* Astronomy

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RE: Cassiopeia A

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Astronomers have at last found definitive evidence that the universe's first dust -- the celestial stuff that seeded future generations of stars and planets -- was forged in the explosions of massive stars.
The findings, made with NASA's Spitzer Space Telescope, are the most significant clue yet in the longstanding mystery of where the dust in our very young universe came from. Scientists had suspected that exploding stars, or supernovae, were the primary source, but nobody had been able to demonstrate that they can create copious amounts of dust -- until now. Spitzer's sensitive infrared detectors have found 10,000 Earth masses worth of dust in the blown-out remains of the well-known supernova remnant Cassiopeia A.

"Now we can say unambiguously that dust -- and lots of it -- was formed in the ejecta of the Cassiopeia A explosion. This finding was possible because Cassiopeia A is in our own galaxy, where it is close enough to study in detail" - Jeonghee Rho of NASA's Spitzer Science Centre at the California Institute of Technology in Pasadena. Rho is the lead author of a new report about the discovery appearing in the Jan. 20 issue of the Astrophysical Journal.

Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe. Developing stars need dust to cool down enough to collapse and ignite, while planets and living creatures consist of the powdery substance. In our nearby universe, dust is pumped out by dying stars like our sun. But back when the universe was young, sun-like stars hadn't been around long enough to die and leave dust.
That's where supernovae come in. These violent explosions occur when the most massive stars in the universe die. Because massive stars don't live very long, theorists reasoned that the very first exploding massive stars could be the suppliers of the unaccounted-for dust. These first stars, called Population III, are the only stars that formed without any dust.
Other objects in addition to supernovae might also contribute to the universe's first dust. Spitzer recently found evidence that highly energetic black holes, called quasars, could, together with supernovae, manufacture some dust in their winds.

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Title: The Shape of Cas A
Authors: J. Craig Wheeler, Justyn R. Maund, Sean M. Couch

Based on optical, IR and X-ray studies of Cas A, we propose a geometry for the remnant based on a "jet-induced" scenario with significant systematic departures from axial symmetry. In this model, the main jet axis is oriented in the direction of strong blue-shifted motion at an angle of 110 - 120 degrees East of North and about 40 - 50 degrees to the East of the line of sight. Normal to this axis would be an expanding torus as predicted by jet-induced models. In the proposed geometry, iron-peak elements in the main jet-like flow could appear "beyond" the portions of the remnant rich in silicon by projection effects, not the effect of mixing. In the context of the proposed geometry, the displacement of the compact object from the kinematic centre of the remnant at a position angle of ~169 degrees can be accommodated if the motion of the compact object is near to, but slightly off from, the direction of the main "jet" axis by of order 30 degrees. In this model, the classical NE "jet," the SW "counter-jet" and other protrusions, particularly the "hole" in the North, are non-asymmetric flows approximately in the equatorial plane, e.g., out through the perimeter of the expanding torus, rather than being associated with the main jet. We explore the spoke-like flow in the equatorial plane in terms of Rayleigh-Taylor, Richtmyer-Meshkov and Kelvin-Helmholz instabilities and illustrate these instabilities with a jet-induced simulation.

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Brief video by ESA regarding supernovas (specifically Cas A SN in the constellation Cassiopeia 10000 Ly's from Earth).

[youtube=http://youtube.com/watch?v=2duy24P2sZU]

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Tycho's Supernova Remnant

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Title: The blast wave of Tycho's supernova remnant
Authors: Gamil Cassam-Chenai, John P. Hughes, Jean Ballet (AIME), Anne Decourchelle (AIME)
(version, v2))

We use the Chandra X-ray Observatory to study the region in the Tycho supernova remnant between the blast wave and the shocked ejecta interface or contact discontinuity. This zone contains all the history of the shock-heated gas and cosmic-ray acceleration in the remnant. We present for the first time evidence for significant spatial variations of the X-ray synchrotron emission in the form of spectral steepening from a photon index of 2.6 right at the blast wave to a value of 3.0 several arcseconds behind. We interpret this result along with the profiles of radio and X-ray intensity using a self-similar hydrodynamical model including cosmic ray backreaction that accounts for the observed ratio of radii between the blast wave and contact discontinuity. Two different assumptions were made about the post-shock magnetic field evolution: one where the magnetic field (amplified at the shock) is simply carried by the plasma flow and remains relatively high in the post-shock region (synchrotron losses limited rim case), and another where the amplified magnetic field is rapidly damped behind the blast wave (magnetic damping case). Both cases fairly well describe the X-ray data, however both fail to explain the observed radio profile. The projected synchrotron emission leaves little room for the presence of thermal emission from the shocked ambient medium. This can only be explained if the pre-shock ambient medium density in the vicinity of the Tycho supernova remnant is below 0.6 cm^-3.

arXiv:astro-ph/0703239v2 (PDF)

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Tycho's supernova

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RE: Cassiopeia A

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Spitzer IRAC Images and Sample Spectra of Cassiopeia A's Explosion