* Astronomy

Blobrana · L

RE: Type Ia supernovae

Blobrana · L

Title: Discovery of 100 supernovae among 700,000 Sloan spectra: the Type-Ia supernova rate versus galaxy mass and star-formation rate at redshift ~0.1
Authors: Or Graur, Dan Maoz

Using a method to discover and classify supernovae (SNe) in galaxy spectra, we find 90 Type Ia SNe (SNe Ia) and 10 Type IIP SNe among the ~700,000 galaxy spectra in the Sloan Digital Sky Survey Data Release 7 that have VESPA-derived star-formation histories (SFHs). We use the SN Ia sample to measure SN Ia rates per unit stellar mass. We confirm, at the median redshift of the sample, z = 0.1, the inverse dependence on galaxy mass of the SN Ia rate per unit mass, previously reported by Li et al. (2011b) for a local sample. We further confirm, following Kistler et al. (2011), that this relation can be explained by the combination of galaxy "downsizing" and a power-law delay-time distribution (DTD; the distribution of times that elapse between a hypothetical burst of star formation and the subsequent SN Ia explosions) with an index of -1, inherent to the double-degenerate progenitor scenario. We use the method of Maoz et al. (2011) to recover the DTD by comparing the number of SNe Ia hosted by each galaxy in our sample with the VESPA-derived SFH of the stellar population within the spectral aperture. In this galaxy sample, which is dominated by old and massive galaxies, we recover a "delayed" component to the DTD of 4.5 ± 0.6 X 10^-14 SNe Msun^-1 yr^-1 for delays in the range > 2.4 Gyr. The mass-normalised SN Ia rate, averaged over all masses and redshifts in our galaxy sample, is R(Ia,M,z=0.1) = 0.10 ± 0.01 SNuM, and the volumetric rate is R(Ia,V,z=0.1) = 0.25 ± 0.03 X 10^-4 SNe yr^-1 Mpc^-3. This is the most precise SN Ia rate measurement at this redshift, and is consistent with rates and the rate evolution from other SN Ia surveys.

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Peering Into the Heart of a Supernova

Each century, about two massive stars in our own galaxy explode, producing magnificent supernovae. These stellar explosions send fundamental, uncharged particles called neutrinos streaming our way and generate ripples called gravitational waves in the fabric of space-time. Scientists are waiting for the neutrinos and gravitational waves from about 1,000 supernovae that have already exploded at distant locations in the Milky Way to reach us. Here on Earth, large, sensitive neutrino and gravitational-wave detectors have the ability to detect these respective signals, which will provide information about what happens in the core of collapsing massive stars just before they explode.
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Title: Evidence for asymmetric distribution of circumstellar material around Type Ia Supernovae
Authors: Francisco Förster, Santiago González-Gaitán, Joseph Anderson, Sebastián Marchi, Claudia P. Gutiérrez, Mario Hamuy, Giuliano Pignata, Régis Cartier

We study the properties of low-velocity material in the line of sight towards nearby Type Ia Supernovae (SNe Ia) that have measured late phase nebular velocity shifts (v_neb), thought to be an environment-independent observable. We have found that the distribution of equivalent widths of narrow blended Na I D1 & D2 and Ca II H & K absorption lines differs significantly between those SNe Ia with negative and positive v_neb, with generally stronger absorption for SNe Ia with v_neb > 0. A similar result had been found previously for the distribution of colours of SNe Ia, which was interpreted as a dependence of the temperature of the ejecta with viewing angle. Our work suggests that: 1) a significant part of these differences in colour should be attributed to extinction, 2) this extinction is caused by an asymmetric distribution of circumstellar material (CSM) and 3) the CSM absorption is generally stronger on the side of the ejecta opposite to where the ignition occurs. Since it is difficult to explain 3) via any known physical processes that occur before explosion, we argue that the asymmetry of the CSM is originated after explosion by a stronger ionising flux on the side of the ejecta where ignition occurs, probably due to a stronger shock breakout and/or more exposed radioactive material on one side of the ejecta. This result has important implications for both progenitor and explosion models.

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One Supernova Type, Two Different Sources

The exploding stars known as Type Ia supernovae serve an important role in measuring the universe, and were used to discover the existence of dark energy. They're bright enough to see across large distances, and similar enough to act as a "standard candle" - an object of known luminosity. The 2011 Nobel Prize in Physics was awarded for the discovery of the accelerating universe using Type Ia supernovae. However, an embarrassing fact is that astronomers still don't know what star systems make Type Ia supernovae.
Two very different models explain the possible origin of Type Ia supernovae, and different studies support each model. New evidence shows that both models are correct - some of these supernovae are created one way and some the other.
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Title: Normal Type Ia supernovae from violent mergers of white dwarf binaries
Authors: R. Pakmor, M. Kromer, S. Taubenberger, S. A. Sim, F. K. Roepke, W. Hillebrandt

One of the most important questions regarding the progenitor systems of Type Ia supernovae (SNe Ia) is whether mergers of two white dwarfs can lead to explosions that reproduce observations of normal events. Here we present a fully three-dimensional simulation of a violent merger of two carbon-oxygen white dwarfs with masses of 0.9 solar masses and 1.1 solar masses combining very high resolution and exact initial conditions. A well-tested combination of codes is used to study the system. We start with the dynamical inspiral phase and follow the subsequent thermonuclear explosion under the plausible assumption that a detonation forms in the process of merging. We then perform detailed nucleosynthesis calculations and radiative transfer simulations to predict synthetic observables from the homologously expanding supernova ejecta. We find that synthetic colour lightcurves of our merger, which produces about 0.62 solar masses of ^{56}Ni, show good agreement with those observed for normal SNe Ia in all wave bands from U to K. Line velocities in synthetic spectra around maximum light also agree well with observations. We conclude, that violent mergers of massive white dwarfs can closely resemble normal SNe Ia. Therefore, depending on the number of such massive systems available these mergers may contribute at least a small fraction to the observed population of normal SNe Ia.

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SNR 0509-67.5

Type 1a supernovae, exploding stars that can outshine entire galaxies, were instrumental to the Nobel Prize-winning discovery that a mysterious "dark energy" is fuelling the expansion of the universe. But astronomers haven't been able to pin down what causes these massive stellar explosions.
Now, after studying a Type 1a supernova in a nearby galaxy, two researchers say that they must be the result of a collision between two white dwarf stars. They made their case this week in the journal Nature.
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Most Ancient Supernovas Are Discovered

Supernovas - stars in the process of exploding - open a window onto the history of the elements of Earth's periodic table as well as the history of the universe. All of those heavier than oxygen were formed in nuclear reactions that occurred during these explosions.
The most ancient explosions, far enough away that their light is reaching us only now, can be difficult to spot. A project spearheaded by Tel Aviv University researchers has uncovered a record-breaking number of supernovas in the Subaru Deep Field, a patch of sky the size of a full moon. Out of the 150 supernovas observed, 12 were among the most distant and ancient ever seen.
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Universe's "Standard Candles" Are White Dwarf Mergers

The largest survey to date of distant exploding stars is giving astronomers new clues to what's behind the Type Ia supernovae they use to measure distances across the cosmos.
These stellar explosions helped astronomers conclude more than a decade ago that dark energy is accelerating the expansion of the universe. But what caused them was a mystery. Many astronomers thought white dwarf stars were pulling matter from their normal stellar companions and growing so fat they exploded.
But the new study by American, Israeli and Japanese astronomers using Subaru and Keck telescopes in Hawaii instead suggests that many, if not most, of the Type Ia supernovae result when two white dwarf stars merge and annihilate in a thermonuclear explosion.
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Israelis, Japanese discover ancient supernovae

The largest sample ever found of the most distant exploding stars called supernovae have been discovered by a team of Israeli and Japanese astronomers, who used a device called the Subaru Telescope, located at the 4,200 meter-high summit of Mauna Kea on the island of Hawaii.
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