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TOPIC: Type Ia supernovae


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3D Supernovae Simulations
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New computer models show in detail how supernovae obtain their shape

Researchers at the Max Planck Institute for Astrophysics in Garching have for the first time managed to reproduce the asymmetries and fast-moving iron clumps of observed supernovae by complex computer simulations in all three dimensions. To this end they successfully followed the outburst in their models consistently from milliseconds after the onset of the blast to the demise of the star several hours later. (Astrophysical Journal, May10th, 2010)
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Title: Three-Dimensional Simulations of Mixing Instabilities in Supernova Explosions
Authors: N. J. Hammer, H.-Th. Janka and E. Müller

We present the first three-dimensional (3D) simulations of the large-scale mixing that takes place in the shock-heated stellar layers ejected in the explosion of a 15.5 solar mass blue supergiant star. The blast is initiated and powered by neutrino-energy deposition behind the stalled shock by means of choosing sufficiently high neutrino luminosities from the contracting, nascent neutron star, whose high-density core is excised and replaced by a retreating inner grid boundary. The outgoing supernova shock is followed beyond its breakout from the stellar surface more than 2 hr after the core collapse. Violent convective overturn in the post-shock layer causes the explosion to start with significant large-scale asphericity, which acts as a trigger of the growth of Rayleigh-Taylor instabilities at the composition interfaces of the exploding star. Despite the absence of a strong Richtmyer-Meshkov instability at the H/He interface, which only a largely deformed shock could instigate, deep inward mixing of hydrogen is found as well as fast-moving, metal-rich clumps penetrating with high velocities far into the hydrogen envelope of the star as observed, for example, in the case of Supernova 1987A. Also individual clumps containing a sizeable fraction of the ejected iron-group elements (up to several 10^-3 solar masses) are obtained in some models. The metal core of the progenitor is partially turned over with nickel-dominated fingers overtaking oxygen-rich bullets and both nickel and oxygen moving well ahead of the material from the carbon layer. Comparing with corresponding two-dimensional (axially symmetric; 2D) calculations, we determine the growth of the Rayleigh-Taylor fingers to be faster, the deceleration of the dense metal-carrying clumps in the helium and hydrogen layers to be reduced, the asymptotic clump velocities in the hydrogen shell to be higher (up to ~4500 km s^-1 for the considered progenitor and an explosion energy of 10^51 erg, instead of l~2000 km s^-1 in 2D), and the outward radial mixing of heavy elements and inward mixing of hydrogen to be more efficient in 3D than in 2D. We present a simple argument that explains these results as a consequence of the different action of drag forces on moving objects in the two geometries.

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RE: Type Ia supernovae
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From the Clash of White Dwarfs, the Birth of a Supernova

For the last 20 years, astronomers seeking to measure the cosmos have used a special type of exploding star, known as Type 1a supernovas, as distance markers. They are thought to result when stars known as white dwarfs grow beyond a certain weight limit, setting off a thermonuclear cataclysm that is not only bright enough to be seen across the universe but is also remarkably uniform from one supernova to the next. Using them, two teams of astronomers a little more than a decade ago reached the startling and now widely held conclusion that some "dark energy" was speeding up the expansion of the universe.
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Clashing stellar couples trigger cosmic blasts

Ill-fated encounters between stellar couples may be responsible for the spectacular explosions used to measure the effects of dark energy, a new study suggests.
Stellar explosions called type Ia supernovae are remarkably consistent in brightness. Astronomers have long used them as standard candles to measure cosmic distances and the expansion history of the universe.
These measurements allowed them to discover that the expansion is accelerating, propelled by a mysterious dark energy.

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NASA will hold a teleconference with reporters at 1 p.m. EST on Wednesday, Feb. 17, to discuss the latest Chandra X-ray Observatory findings that advance our understanding of certain supernovae.

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Astronomers simulate how white dwarf stars merge and become a supernova

Supernovae are spectacular events: Suddenly somewhere in the heavens a "new star" lights up and shines as bright as a whole galaxy consisting of billions of stars. The mechanisms behind these cosmic catastrophes are varied. Researchers at the Max Planck Institute for Astrophysics in Garching have now used computer simulations to confirm that some of these bright supernovae are due to the merger of two white dwarfs, compact massive stars at the end of their lifetime. As supernovae are used by astronomers to measure cosmic distances and study the expansion history of our Universe, understanding their mechanism is one of the key challenges in astrophysics.
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Type Ia Supernovae Progenitors
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Title: The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?
Authors: Rosanne Di Stefano

In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100 eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous supersoft X-ray sources (SSSs). During the past several years, surveys for soft X-ray sources in external galaxies have been conducted. This paper shows that the results falsify the hypothesis that a large fraction of progenitors are NBWDs which are presently observable as SSSs. The data also place limits on sub-M_C models. While Type Ia supernova progenitors may pass through one or more phases of SSS activity, these phases are far shorter than the time needed to accrete most of the matter that brings them close to M_C.

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Collisions of white dwarfs
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Title: Collisions of white dwarfs as a new progenitor channel for type Ia supernovae
Authors: Stephan Rosswog, Daniel Kasen, James Guillochon and Enrico Ramirez-Ruiz
(Version v3)

We present the results of a systematic numerical study of an alternative progenitor scenario to produce Type Ia supernova explosions, which is not restricted to the ignition of a CO white dwarf (WD) near the Chandrasekhar mass. In this scenario, a shock-triggered thermonuclear explosion ensues from the collision of two WDs. Consistent modelling of the gas dynamics together with nuclear reactions using both a smoothed particle and a grid-based hydrodynamics code are performed to study the viability of this alternative progenitor channel. We find that shock-triggered ignition and the synthesis of Ni are in fact a natural outcome for moderately massive WD pairs colliding close to head-on. We use a multi-dimensional radiative transfer code to calculate the emergent broadband light curves and spectral time series of these events. The synthetic spectra and light curves compare well to those of normal Type Ia supernovae over a similar B-band decline rate and are broadly consistent with the Phillips relation, although a mild dependence on viewing angle is observed due to the asymmetry of the ejected debris. While event rates within galactic centers and globular clusters are found to be much too low to explain the bulk of the Type Ia supernovae population, they may be frequent enough to make as much as a one percent contribution to the overall rate. Although these rate estimates are still subject to substantial uncertainties, they do suggest that dense stellar systems should provide upcoming supernova surveys with hundreds of such collision-induced thermonuclear explosions per year.

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RE: Type Ia supernovae
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Computer code gives astrophysicists first full simulation of star's final hours
The precise conditions inside a white dwarf star in the hours leading up to its explosive end as a Type Ia supernova are one of the mysteries confronting astrophysicists studying these massive stellar explosions. But now, a team of researchers, composed of three applied mathematicians at the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory and two astrophysicists, has created the first full-star simulation of the hours preceding the largest thermonuclear explosions in the universe.
In a paper to be published in the October issue of Astrophysical Journal, Ann Almgren, John Bell and Andy Nonaka of Berkeley Lab's Computational Research Division, with Mike Zingale of Stony Brook University and Stan Woosley of University of California, Santa Cruz, describe the first-ever three-dimensional, full-star simulations of convection in a white dwarf leading up to ignition of a Type Ia supernova. The project was funded by the DOE Office of Science.
Type Ia supernovae are of particular interest to astrophysicists as they are all believed to be surprisingly similar to each other, leading to their use as "standard candles" which scientists use to measure the expansion of the universe. Based on observations of these massive stellar explosions - a single supernova is as bright as an entire galaxy - scientists believe our universe is expanding at an accelerating rate. But what if Type Ia supernovae have not always exploded in the same way? What if they aren't standard?

"We're trying to understand something very fundamental, which is how these stars blow up, but it has implications for the fate of the universe" - Ann Almgren.

The problem is that astrophysicists still don't know exactly how a star of this type explodes. Over the years, several simulations have tried to answer the problem, but the traditional methods and available supercomputing power haven't been up to the task.
For the past three years, Almgren, Bell and Nonaka, along with their collaborators, have been developing a simulation code known as MAESTRO. The code simulates the flow of mass and heat throughout the star over time, and requires supercomputers to model the entire star. It's unique in that it is intended for processes that occur at speeds much lower than the speed of sound, which allows the simulation to produce detailed results using much less supercomputing time than traditional codes. What makes MAESTRO's approach different from the traditional methods is that the sound waves have been stripped out, which allows the code to run much more efficiently.
The team ran their simulations on Jaguar, a Cray XT4 supercomputer at the Oak Ridge Leadership Computing Facility in Tennessee, using an allocation under DOE's Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program.

"The INCITE allocation on Jaguar was crucial in enabling the successful runs leading to these groundbreaking results. And the continuing support of the Department of Energy Office of Science is critical to advancing our research" - Stan Woosley, leader of the SciDAC supernova project, which has fostered successful collaborations like this one between applied mathematicians and astrophysicists.

The simulation provided a valuable glimpse into the end of a process that started several billion years ago. A Type Ia supernova begins as a white dwarf, the compact remnant of a low-mass star that never got hot enough to fuse its carbon and oxygen. But if another star is near enough, the white dwarf may start taking on mass ("accreting") from its neighbour until it reaches a critical limit, known as the Chandrasekhar mass. Eventually, enough heat and pressure build up and the star begins to simmer, a process that lasts several centuries. During this simmering phase, fluid near the center of the star becomes hotter and more buoyant, and the buoyancy-driven convection "floats" the heat away from the center. During the final few hours, the convection can't move the heat away from the center fast enough, and the star gets hotter, faster. The fluid flow becomes stronger and more turbulent, but even so, at some point or points in the star, the temperature finally reaches about 1,000,000,000 degrees Kelvin ( about 1.8 million degrees F), and ignites. A burning front then moves through the star, slowly at first, but gaining speed as it goes. From ignition to explosion is only a matter of seconds.
The team's simulations show that at the early stages, the motion of the fluid appears as random swirls. But as the heating in the center of the star increases, the convective flow clearly moves into the star's core on one side and out the other, a pattern known as a dipole. But the flow also becomes increasingly turbulent, with the orientation of the dipole bouncing around inside the star. While others have also seen this dipole pattern, the simulations using MAESTRO are the first to have captured the full star in three dimensions.

This, according to the paper written by the team, could be a critical piece in our understanding of how the final explosion happens.

"As calculations have become more sophisticated, it has only become more clear that the outcome of the explosion is extremely sensitive to exactly how the burning fronts are initiated. As seen from the wide range of explosion outcomes in the literature, realistic initial conditions are a critical part of SNe Ia modelling. Only simulations of this convective phase can yield the number, size, and distribution of the initial hot spots that seed the flame. Additionally, the initial turbulent velocities in the star are at least as large as the flame speed, so accurately representing this initial flow may be an important component to explosion models"

Almgren and Nonaka caution against reading too much into results from a single calculation. While the work described in this paper - their fourth in the Astrophysical Journal about MAESTRO - is an important step towards understanding this problem, more work is needed to be confident in the results.

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Type 1a supernovae
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Variability of type 1a supernovae has implications for dark energy studies
The stellar explosions known as type 1a supernovae have long been used as "standard candles", their uniform brightness giving astronomers a way to measure cosmic distances and the expansion of the universe. But a new study published this week in Nature reveals sources of variability in type 1a supernovae that will have to be taken into account if astronomers are to use them for more precise measurements in the future.
The discovery of dark energy, a mysterious force that is accelerating the expansion of the universe, was based on observations of type 1a supernovae. But in order to probe the nature of dark energy and determine if it is constant or variable over time, scientists will have to measure cosmic distances with much greater precision than they have in the past.

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Type Ia Supernovae
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Title: On Type Ia Supernovae From The Collisions of Two White Dwarfs
Authors: Cody Raskin, F.X. Timmes, Evan Scannapieco, Steven Diehl, Chris Fryer
(Version v2)

We explore collisions between two white dwarfs as a pathway for making Type Ia Supernovae (SNIa). White dwarf number densities in globular clusters allow 10-100 redshift <1 collisions per year, and observations by (Chomiuk et al. 2008) of globular clusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIa remnant. We carry out simulations of the collision between two 0.6 solar mass white dwarfs at various impact parameters and mass resolutions. For impact parameters less than half the radius of the white dwarf, we find such collisions produce approximately 0.4 solar masses of Ni56, making such events potential candidates for underluminous SNIa or a new class of transients between Novae and SNIa.

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