* Astronomy

Title: The Origin of the Iron-Rich Knot in Tycho's Supernova Remnant
Author: Hiroya Yamaguchi, John P. Hughes, Carles Badenes, Eduardo Bravo, Ivo R. Seitenzahl, Hector Martnez-Rodrguez, Sangwook Park, Robert Petre

X-ray observations of supernova remnants (SNRs) allow us to investigate the chemical inhomogeneity of ejecta, offering unique insight into the nucleosynthesis in supernova explosions. Here we present detailed imaging and spectroscopic studies of the "Fe knot" located along the eastern rim of the Type Ia SNR Tycho (SN 1572) using Suzaku and Chandra long-exposure data. Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn, or Ni, which is unusual for the Fe-rich regions in this SNR. Within the framework of the canonical delayed-detonation models for SN Ia, the observed mass ratios M_Cr/M_Fe < 0.023, M_Mn/M_Fe < 0.012, and M_Ni/M_Fe < 0.029 (at 90% confidence) can only be achieved for a peak temperature of (5.3-5.7) x 10^9 K and a neutron excess of < 2.0 x 10^-3. These constraints rule out the deep, dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot, and favors either incomplete Si burning or the alpha-rich freeze-out regime, probably close to their boundary. An explosive He burning regime is a possible alternative, although this hypothesis is in conflict with the main properties of this SNR.

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Title: Azimuthal Density Variations Around the Rim of Tycho's Supernova Remnant
Authors: Brian J. Williams, Kazimierz J. Borkowski, Parviz Ghavamian, John W. Hewitt, S. Alwin Mao, Robert Petre, Stephen P. Reynolds, John M. Blondin

Spitzer images of Tycho's supernova remnant in the mid-infrared reveal limb-brightened emission from the entire periphery of the shell and faint filamentary structures in the interior. As with other young remnants, this emission is produced by dust grains, warmed to ~ 100 K in the post-shock environment by collisions with energetic electrons and ions. The ratio of the 70 to 24 µm fluxes is a diagnostic of the dust temperature, which in turn is a sensitive function of the plasma density. We find significant variations in the 70/24 flux ratio around the periphery of Tycho's forward shock, implying order-of-magnitude variations in density. While some of these are likely localised interactions with dense clumps of the interstellar medium, we find an overall gradient in the ambient density surrounding Tycho, with densities 3-10 times higher in the NE than in the SW. This large density gradient is qualitatively consistent with the variations in the proper motion of the shock observed in radio and X-ray studies. Overall, the mean ISM density around Tycho is quite low (~ 0.1-0.2 cm^{-3}), consistent with the lack of thermal X-ray emission observed at the forward shock. We perform two-dimensional hydrodynamic simulations of a Type Ia SN expanding into a density gradient in the ISM, and find that the overall round shape of the remnant is still easily achievable, even for explosions into significant gradients. However, this leads to an offset of the center of the explosion from the geometric center of the remnant of up to 20%, although lower values of 10% are preferred. The best match with hydrodynamical simulations is achieved if Tycho is located at a large (3-4 kpc) distance in a medium with a mean preshock density of ~ 0.2 cm^{-3}. Such preshock densities are obtained for highly (\ga 50%) porous ISM grains.

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Title: A Quadruple or Triple Origin For Tycho B and SN 1572
Authors: Todd A. Thompson, Andrew Gould

Kerzendorf et al. (2012) recently reported the startling discovery of a metal-poor ([Fe/H]=-1 ± 0.4) A-type star near the center of the Tycho supernova remnant. We propose two possible explanations. In the first, Tycho B is a blue straggler, formed from the merger of a close K- or G-type binary system, which was previously in a quadruple system with the binary that produced SN 1572. Both binaries were likely brought to tidal contact by Kozai-Lidov oscillations acting in concert with tidal friction. Analogous progenitor systems may include CzeV343, VW LMi, and KIC 4247791. In the second, Tycho B is the surviving tertiary component of a triple system, which was also likely affected by Kozai-Lidov oscillations. Rates are briefly discussed. Problems with each evolutionary scenario are presented. Finally, a chance alignment between Tycho B and the supernova remnant is not excluded.

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NASA's Fermi Shows That Tycho's Star Shines in Gamma Rays

In early November 1572, observers on Earth witnessed the appearance of a "new star" in the constellation Cassiopeia, an event now recognised as the brightest naked-eye supernova in more than 400 years. It's often called "Tycho's supernova" after the great Danish astronomer Tycho Brahe, who gained renown for his extensive study of the object. Now, years of data collected by NASA's Fermi Gamma-Ray Space Telescope reveal that the shattered star's remains shine in high-energy gamma rays.
The detection gives astronomers another clue in understanding the origin of cosmic rays, subatomic particles -- mainly protons -- that move through space at nearly the speed of light. Exactly where and how these particles attain such incredible energies has been a long-standing mystery because charged particles speeding through the galaxy are easily deflected by interstellar magnetic fields. This makes it impossible to track cosmic rays back to their sources.
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