10 Jan 2012

OU telescope captures breakthrough image of supernova’s status

Image of the star field captured by the OU telescope

Image of the star field captured by the OU telescope

A robotic astronomical telescope operated by The Open University on the Spanish island of Mallorca was responsible for a one-off image of a supernova, leading to crucial information on the nature of these exploding stars.

The image was taken by a PhD student who happened to be “in the right place at the right time” and was able to gain sophisticated detail of these captivating stars.

Despite the fascination with ‘Type Ia’ supernovae and their role in cosmology, little is known about what kind of stars - in scientific terms the progenitors - actually become supernovae or ‘exploding stars’ as they are commonly known.

It was therefore a lucky night for student Stefan Holmes sitting at the helm of the OU’s PIRATE facility in August 2011, who captured an image of the M101 (the spiral galaxy in which the supernova was seen) at 9pm BST on 23 August 2011, only 4 hours after the explosion of the supernova. The explosion - at 20 million light years distance - represented the closest such explosion for decades, and is the first to be available for detailed investigations with modern-day astronomical detectors.

The supernova itself was not visible in the PIRATE image, but it did capture the upper limit on the early brightness of the supernova. As a result scientists working on the PIRATE facility, in collaboration with an international team led by colleagues at the University of California at Berkeley, can say with a high degree of certainty that it was a ‘white dwarf’ supernova, the name given to dead or ‘end state’ stars.

This breakthrough is now the subject of an article in the January 10th issue of the prestigious Astrophysical Journal Letters series (2012, ApJ, Issue 744, L17) and has been presented this week at the meeting of the American Astronomical Society in Austin, Texas.

Dr Ulrich Kolb, Senior Lecturer at the OU’s Department of Physical Sciences and Director of the PIRATE facility, said: “This a great advancement – the spectral appearance of type Ia supernovae have long suggested exploding white dwarfs as the culprits responsible for the explosion, but this new research is effectively proof of their white dwarf nature.

“It demonstrates the capabilities of small to medium-aperture telescopes to contribute to world-leading research.”

Stefan, an astronomy research student at the OU, who was observing the skies on the night in question, is among the team conducting the PIRATE research programme.

He said: “It was great to have been able to capture this image and be part of such an exciting outcome. It was a case of being at the right place at the right time. It gives the most definite proof yet of the white dwarf nature of ‘Type Ia’ supernovae and is independent of any other evidence on this subject.”

The authors of the Astrophysical Journal Letters paper are Joshua S. Bloom, Daniel Kasen, Ken J. Shen and Peter E Nugent of University of Berkeley; Nathaniel R. Butler of Arizona State University; Melissa L. Graham and D. Andrew Howell of UC Santa Barbara; Ulrich Kolb, Stefan Holmes and Carole Haswell of The Open University; Vadim Burwitz of the Max Planck Institute for Extraterrestrial Physics in Germany; Juan Rodriguez of the Astronomical Observatory of Mallorca in Spain; and Mark Sullivan of the University of Oxford.

Notes for Editors

‘Type Ia’ supernovae are exploding stars which increase their brightness dramatically to shine brighter than a billion Suns combined, for several weeks. They are known to reach a common maximum luminosity, so when they are seen in faraway galaxies it is possible to determine the distance to the galaxy from the apparent brightness of the exploding star: more distant supernovae will appear fainter than closer ones.

This is how astrophysicists measured the distance of numerous galaxies with known speed, only to find that the expansion of the Universe is accelerating, not slowing down, as previously thought. Their work revolutionised cosmology and was awarded the Physics Nobel Prize in 2011.

Supernovae are thought to occur when a white dwarf star, gaining mass by accretion from a companion or partner star is pushed over its stable mass limit.

Shock-heated material ejected in the explosion contributes to the early light output of a supernova. The energy available to be radiated away is proportional to the size of the exploding object. The PIRATE observations therefore place an upper limit on the radius of the progenitor: the radius of the object that exploded must have been smaller than 2/100 of the radius of the Sun, consistent with it being a white dwarf, the end state of a star.

A similar argument restricts the radius of the companion star radius to less than about a tenth of the radius of the Sun, thus excluding most normal stars and therefore a large group of potential progenitor systems.

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