987A
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0.3 x 1051 erg 0.6 0.9 1.2 1.5 1.8 2.4 3.0 5.0
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Masses of 56Ni (solar masses)
0.048 0. 0.057
0.003 0.065 0.22 0.072
0.23 0.078 0.24 0.082
0.25 0.090 0.27 0.095
0.29 -- 0.34 --
0.44
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0.3 x 1051 erg 0.6 0.9 1.2 1.5 1.8 2.4 3.0 5.0
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Masses of 56Ni (solar masses)
0. 0. 0. --
0. -- 0.
0.02 0.27 -- 0.28
0.40 0.31 0.42 0.33
0.44 0.37 0.49 0.45
0.59
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Solar metallicity
mass cut at Fe-core
(after fall back)
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PreSN Models
Black hole formation may have been more frequent
early in the universe
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Gravitational Binding Energy of the Presupernova
Star
solar
low Z
This is just the binding energy outside the iron
core. Bigger stars are more tightly bound and
will be harder to explode. The effect is
more pronounced in metal-deficient stars.
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Summary M lt 100

• Nucleosynthesis overall is reasonably
  consistent with what is seen in the most metal
  deficient stars in our Galaxy. No clear need
  for a separate (e.g. supermassive) component
  at the metallicities studied so far
• Supernovae like 87A brighter if much primary
  nitrogen is made
• Efficient at making black holes for M above
  about 30
• May be more efficient at making GRBs in the
  collapsar model

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Good

• Explosion mechanism well understood
• Mass loss may be negligible
• Initial composition well known
• Pulsationally stable

Many Studies in 1970s and 1980s Rakavy,
Shaviv Fraley Barkat Arnett, Bond,
Carr Ober, El Eid, Fricke Talbot
Appenzeller .
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Problematic

• Their existence
• Mixing between H envelope and He convective
  core makes primary nitrogen resulting in
  radical restructuring of the star
  Sensitive to overshoot mixing, rotation, and
  zoning Determines whether star is BSG
  or RSG at death
• Rotation (no observations for guidance)
• Lack of opacity tables for CNO rich Fe deficient
  matter

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shortly thereafter star becomes a red supergiant
with R gt 1014 cm.
With rotation and standard overshoot and
semiconevcetive parameter settings this happens
for all the Z 0 stars over 100 solar masses
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Survey 2
Helium Cores
Full Stars
Nucleosynthesis and light curves
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Initial mass 150M?
After explosion
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Initial mass 150M?
Si
O
S Ar
Mg
Ca
C
Fe-deficient by 103
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Initial mass 250M?
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Initial mass 250M?
Fe
Si S Ar Ca
Mg
O
iron-rich
C
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Si
O
C
N
30
N?
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N ?
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Bright Supernovae at the edge of the Universe?
Scannapieco et al (2005) astroph 0507182

• Explosion energy up to 1053erg(50-100x that of
  normal supernovae)
• Up to 50 solar masses of radioactive
  56Ni(50-100x that of normal supernovae)

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Calculations by Sergei Blinnikov
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130 solar mass helium star
Should the stars lose their hydrogen envelopes
they could be even brighter (or fainter),
60 solar mass helium star
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Scannapieco et al conclude that one should be
able to limit the fraction of stars in the 140
260 solar mass range to less than 1 of the star
forming mass density to redshift 2 using current
ongoing searches. With JDAM, the limit might be
pushed to z 6 Note that the evolution of
metallicity in the universe is not homogeneous.
Pockets of low Z material might persist up to
observable redshifts.
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Temperature in 109 K just prior to black hole
formation. about 90 solar masses quickly accretes
into the black hole.
radial velocity 6.5 s after black hole formation
Pair-instability collapse for M 300 solar
masses (Fryer, Woosley, Heger ApJ,
550, 372, 2001)
Possible observational challenge long time
scale, soft spectrum.
Is the envelope on or off?Does the star have
enough rotation?
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III. Gamma-Ray Bursts

• GRBs (at least a lot of those of the long-soft
  variety) come from the deaths of massive
  stars
• At least some of these eject about 0.5 solar
  masses of 56Ni an important diagnostic of
  the central engine
• The supernovae may be, on the average,
  hyperenergetic (1052 erg) and asymmetric.
  They are Type Ib/c.
• Unlike ordinary supernovae, those that make GRBs
  eject an appreciable and highly variable -
  fraction of their energy in relativistic
  ejecta (G gt 200)
• The fraction of all supernova-like events that
  make GRBs is small. Typical Ib/c supernovae
  have progenitor masses 3 5 solar masses
  and do not make GRBs.

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The GRB rate is a very small fraction of the
total supernova rate
Madau, della Valle, Panagia, MNRAS,
1998 Supernova rate per 16 arc min squared per
year 20
This corresponds to an all sky supernova rate of
6 SN/sec For comparison the universal
GRB rate is about 3 /day 300 forbeaming or
0.02 GRB/sec
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Today, after times when over 150 GRB models
could be defended, only two are left
standing (for long-soft bursts)

• The collapsar model
• The millisecond magnetar model

Both rely on the existence of situations
where some fraction of massive stars die with an
unsually large amount of rotation.
The degree of rotation and the distribution of
angular momentum is what distinguishes GRBs from
ordinary supernovae.
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Common theme (and a potential difficulty)
Need iron core rotation at death to correspond
to a pulsar of lt 5 ms period if rotation
and B-fields are to matter at all. Need a
period of 1 ms or less to make GRBs. This
is much faster than observed in common pulsars.
To make a disk around a 3 solar mass black
hole need j 5 x 1016 cm2 sec-1
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Calculations agree that without magnetic torques
it is easy to make GRBs
This is plenty of angular momentum to make
either a ms neutron star or a collapsar.
Heger, Langer, Woosley (2002)
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Much of the spin down occurs as the star
evolves from H depletion toHe ignition, i.e.
as a RSG.
Heger, Woosley, Spruit (2004)
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Good news for pulsars Bad news for GRBs!
Heger, Woosley, Spruit (2004) using magnetic
torques as derived inSpruit (2002)
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never a red giant
vrot 400 km/s
He
C,Ne
He
H
C,Ne
O
Si
O
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PreSN
He-depl
GRB
C-depl
H
8 ms pulsar
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Woosley Heger astroph - 0508175
And so maybe .
GRBs come from single stars on the high- velocity
tail of the rotational velocity
distribution Such stars mix completely on the
main sequence The WR mass loss rate is low
(because of metallicity)
See also Yoon and Langer astroph - 0508242
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Effect of Mass Losson Burst Properties
The wind mass required to decelerate a
relativistic jet of equivalent isotropic energy E
and Lorentz factor G is the mass loss rate times
the time before the burst
For typical GRB (equivalent isotropic) energies,
E53 1 the relativistic jet with G 100 gives
up its energyat around 1015 cm.
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Mass Loss
Burning Phase Duration Wind Radius
Probed by
t 108 x t
Hydrogen 19 My
Optical observation
Helium 0.5 My
Optical observation
Carbon/ 3400 y lt
1019 cm SN Ib, GRB afterglow Neon

Oxygen
7 mo lt 2 x 1015 cm
GRB
Si
2 weeks lt 2 x 1014
cm GRB
For helium burning and beyond the wind radius is
taken to be 1000 km/s times the duration of the
burning phase. No WR star has ever been observed
in any of the burning phasesmost appropriate to
GRBs.
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Summary

• Credible, though uncertain models can give
  approximately the observed rotation rate of
  young pulsars for stars that become red
  supergiants and have their differential rotation
  partially braked by internal magnetic
  torques.
• Using the same torques, but reducing WR mass
  loss by a factor of a few can give credible
  GRB progenitors if the stars thoroughly mix
  during hydrogen and helium burning. This may
  occur if the stars rotate on the main sequence
  considerably faster than usual about 35
  Keplerian, 400 km/s.
• GRBs will be favored by low metallicity. The
  threshold metallicity for making a GRB
  depends critically upon the size and
  Z-dependence of the mass loss rate.