Astrophysics
Calculating electron capture rates during core collapse
Supernova simulations to date have assumed that during core collapse, electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli blocked and are ignored. Langanke et al. have calculated rates for electron capture on nuclei with mass numbers A = 65–112 for the temperatures and densities appropriate for core collapse. They found that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This result leads to significant changes in core collapse simulations.
K. Langanke, G. Martinez-Pinedo, J. M. Sampaio, D. J. Dean, W. R. Hix, O. E. B. Messer, A. Mezzacappa, M. Liebendorfer, H.-Th. Janka, and M. Rampp, “Electron capture rates on nuclei and implications for stellar core collapse,” Phys. Rev. Lett. 90, 241102 (2003). NP, SciDAC, DRA, JIHIR, MCYT, NASA, NSF, PECASE
Simulating binary star formation
Developing a comprehensive theory of star formation remains one of the most elusive goals of theoretical astrophysics, partly because gravitational collapse depends on initial conditions within giant molecular cloud cores that only recently have been observed with sufficient accuracy to permit a realistic attack on the problem. Klein et al. have simulated the gravitational collapse and fragmentation of marginally stable turbulent molecular cloud cores and followed the collapse of high-mass fragments as they interact with the radiation of the protostars forming at their centers. Their results show excellent agreement with observations of real cores, including their aspect ratios, rotation rates, linewidth-size relations, and formation of binary systems (Figure 3).
R. I. Klein, R. T. Fisher, M. R. Krumholz, and C. F. McKee, “Recent advances in the collapse and fragmentation of turbulent molecular cloud cores,” RevMexAA 15, 92 (2003). NP, NASA
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Figure 3 |
Looking down the hole in a supernova
Type Ia supernovae are born from a white dwarf accreting material from a nearby companion star. Soon after the white dwarf explodes, the ejected material runs over the companion star, and in the interaction a substantial asymmetry can be imprinted on the supernova ejecta. Kasen et al. performed the first-ever calculation of a 3D atmosphere model of a Type Ia colliding with its companion star. They explored the observable consequences of ejecta-hole asymmetry, using the simulation results to explain the diversity of observed supernovae.
D. Kasen, P. Nugent, R. Thomas, and L. Wang, “Filling a hole in our understanding of Type Ia supernovae,” Astrophysical Journal (in press). NP, NASA.
Probing the chemical evolution of the Universe
Type II supernovae have a very large spread in their intrinsic brightness—greater than a factor of 500—because of their diverse progenitors. Despite this diversity, Baron et al. have shown that their atmospheres can be well understood with detailed synthetic spectral modeling that can determine the stellar compositions, degree of mixing, and kinetic energy of the explosion. These results make Type II supernovae attractive probes of chemical evolution in the Universe and potentially useful independent checks on the cosmological results derived from Type 1a supernovae.
E. Baron, P. Nugent, D. Branch, P. Hauschildt, M. Turatto, and E. Cappellaro, “Determination of primordial metallicity and mixing in the Type IIP supernova 1993W,” Astrophysical Journal 586, 1199 (2003). NP, IBM SUR, NASA, NSF
Checking the foreground of the cosmic microwave background
Observations of the cosmic microwave background (CMB) can be contaminated by diffuse foreground emission from sources such as galactic dust and synchrotron radiation, but an international team of researchers has developed a technique for quantifying this effect. They applied this technique to CMB data from the MAXIMA-1 experiment and found that the effect on CMB power spectrum observations is negligible.
A. H. Jaffe, A. Balbi, J. R. Bond, J. Borrill, P. G. Ferreira, D. Finkbeiner, S. Hanany, A. T. Lee, B. Rabii, P. L. Richards, G. F. Smoot, R. Stompor, C. D. Winant, and J. H. P. Wu, “Determining foreground contamination in CMB observations: Diffuse galactic emission in the MAXIMA-I field,” Astrophysical Journal (in press); astro-ph/0301077. HEP, NASA, NSF
Simulating black hole mergers and gravitational waves
Simulations of the gravitational waves resulting from the collision of two black holes will provide patterns that new gravitational wave detectors can look for as they attempt to verify the predictions of General Relativity. A research team led by Edward Seidel of the Albert Einstein Institute/Max Planck Institute for Gravitation Physics in Potsdam, Germany, has determined effective computational and theoretical (gauge) parameters needed to carry out binary black hole evolutions for time scales never before achieved. Another research group, led by Carlos Lousto of the University of Texas at Brownsville, has achieved a full numerical computation of the gravitational radiation generated by the evolution of a series of binary black hole configurations with aligned and counter-aligned spins.
M. Alcubierre, B. Bruegmann, P. Diener, M. Koppitz, D. Pollney, E. Seidel, and R. Takahashi, “Gauge conditions for long-term numerical black hole evolutions without excision,” Phys. Rev. D 67, 084023 (2003). NP, AEI, EU, LSU
J. Baker, M. Campanelli, C. O. Lousto, and R. Takahashi,
“The final plunge of spinning binary black holes,” Phys. Rev.
D (submitted); astro-ph/0305287 (2003). NP, NASA, NRC, NSF