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The Origin and Heating Mechanism of Supernova Remnants

supernova remnant Cassiopeia A

Million-second observation of supernova remnant Cassiopeia A taken by the Chandra X-ray Observatory in 2004. (Credit: NASA/CXC/GSFC/U. Hwang et al.)

All stars fuse lighter elements into heavier ones, producing energy to combat the force of gravity. Stars more massive than about 8 times the mass of the Sun will go through many stages of nuclear fusion in the core from hydrogen to helium, from helium to carbon, and on and on until it accumulates a core of iron. This is where energy-generating fusion reaction stops. Once the iron core grows beyond a certain mass (about 1.4 times the mass of the Sun, called the Chandrasekhar limit), it cannot support its own weight and collapses. The envelope of the star falls onto the collapsing core, and rebounds: this is a core-collapse supernova.

In a star like the Sun, on the other hand, nuclear fusion stops far short of iron. A carbon-oxygen core is created, which does not achieve a high enough temperature and pressure to proceed to the next stage of fusion. Such a star sheds the outer layers far more gently, leaving behind a white dwarf star. However, if a white dwarf is in a binary system, it can explode as a different type (thermonuclear, or Type Ia) supernova. Over the last 20 years or so, cosmologists have used a comparison of the apparent brightness and the cosmological redshift of Type Ia supernovae to infer that the expansion of the universe is accelerating.

But how exactly do white dwarfs become Type Ia supernovae? There are two broad categories of models. In one, two white dwarf stars in a tight binary are brought together, as their orbit decays due to gravitational radiation. In the other, the binary consists of a white dwarf and a relatively normal star, and the white dwarf accretes matter from its companion; as its mass grows to near the Chandrasekhar limit, its core temperature and pressure increase till it explodes. Is one scenario right and the other wrong, or are both scenarios possible? If the latter, could that potentially skew the cosmological results?

In both types, the supernova leaves behind an expanding cloud of gas, which collides with the interstellar medium and gets heated to X-ray emitting temperatures over the subsequent centuries and millennia. X-ray observations of these supernova remnants (SNRs) can help us understand the details of the explosions and subsequent collisions, as well as about the progenitors – the stars that became the supernovae. In particular, high temperature leads to ionization – stripping of electrons from atoms as they collide with each other and with electrons. Each ionization stage of each element has a characteristic energy – X-rays are frequently emitted at that energy, leading to "emission lines". The Suzaku XIS can detect weak lines better than Chandra or XMM-Newton, and it can also determine the precise energy of lines better than other X-ray telescopes.

This improved ability of Suzaku has led to several major discoveries, including these three top "hits".

X-ray emission from the Jellyfish Nebula

Close-up image of the X-ray emission in the supernova remnant known as the Jellyfish Nebula. (Credit: JAXA/NASA/Suzaku, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF)

Usually, a supernova is gradually heated as the ejecta runs into interstellar medium. Because the density of a supernova remnant is extremely low in terrestrial standards, it takes time for the ionization level to catch up with the new temperature. This leaves plasmas in supernova remnants under-ionized (more electrons are retained by atoms than one would expect for the temperature). However, with Suzaku data, a team based in Japan has identified several supernova remnants in which atoms are over-ionized – likely the result of an early fireball stage and rapid cooling.
supernova remnant images and spectra reveal a difference	between supernova types

Suzaku observations of 23 supernova remnants, including those shown here, reveal a distinction between those from massive stars and those from white dwarfs. (Credits: NASA's Goddard Space Flight Center)

If we observe a supernova explosion, it is a routine matter to determine whether it is a core-collapse supernova or a thermonuclear (Type Ia) supernova. If you just observe a remnant, it is far trickier to do so, until now. These authors found a systematic difference in how ionized the iron atoms are between Type Ia and core-collapse supernova remnants, presumably because the ejecta from core collapse supernovae collide with the slower wind from the progenitor star.
Kepler's supernova

Composite of image of Kepler's Supernova in low (red), intermediate (green) and high-energy (blue) X-rays. The background is an optical star field taken from the Digitized Sky Survey. (Credit: X-ray: NASA/CXC/NCSU/M.Burkey et al.; optical: DSS)

Type Ia supernovae are thermonuclear explosions of white dwarf stars. They produce a lot of iron, and a lesser amount of other elements such as chromium and manganese. The yields of such trace elements turn out to provide valuable information about the progenitors of these explosions. Scientists used Suzaku to measure the chromium to manganese ratios first for Tycho's supernova remnant, then for Kepler's supernova remnant, to infer the metallically (the fraction of elements other than hydrogen and helium) of the stars that became the white dwarfs that exploded. A later related study used the amount of nickel and manganese in a third supernova remnant to infer that this supernova likely happened in a binary of a white dwarf and a normal star.

Publication Date: July 2015