ScienceWise - Nov/Dec 2007

Smoking gun

Searching for the donor stars left over from type Ia supernovae

We’ve covered a few stories in this edition of ScienceWise about the versatility that pure science degrees offer graduates and one such graduate is Wolfgang Kerzendorf who having completed his degree in physics is now performing research towards a PhD in Astronomy in the group led by Professor Brian Schmidt. The group are researching the apparent acceleration in the expansion of the universe (see previous story) and Wolfgang’s part of this work is to investigate the nature of the type the Ia supernovae that provide the distance measurement data. He explains, “I’m interested to know what supernovae actually are because as much as we use them as tools in astronomy, there are still many things we don’t know about them.”

The widely accepted theory about type Ia supernovae is that they occur when a white dwarf star and a larger donor star are in orbit around each other. The white dwarf’s nuclear reactions stop at carbon and oxygen because its mass isn’t great enough to compress it sufficiently for fusion to progress all the way to iron as with larger stars. Astronomers believe matter is gradually transferred from the donor to the exhausted white dwarf until a critical mass is reached at which point it collapses and fuses heavy elements producing a sudden and enormous release of energy. A type Ia supernova can for a brief period, outshine an entire galaxy.

“If our current theories are correct about the mechanism of type Ia supernovae, then for the nearer ones, we should be able to see the left over donor star” Wolfgang explains. “Because the white dwarf and its donor are in orbit around each other their rotation should be tidally locked just like the earth and the moon. This means that in theory, we should be able to identify a left over donor star by either its velocity through space (increased by the kick of a nearby supernova), or its unusually high spin (because it was once tidally locked to a partner star).”

Part of Wolfgang’s PhD project is to use some of the world’s largest telescopes such as the Very Large Telescope (VLT) in Chile to investigate potential left over donor stars. Many of these are linked to historical observations such as Tycho’s supernova of 1572 and the famous 1006 supernova that was recorded by Chinese astronomers in AD1006 who said that it cast as much light at night as the moon. However unlike the events themselves, the stars and gas envelopes left over are often incredibly dim. So much so that it wasn’t until 1965 that the gas envelope associated with the 1006 supernova was identified using the Parkes radio telescope in rural Australia. Part of Wolfgang’s’ work will be to measure the radial and linear velocities of the very faint stars within the gas shells left over from known type Ia supernovae in order to identify potential donor stars.

Another aspect of his work is to look at the light curve of modern type Ia supernovae in other words how they vary in brightness with time. One valuable source of data for this is the increasingly excellent images created by amateur astronomers. Correlating brightness data with various theoretical models of the mechanism of supernovae can yield vital clues as to what is actually going on. “At the end of the day,” Wolfgang says, “the thrust of this work is better understanding of the mechanisms of supernova phenomena so that we can make better predictions about their absolute brightness. When I look at supernovae exploding, there’s a lot of particle physics involved, so by undergraduate physics background comes in quite useful.”

For those with an aptitude for sciences, the journey from high school graduation to involvement in the forefront of major scientific discovery can by accomplished in surprisingly short times.

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Professional development workshops for science teachers at the Centre for the Public Awareness of Science
Searching for the donor stars left over from type Ia supernovae
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The cool thing about science
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