* Astronomy

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RE: Cassiopeia A

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Title: The Bubble-like Interior of the Core-Collapse Supernova Remnant Cassiopeia A
Author: Dan Milisavljevic, Robert A. Fesen

The death of massive stars is believed to involve aspheric explosions initiated by the collapse of an iron core. The specifics of how these catastrophic explosions proceed remain uncertain due, in part, to limited observational constraints on various processes that can introduce asymmetries deep inside the star. Here we present near-infrared observations of the young Milky Way supernova remnant Cassiopeia A, descendant of a type IIb core-collapse explosion, and a three-dimensional map of its interior, unshocked ejecta. The remnant's interior has a bubble-like morphology that smoothly connects to and helps explain the multi-ringed structures seen in the remnant's bright reverse shocked main shell of expanding debris. This internal structure may have originated from turbulent mixing processes that encouraged the development of outwardly expanding plumes of radioactive 56Ni-rich ejecta. If this is true, substantial amounts of its decay product, 56Fe, may still reside in these interior cavities.

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CAT Scan of Nearby Supernova Remnant Reveals Frothy Interior

Cassiopeia A, or Cas A for short, is one of the most well studied supernova remnants in our galaxy. But it still holds major surprises. Harvard-Smithsonian and Dartmouth College astronomers have generated a new 3-D map of its interior using the astronomical equivalent of a CAT scan. They found that the Cas A supernova remnant is composed of a collection of about a half dozen massive cavities - or "bubbles."
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Title: X-ray observation of the shocked red supergiant wind of Cassiopeia A
Author: Jae-Joon Lee, Sangwook Park, John P. Hughes, Patrick O. Slane

Cas A is a Galactic supernova remnant whose supernova explosion is observed to be of Type IIb from spectroscopy of its light echo. Having its SN type known, observational constraints on the mass-loss history of Cas A's progenitor can provide crucial information on the final fate of massive stars. In this paper, we study X-ray characteristics of the shocked ambient gas in Cas A using the 1 Ms observation carried out with the Chandra X-Ray Observatory and try to constrain the mass-loss history of the progenitor star. We identify thermal emission from the shocked ambient gas along the outer boundary of the remnant. Comparison of measured radial variations of spectroscopic parameters of the shocked ambient gas to the self-similar solutions of Chevalier show that Cas A is expanding into a circumstellar wind rather than into a uniform medium. We estimate a wind density nH ~ 0.9 ± 0.3 cm-3 at the current outer radius of the remnant (~3 pc), which we interpret as a dense slow wind from a red supergiant (RSG) star. Our results suggest that the progenitor star of Cas A had an initial mass around 16 Msun, and its mass before the explosion was about 5 Msun, with uncertainties of several tens of percent. Furthermore, the results suggest that, among the mass lost from the progenitor star (~11 Msun), a significant amount (more than 6 Msun) could have been via its RSG wind.

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Title: LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
Authors: Ashish Asgekar (1), J. B. R. Oonk (1), S. Yatawatta (1,2), R. J. van Weeren (3,1,8), J. P. McKean (1), G. White (4,38), N. Jackson (25), J. Anderson (32), I. M. Avruch (15,2,1), F. Batejat (11), R. Beck (32), M. E. Bell (35,18), M. R. Bell (29), I. van Bemmel (1), M. J. Bentum (1), G. Bernardi (2), P. Best (7), L. Birzan (3), A. Bonafede (6), R. Braun (37), F. Breitling (30), R. H. van de Brink (1), J. Broderick (18), W. N. Brouw (1,2), M. Bruggen (6), H. R. Butcher (1,9), W. van Cappellen (1), B. Ciardi (29), J. E. Conway (11), F. de Gasperin (6), E. de Geus (1), A. de Jong (1), M. de Vos (1), S. Duscha (1), J. Eisloffel (28), H. Falcke (10,1), R. A. Fallows (1), C. Ferrari (20), W. Frieswijk (1), M. A. Garrett (1,3), J-M. Griesmeier (23,36), T. Grit (1), A. W. Gunst (1), T. E. Hassall (18,25), et al. (48 additional authors not shown)

Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15-80 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 10^-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band.

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Title: Cooling neutron star in the Cassiopeia~A supernova remnant: Evidence for superfluidity in the core
Authors: Peter S. Shternin, Dmitry G. Yakovlev, Craig O. Heinke, Wynn C. G. Ho, Daniel J. Patnaude

According to recent results of Ho & Heinke (2009) and Heinke & Ho (2010), the Cassiopeia A supernova remnant contains a young neutron star which has carbon atmosphere and shows noticeable decline of the effective surface temperature. We report a new (November 2010) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet-state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature T_{cn}(rho) for the onset of neutron superfluidity [T_{cn}(rho) should have a wide peak with maximum \approx (7-9) x 10^8 K], on the reduction factor q of CPF process by collective effects in superfluid matter (q > 0.4), and on the intensity of neutrino emission before the onset of neutron superfluidity (30--100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs.

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A Star Explodes and Turns Inside Out

A new X-ray study of the remains of an exploded star indicates that the supernova that disrupted the massive star may have turned it inside out in the process. Using very long observations of Cassiopeia A (or Cas A), a team of scientists has mapped the distribution of elements in the supernova remnant in unprecedented detail. This information shows where the different layers of the pre-supernova star are located three hundred years after the explosion, and provides insight into the nature of the supernova.
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Title: Spectroscopic detection of Carbon Monoxide in the Young Supernova Remnant Cassiopeia A
Authors: Jeonghee Rho, Takashi Onaka, Jan Cami, William Reach

We report the detection of carbon monoxide (CO) emission from the young supernova remnant Cassiopeia A (Cas A) at wavelengths corresponding to the fundamental vibrational mode at 4.65 micron. We obtained AKARI Infrared Camera spectra towards 4 positions which unambiguously reveal the broad characteristic CO ro-vibrational band profile. The observed positions include unshocked ejecta at the center, indicating that CO molecules form in the ejecta at an early phase. We extracted a dozen spectra across Cas A along the long 1 arcmin slits, and compared these to simple CO emission models in Local Thermodynamic Equilibrium to obtain first-order estimates of the excitation temperatures and CO masses involved. Our observations suggest that significant amounts of carbon may have been locked up in CO since the explosion 330 years ago. Surprisingly, CO has not been efficiently destroyed by reactions with ionised He or the energetic electrons created by the decay of the radiative nuclei. Our CO detection thus implies that less carbon is available to form carbonaceous dust in supernovae than is currently thought and that molecular gas could lock up a significant amount of heavy elements in supernova ejecta.

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Title: A Chandra X-ray Survey of Ejecta in the Cassiopeia A Supernova Remnant
Authors: Una Hwang, J. Martin Laming

We present a survey of the X-ray emitting ejecta in the Cassiopeia A supernova remnant based on an extensive analysis of over 6000 spectral regions extracted on 2.5-10" angular scales using the Chandra 1 Ms observation. We interpret these results in the context of hydrodynamical models for the evolution of the remnant. The distributions of fitted temperature and ionisation age, and the implied mass coordinates, are highly peaked and suggest that the ejecta were subjected to multiple secondary shocks following reverse shock interaction with ejecta inhomogeneities. Based on the fitted emission measure and element abundances, and an estimate of the emitting volume, we derive masses for the X-ray emitting ejecta and also show the distribution of the mass of various elements over the remnant. An upper limit to the total shocked Fe mass visible in X-rays appears to be roughly 0.13 Solar masses, which accounts for nearly all of the mass expected in Fe ejecta. We find two populations of Fe ejecta, that associated with normal Si-burning and that possibly associated with alpha-rich freeze-out, with a mass ratio of approximately 2:1. Essentially all of the observed Fe (both components) lies well outside the central regions of the SNR, possibly having been ejected by hydrodynamic instabilities during the explosion. We discuss this, and its implications for the neutron star kick.

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Title: A Spallation Model for the Titanium-rich Supernova Remnant Cassiopeia A
Authors: Rachid Ouyed (1), Denis Leahy (1), Amir Ouyed (1), Prashanth Jaikumar (2) ((1) Physics&Astronomy, University of Calgary, AB, Canada, (2) Physics&Astronomy, California State University Long Beach, CA, USA)

Titanium-rich subluminous supernovae are rare and challenge current SN nucleosynthesis models. We present a model in which ejecta from a standard Supernova is impacted by a second explosion of the neutron star (a Quark-nova), resulting in spallation reactions that lead to 56Ni destruction and 44Ti creation under the right conditions. Basic calculations of the spallation products shows that a delay between the two explosions of ~ 5 days reproduces the observed abundance of 44Ti in Cas A and explains its low luminosity as a result of the destruction of 56Ni. Our results could have important implications for lightcurves of subluminous as well as superluminous supernovae.

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