Space Forensics of a Twice-Dead Corpse

The vast majority of stars will end up as white dwarfs. A tiny fraction of these white dwarfs will "die" a second time, exploding as a Type Ia supernova. A team of scientists, led by Dr. Carles Badenes of Princeton, have figured out a way to study the elemental composition of the original star - before it became a white dwarf - by observing the debris from the supernova explosion.

Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al. A Chandra image of Tycho's supernova remnant. See a description at the Chandra X-ray Observatory site.

Nuclear fusion in stars proceed in stages. The more massive the star, the more stages it can go through. Our Sun today shines by fusing hydrogen to make helium. Later in its life, it will fuse helium to make carbon and oxygen. It doesn't have the mass necessary to create high enough temperature and pressure to take fusion to the next stage. So nuclear fusion stops; the Sun will shed the outer layers, and leave behind a extremely dense star, called a white dwarf. It will still have about 60% of the current mass of the Sun, but will shrink to the size of the Earth. The white dwarf will be made mostly of carbon and oxygen, the ashes of nuclear fusion in the Sun.

That will be the end of the story for the Sun. However, if such a white dwarf is in a binary star system of the right kind - and we don't know yet what "the right kind" is - then something spectacular happens. It must first steal enough mass from its binary companion and grow to about 1.4 times the mass of the Sun. At that point, the pressure inside becomes so high that carbon and oxygen start a frenzied activity of nuclear fusion. It happens so quickly that the entire star explodes, leaving nothing behind but a rapidly expanding shell of gas.

One such explosion was witnessed by the famous astronomer Tycho Brahe (among others) in 1572. He called it Stella Nova (or "new star"). Astronomers today call it Tycho's supernova - "super" was added during the 20th century to distinguish these from lesser explosions. The expanding shell of gas left by the explosion is a beautiful X-ray emitting nebula.

A part of the Suzaku spectrum of Tycho's supernova remnant, showing the features due to chromium, manganese, and iron. (Figure 3 of Dr. Badenes et al.'s paper.)

X-ray astronomers can measure the amount of various elements in such hot gas, as was done for clusters of galaxies by Dr. Kosuke Sato and his colleagues. So, Tycho's supernova remnant was an obvious target for a Suzaku observation. In the resulting X-ray spectrum, a team led by Dr. Toru Tamagawa of Riken, Japan, has discovered weak features due to the elements chromium and manganese. These elements are relatively rare (compared to iron or silicon, for example), producing weak features. The spectral sensitivity of Suzaku enabled the detection of these features for the first time. Enter Dr. Badenes and his colleagues, who asked themselves: "what do these features tell us about this object?"

At one level, all Type Ia supernovae are alike - they all are a carbon/oxygen fusion bomb of about 1.4 times the mass of the Sun. They are sufficiently alike that cosmologists today use them as "standard candles" to figure out the expansion history of the universe. If you look at the details, though, there are differences. For instance, "impurities" in the white dwarf - the amount of elements other than carbon and oxygen - can affect how much of each element is synthesized in the explosion. And it turns out that the amount of manganese after the explosion depends sensitively to the amount of neon in the white dwarf (to be precise, it's the ratio of amounts of manganese and chromium that gives you the clue).

The amount of neon in the white dwarf, in turn, is related to the amount of "metals" (elements heavier than hydrogen and helium) that was present in the star when it formed. Dr. Badenes infers that the star that became Tycho's supernova remnant had more metals than our Sun. This probably means it was formed later than the Sun: Each generation of stars add more metals to the interstellar gas, from which the next generation of stars are formed.

Cosmologists are planning to use improved observations of distant Type Ia supernovae to make a more precise measurement of the expansion history of the universe. This also demands that we improve our understanding of the subtle differences among Type Ia supernovae. Dr. Badenes' research has pioneered a new way to learn about the history of nearby Type Ia supernovae. We might eventually learn what, if any, variations there are in the paths leading up to Type Ia explosions, and what influences they may have on the exact properties of these explosions.

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