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TOPIC: Type Ia supernovae


L

Posts: 131433
Date:
Murphy's Supernova
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I confidently predict when the next galactic supernova will take place...
Supernovae occur on average once or twice per century in the Milky Way.
But we have not seen one for over 300 years now.


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L

Posts: 131433
Date:
RE: Type Ia supernovae
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Title: Progenitors of Type Ia Supernovae: Binary Stars with White Dwarf Companions
Authors: M. Parthasarathy, David Branch, David J. Jeffery, E. Baron

Type Ia SNe (SNe Ia) are thought to come from carbon-oxygen white dwarfs that accrete mass from binary companions until they approach the Chandrasekhar limit, ignite carbon, and undergo complete thermonuclear disruption. A survey of the observed types of binaries that contain white dwarfs is presented. We propose that certain systems that seem most promising as SN Ia progenitors should be more intensively observed and modelled, to determine whether the white dwarfs in these systems will be able to reach the Chandrasekhar limit. In view of the number of promising single-degenerate systems and the dearth of promising double-degenerate systems, we suspect that single-degenerates produce most or perhaps all SNe Ia, while double-degenerates produce some or perhaps none.

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L

Posts: 131433
Date:
Cosmic lighthouses
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Supernovae stand out in the sky like cosmic lighthouses. Scientists at the Max Planck Institute for Astrophysics and at the National Astronomical Institute of Italy have now found a way to use these cosmic beacons to measure distances in space more accurately. The researchers have been able to show that all supernovae of a certain type explode with the same mass and the same energy - the brightness depends only on how much nickel the supernova contains. This knowledge has allowed the researchers to calibrate the brightness of supernovae with greater precision. This means that in the future, they will use the brightness of a supernova that they are observing through their telescopes to determine more accurately how far away from the Earth the cosmic lighthouse is emitting its rays.

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L

Posts: 131433
Date:
Type Ia supernovae
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Astronomers Study Shape of Stellar Candles
Astronomers are reporting remarkable new findings that shed light on a decade-long debate about one kind of supernovae, the explosions that mark a star's final demise: does the star die in a slow burn or with a fast bang? From their observations, the scientists find that the matter ejected by the explosion shows significant peripheral asymmetry but a nearly spherical interior, most likely implying that the explosion finally propagates at supersonic speed.
These results are reported today in Science Express, the online version of the research journal Science, by Lifan Wang, Texas A&M University (USA), and colleagues Dietrich Baade and Ferdinando Patat from ESO.

"Our results strongly suggest a two-stage explosion process in this type of supernova. This is an important finding with potential implications in cosmology" - Lifan Wang.

Using observations of 17 supernovae made over more than 10 years with ESO's Very Large Telescope and the McDonald Observatory's Otto Struve Telescope, astronomers inferred the shape and structure of the debris cloud thrown out from Type Ia supernovae. Such supernovae are thought to be the result of the explosion of a small and dense star - a white dwarf - inside a binary system. As its companion continuously spills matter onto the white dwarf, the white dwarf reaches a critical mass, leading to a fatal instability and the supernova. But what sparks the initial explosion, and how the blast travels through the star have long been thorny issues.
The supernovae Wang and his colleagues observed occurred in distant galaxies, and because of the vast cosmic distances could not be studied in detail using conventional imaging techniques, including interferometry. Instead, the team determined the shape of the exploding cocoons by recording the polarisation of the light from the dying stars.
Polarimetry relies on the fact that light is composed of electromagnetic waves that oscillate in certain directions. Reflection or scattering of light favours certain orientations of the electric and magnetic fields over others. This is why polarising sunglasses can filter out the glint of sunlight reflected off a pond. When light scatters through the expanding debris of a supernova, it retains information about the orientation of the scattering layers. If the supernova is spherically symmetric, all orientations will be present equally and will average out, so there will be no net polarisation. If, however, the gas shell is not round, a slight net polarisation will be imprinted on the light.
This is what broad-band polarimetry can accomplish. If additional spectral information is available ('spectro-polarimetry'), one can determine whether the asymmetry is in the continuum light or in some spectral lines. In the case of the Type Ia supernovae, the astronomers found that the continuum polarisation is very small so that the overall shape of the explosion is crudely spherical. But the much larger polarization in strongly blue-shifted spectral lines evidences the presence, in the outer regions, of fast moving clumps with peculiar chemical composition.

"Our study reveals that explosions of Type Ia supernovae are really three-dimensional phenomena. The outer regions of the blast cloud is asymmetric, with different materials found in 'clumps', while the inner regions are smooth. This study was possible because polarimetry could unfold its full strength thanks to the light-collecting power of the Very Large Telescope and the very precise calibration of the FORS instrument" - Dietrich Baade.

The research team first spotted this asymmetry in 2003, as part of the same observational campaign (ESO PR 23/03 and ESO PR Photo 26/05). The new, more extensive results show that the degree of polarisation and, hence, the asphericity, correlates with the intrinsic brightness of the explosion. The brighter the supernova, the smoother, or less clumpy, it is.

"This has some impact on the use of Type Ia supernovae as standard candles. This kind of supernovae is used to measure the rate of acceleration of the expansion of the Universe, assuming these objects behave in a uniform way. But asymmetries can introduce dispersions in the quantities observed" - Ferdinando Patat.

"Our discovery puts strong constraints on any successful models of thermonuclear supernova explosions" - Lifan Wang.

Models have suggested that the clumpiness is caused by a slow-burn process, called 'deflagration', and leaves an irregular trail of ashes. The smoothness of the inner regions of the exploding star implies that at a given stage, the deflagration gives way to a more violent process, a 'detonation', which travels at supersonic speeds - so fast that it erases all the asymmetries in the ashes left behind by the slower burning of the first stage, resulting in a smoother, more homogeneous residue.

Source

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