Joining the Rungs on the Cosmic Distance Ladder

By tying down exactly how...white dwarfs explode, and with what brightness, we can get a better cosmic ruler to measure the Universe with.

Researchers at The Australian National University (ANU) Research School of Astronomy and Astrophysics have performed a detailed study of a critical new supernova used for measuring distances in the Universe.

White dwarf stars are the remnants of Sun-like stars that have burned all of their hydrogen and helium and shed their outer layers, leaving a central core of carbon and oxygen. If these stars are paired with another star in a binary system, the white dwarf can steal material from the companion star, increasing its mass. However, white dwarf stars have an upper mass limit, known as the Chandrasekhar limit. Once the growing white dwarf reaches this mass limit, it undergoes a massive nuclear detonation and explodes in what astronomers refer to as a Type Ia supernova.

Because white dwarf stars always explode as Type Ia supernovae when they reach the Chandrasekhar limit, the brightness of the explosion is always the same. “This allows Type Ia supernovae to be used as ‘standard candles’,” explains Dr Michael Childress of the Research School of Astronomy and Astrophysics. “We know how the brightness of a particular candle grows fainter as it moves away from us. So, if we know the true brightness of the candle, which we do for Type Ia supernovae, then we can calculate how far away it is.”

Dr Childress and his team analysed data from the Type Ia supernova SN 2012fr. This supernova appeared last year in the nearby galaxy NGC 1365. “Because NGC 1365 is reasonably close to us, we can measure its distance from us using a special kind of star called a Cepheid variable,” says Dr Childress. “By calibrating the brightness of the supernova to the independently-known distance to the galaxy, we can increase the precision of which we can measure distances to far-off Type 1a supernova, where we can’t see Cepheid variables. This means we can now determine distances to Type Ia supernovae with much higher accuracy.”

While analysing their data of SN 2012fr, Dr Childress’ team noticed that the layering of the burnt material in the supernova wasn’t quite what they were expecting. “In particular, the way that the silicon and iron from the explosion were layered in its aftermath was unprecedented,” he explains. “We found an outer, thick layer of silicon that had faded once the supernova brightness peaked, and a deeper layer that hardly changed for several weeks after the explosion.” This kind of careful analysis has allowed Dr Childress and his colleagues to place constraints on the exact explosion mechanism of the supernova, and determine that the supernova must have undergone a very fast detonation. They also suspect that the surface of the white dwarf must have burned very rapidly once the supernova began.

Type 1a supernovae play an important role in measuring of the expansion of the Universe. Due to their immense brightness, they can be seen from great distances, and can be used to measure those distances. Indeed, Type Ia supernovae were used by Nobel prize winner Professor Brian Schmidt to co-discover the accelerating expansion of the Universe.  “We need to keep refining our measurements of Type Ia supernovae to hunt for dark energy, the source of this acceleration,” says Dr Childress. “By tying down exactly how the white dwarfs explode, and with what brightness, we can get a better cosmic ruler to measure the Universe with.”

Dr Childress’ work was recently published in The Astrophysical Journal, Volume 770, Issue 1, article id. 29. You can also watch a video about Dr Childress’ work, courtesy of the ARC Centre of Excellence for All-Sky Astrophysics.

Image: NGC 1365. SN 2012fr is marked with white dashes. Image credit and copyright: Martin Pugh.

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