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z 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 Z 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.