Stellar Ghosts Haunt Stars
Observing Type Ia supernovae must be accomplished quickly. This is because they flare into existence, and then dim to darkness, within only a few months. In order to study supernovae that have long since faded, and the stellar white dwarf ghosts that created them, Dr. Kirby and his colleagues used a technique informally named galactic archaeology.
The term galactic archaeology refers to the search for lingering chemical signatures of ancient explosions in other, more youthful stars. When a white dwarf explodes in a Type Ia supernova, it “pollutes” its galactic surroundings with the elements forged during the blast–metals like nickel and iron. Then those heavy atomic elements wander through space, eventually to be incorporated into any newly forming baby stars located in that region. This is comparable to the way that fossils currently provide scientists with important clues about animals that have long been extinct. The quantities of nickel and iron show how massive their long-gone predecessors must have been.
Using the Keck II telescope in Hawaii, Dr. Kirby and his team first went on the hunt for certain ancient galaxies, that had run out of material to form baby stars during the first billion years of the Universe’s existence. The astronomers discovered that most of the stars within these galaxies had relatively low nickel content. This was a tattle-tale clue that the white dwarf ghosts that provided them with nickel must have been of relatively low mass–likely lower than the Chandrasekhar mass.
However, the team of galactic archaeologists found that the nickel content was higher in more recently formed gas. This means that, as time passed after the Big Bang, white dwarfs started to explode with higher masses.
It is important for scientists to understand the mysterious processes that trigger Type Ia supernovae. This is because the explosions themselves have been successfully used as tools for making cosmological measurements. No matter how they exploded, most Type Ia supernovae display a well-known relationship between luminosity and the time it takes for them to dim.
“We call Type Ia supernovae ‘standardizable candles’,” Dr. Kirby commented in an August 6, 2010 Caltech Press release.
“If you look at a candle at a distance it will look dimmer than when it is up close. If you know how bright it is supposed to be up close, and measure how bright it is at a distance, you can calculate that distance. Type Ia supernovae have been very useful in calculating things like the rate of expansion of the Universe. We use them all the time in cosmology. So, it’s important to understand where they come from, and characterize the white dwarfs that make these explosions,” he added.
The scientists plan to study elements other than nickel next, and are planning to target manganese in particular. Manganese production is extremely sensitive to the mass of the supernova that churns it out, and therefore provides a precise way to validate conclusions derived from the nickel content.
The research paper is published under the title “Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies”.