Tuesday, June 21, 2011

New Types of Supernovae

Supernovae - stars that explode completely? - come in an ever more bewildering variety. They were originally classified by their spectra rather by any understanding of what was actually happening, but we now know that they come in two basic varieties. Spectral Type Ia explosions are the complete thermonuclear detonation of a carbon-rich white-dwarf star that becomes overloaded with more than 1.4 solar masses of material.

All other types seem to result, in one way or another, from the collapse of a much more massive star's core to become a neutron star or black hole. This is thought to be how all stars that begin with more than 8 solar masses ultimately end their lives. But in Nature for May 20th, astronomers describe two stellar explosions that don't fit the categories. They weren't bright enough, and they didn't splatter out nearly enough of their innards. Supernova 2005E blazed forth in the outermost halo of the spiral galaxy NGC 1032 in Cetus. That probably means it was an old, low-mass binary star, not a high-mass core-collapser.

Because it ejected a mere 300 Jupiter's worth of mass (0.3 solar mass), a team led by Hagai B. Perets (Harvard-Smithsonian Center for Astrophysics) concludes that it was an overloaded white dwarf that somehow ignited only part of itself. A second team, led by Koji Kawabata (Hiroshima University, Japan), followed Supernova 2005cz in the elliptical galaxy NGC 4589. This one proved to be helium-rich, suggesting that the core-collapse scenario is more likely. The reason why it became only 20% as bright as it should have, the group suggests, is that the star began life barely above the minimum supernova mass of 8 Suns.

Left: A new type: Supernova 2005E ejected much less mass and was much fainter than known types of exploding stars. Right: Supernova 2005E occurred very far out in its galaxy’s halo.

A mosaic showing 36 of the the 500+ Type Ia supernovae discovered by the Sloan Supernova Survey. Each image is centered on the supernova, which usually stands out as a bright point near or within the galaxy that hosts it. The light of the supernova, powered by the thermonuclear explosion of a single white dwarf star, can outshine that of the tens of billions of stars in its host galaxy. Type Ia supernovae have a constant intrinsic luminosity (after a correction based on the time over which their light rises and falls), so their apparent brightness can be used to infer their distance. The primary goal of the Sloan Supernova Survey was to measure the expansion of the universe with high precision over the last four billion years of cosmic history, to help understand why that expansion is speeding up over time despite the decelerating gravitational effect of atoms and dark matter.
Credit: B. Dilday and the Sloan Digital Sky Survey.

A new image from NASA's Chandra X-ray Observatory shows a supernova remnant with a different look. This object, known as SNR 0104-72.3 (SNR 0104 for short), is in the Small Magellanic Cloud, a small neighboring galaxy to the Milky Way. Astronomers think that SNR 0104 is the remains of a so-called Type Ia supernova caused by the thermonuclear explosion of a white dwarf. In this composite made of X-rays from Chandra shown in purple and infrared data from Spitzer shown in green and red, SNR 0104 looks unlike other likely Type Ia remnants found in our own Galaxy. While objects such as the Kepler and Tycho supernova remnants appear circular, the shape of SNR 0104 in X-rays is not. Instead, the image is dominated by two bright lobes of emission (seen to the upper right and lower left). The large amount of iron in these lobes indicates that SNR 0104 was likely formed by a Type Ia supernova.

This 4-panel compares the Chandra image of DEM L238 with the Chandra image of 3 Type Ia supernova remnants located in the Milky Way. The X-ray emission for Kepler's remnant contains a bright central region similar to DEM L238, while the X-ray emission for Tycho's remnant and SN 1006 are generally much more uniform. These results suggest that the stars that exploded and caused the DEM L238 and Kepler supernova remnants were much younger than the stars that produced the Tycho and SN 1006 remnants.(Credit: NASA/CXC)

Type Ia Timeline

No Ordinary Supernova Remnant:
Most supernova remnants look chaotic and violent. But in the 400 years since its explosive birth in the Large Magellanic Cloud, the shock wave of SNR B0509–67.5 has smoothed into a delicate bubble shell. Apparently the interior material is hot enough, and the outside interstellar medium is dense and uniform enough, that most irregularities have piled up and spread evenly sideways all around the expanding shell. This view combines Hubble images of the shock front in red hydrogen light, visible light images of the star field, and a Chandra X-ray Observatory image(blue) of the X-ray-hot gas inside. The shell is 23 light-years wide.

Type I & II Supernovae Timeline

Type IA (Thermonuclear) Supernova

From April, 2003 until August, 2006, the Canada-France-Hawaii Telescope watched four parts of the sky as often as possible. Armed with the largest digital camera in the known universe, CFHT monitored these four fields for a special type of supernova (called Type Ia supernovae) which are created by the thermonuclear detonation of one or more white-dwarf stars. These explosions are extremely energetic, and can be seen across vast distances in space.These four fields covered roughly 16 times the area of the full Moon on the sky, or roughly 1/10,000 of the entire sky. Even though such a small fraction of the sky was monitored, 241 Type Ia supernovae were seen during the period of observation.
This video is a compilation of the 241 Type Ia supernovae seen in these fields during the CFHT Legacy Survey. The four Deep Fields are shown in color, and the positions of all the supernova are illustrated as time progresses. The animation is rendered at 15 frames per second, and each frame corresponds to just under a single day (one second in the animation corresponds to roughly two weeks of real time). Each supernova is assigned a note to be played:Volume = Distance: The volume of the note is determined by the distance to the supernova, with more distant supernova being quieter and fainter. Pitch = "Stretch:" The pitch of the note was determined by the supernova's "stretch," a property of how the supernova brightens and fades. Higher stretch values played higher notes. The pitches were drawn from a Phrygian dominant scale. Instrument = Mass of Host Galaxy: The instrument the note was played on was determined by the properties of the galaxy which hosted each supernova.

Astronomers have created a 3-D image of the expanding supernova remnant Cassiopeia A, by tracking the motions of all its pieces as they expand away from its center year by year. The remnant, seen in X-rays below, is only 330 years old. The 3-D perspective makes clear that the stellar explosion had two components. The star’s outer mantle, with most of its mass, was ejected spherically, creating a round blast wave. But the star’s inner core blasted out in a flattened plane, especially as high velocity jets that may have been directed along its rotation axis."Now we have to turn this data over to the theorists who simulate supernova explosions and say, ‘Make this!’" says Tracey DeLaney (MIT Kavli Institute). "We don’t understand how we get both the round and flat parts."

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