Hubble provides fresh insights into "standard candle" supernovae

Image of NGC 4424, with the position and close-ups of SN2012cg highlighted in the relief(Credit: NASA/Hubble Space Telescope)

The Hubble Space Telescope has granted astronomers fresh insights into a special family of supernovae that represent a vital tool for measuring vast cosmic distances. The discovery revolved around observing the aftermath of a Type Ia supernova and analyzing the properties of the residual light.

The significance of Type Ia supernovae is that they are predictable. The vast explosions are believed to occur when a white dwarf in a binary system siphons enough stellar material off of its companion to reach what is known as the Chandrasekhar limit.

Once this occurs, a thermonuclear explosion is inevitable. The light from this explosion is somewhat of a known quantity, allowing astronomers to observe the extent to which the light has dimmed before it reaches Earth, and from this, work out the distance to the supernova.

For the new study, Hubble focused on SN 2012cg, a type Ia supernova located roughly 50 million light-years away from Earth in the galaxy NGC 4424. Previous research had indicated that roughly 500 days after the first light of a type Ia supernova reaches our telescopes, the residual light would start to fade.

This is known as the "infrared catastrophe," which was believed to occur as the half life of the nickel isotope 56Ni, a heavy element created in the dramatic supernova, comes to an end, ultimately transitioning into a more stable iron isotope.

However, this drop off has never actually been observed, leading some astronomers to theorize in 2009 that the prolonged light was thrown off from a heavier form of a cobalt isotope with a longer half life known as 57Co.

Observations of SN 2012cg that were carried out three years after the light of the violent explosion reached Earth supported the theory, albeit with an interesting twist. According to an analysis of the residual glow, there would have had to have been twice the amount of 57Co than had previously been believed in order to account for the light output that long after the initial explosion.

There are still a great number of unknowns regarding the supernovae. For example, as it stands astronomers assume that Type Ia supernovae discovered in the local Universe result from the same type of star and type of explosion as those that occured earlier in the lifespan of the cosmos. Should either of these assumptions be proven incorrect, it would introduce a fundamental inconsistency in the way we measure the distances between galaxies.

The new research will attempt to mitigate one area of uncertainty by constraining the explosion models of Type Ia supernovae. The discovery that the powerful explosion of SN 2012cg produced significant quantities of 57Co will allow astronomers to focus on models that successfully predicted higher ratios of the element. This new insight will ultimately allow astronomers to make more accurate measurements of vast cosmic distances, and a better understanding of the expansion of our Universe.

However, there is a possibility that the drop off in light did occur as previously theorized, and that the light detected in recent observations was simply a "light echo." This occurs when light from the original explosion strikes a dust cloud. This cloud refracts the light, scattering it in all directions and causing it to arrive at Earth as an echo years after the first detection of the event.

A paper on the study is available online in the Astrophysical Journal.

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