Nucleosynthesis in Massive Stars,at Low Metallicity

S. E. Woosley and A. Heger

T. Rauscher, R. Hoffman, F. Timmes

Z

Z

Z

z

z

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z

Zz

Zzzzzz...

Zzzzoom

Z(z)

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.

Helium Core Mass

Woosley, Heger, & Weaver, RMP (2002)

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()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.

25 Solar Mass Supernova

15 Solar Mass Supernova

The figures at the right showthe first results of nucleosynthesiscalculations in realistic (albeit1D) models for two supernovaemodelled from the main sequencethrough explosion carrying a network of 2000 isotopes ineach of 1000 zones.

A (very sparse) matrix of 2000 x 2000 was invertedapproximately 8 million timesfor each star studied.

The plots show the log of the final abundances compared to their abundance in the sun.

Fall back absorbs all the 56Ni

light curves without mixing - will be recalculated

30 models

Fe

Co

Cr

Mn

NiTi

Sc

CaSi

Al

N

O

Timmes, Heger, & Woosley (2002)

w/r Fe Cr - excessive Ti - a little deficient Sc, Mn, Co - quite deficient

Zn

Abundances at [Fe/H] ~ -4

Data as summarized by Norris, Ryan, & Beers ApJ, 561, 1034, (2001)

dashed line in right hand framesfrom Timmes et al (1995)

Approximate first results from Timmes, Heger, & Woosley (2002)

?

??

??Cr is made as 52Fe

s-process?

H Big Bang Ar Oxygen burning He Big Bang + stars K Oxygen burning + s-process Li Big Bang, L* + nu process Ca Oxygen burningBe Cosmic rays Sc s-processB Nu-process Ti Expl Si burningC Helium burning, L*+M* V Expl Si burning N CNO cycle, L*+ VMS Cr Expl Si burningO Helium burning Mn Expl Si burning, IaF Nu-process Fe Expl Si burning, IaNe Carbon burning Co alpha-rich freeze outNa Carbon burning Ni alpha-rich freeze outMg Carbon burning Cu alpha-rich freeze out + s-processAl Neon burning Zn Nu-powered windSi Oxygen burning p-proc Explosive neon burning, O-burningP Neon Burning s-proc Helium burning, L* and M*S Oxygen burning r-proc Nu wind, jets?Cl Oxygen burning + s-proc

Species Site Species Site

Summary of Origins

??-rich freeze out

also -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-collapse15 solar mass supernova.

This figure shows the iso-velocitycontours (1000 km/s) 60 ms aftercore bounce in a collapsing massivestar. Calculated by Fryer and Warrenat LANL using SPH (300,000 particles).

Resolution is poor and the neutrinoswere treated artificially (trapped orfreely streaming, no gray region), butsuch calculations will be used toguide 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 zone3,000,000 “ in progress

As the expanding helium core runsinto the massive, but low densityhydrogen envelope, the shock at itsboundary decelerates. The decelerationis in opposition to the radially decreasingdensity gradient of the supernova.

Rayleigh-Taylor instability occurs.

The calculation at the right (Herant andWoosley, ApJ, 1995) shows a 60 degree wedge of a 15 solar mass supernova modelledusing SPH and 20,000 particles. At 9 hours and 36 hours, the growth of thenon-linear RT instability is apparent.

Red is hydrogen, yellow is helium, greenis oxygen, and blue is iron. Radius is insolar radii.

Mixing:

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 etal, Max Planck Institut fuer Astrophysik

Aspiring to realityKifonidis et al. (2001), ApJL, 531, 123

As the Sedov solution shows, a shock wave moving through a regionof decreasing rho r3 will accelerate and, conversely, one moving througha region of increasing rho r3 will slow down.

Fallback

*

Woosley and Weaver, (1995), ApJS, 101, 181

S35B

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 ofmassive stars by imposing artificial mass cuts inone-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.

Nucleonic wind, 1 - 10 seconds

Anti-neutrinos are "hotter" thanthe neutrinos, thus weak equilibriumimplies an appreciable neutron excess,typically 60% neutrons, 40% protons

* favored

r-Process Site #1: The Neutrino-powered Wind

sensitive to the density (entropy)

Hoffman, Woosley, Fuller, & Meyer, ApJ, 460, 478, (1996)

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 ofgravitational collapse.

Thompson, Burrows, and Meyer, (2001), ApJ, 562, 887

So far the necessary highentropy and short time scale for the r-process is not achieved in realistic modelsfor neutron stars (though small radius helps).

Takahashi, Witti, & Janka A&A, (1994), 286, 857

Qian & Woosley, ApJ, (1996), 471, 331

For typical time scales needentropies > 300.

blue lines show contraction fromabout 20 km then evolution atconstant R = 10 km as the luminosity declines.

note models “b” (withB-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

~ 104 – 105 gm cm-3

Compare this to B2/8 with B ~ 1011 gauss.

Also compare wind speed with r 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?

v2 ~ 1020 – 21 erg cm-3

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

Nucleonic disk

0.50 Z = N

Radius

ElectronMole Number

Neutron-rich

1

Lorentzfactor

Radius

The disk responsiblefor rapidly feeding ablack 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 neutronexcess.

r-Process Site #2: Accretion Disk Wind

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 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.

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.