Una supernova di tipo Ia si ha quando una nana bianca accresce materia da una stella compagna finchè essa supera il limite di Chandrasekhar e quindi esplode. Studiando queste esplosioni stellari, gli astronomi sono in grado di misurare l'energia scura e l'espansione dell'Universo. I ricercatori del CfA hanno trovato un modo per correggere piccole variazioni di luminosità apparente da queste supernovae in modo che esse siano considerate ancora di più candele standard. La chiave di questo metodo sta nel classificarle in base al loro colore. 

Credit: Justyn R. Maund (University of Cambridge)

SN 2012fr, an interesting stellar explosion case

Lo scorso anno, una serie di osservazioni relative all’esplosione di una nana bianca nei dintorni nella galassia NGC 1365 ha permesso ad un gruppo di ricercatori della Australian National University a raccogliere una grande quantità di dati su quella che essi ritengono sia una delle migliori ‘candele standard’ che viene utilizzata dagli astronomi come strumento di misura delle distanze cosmiche.

We know how a candle of a particular brightness grows fainter as it is moved further away from us. So, if we know the true brightness of the candle (in this instance, supernova SN 2012fr) and we measure its observed brightness, we can then calculate the interceding distance”, said Michael Childress. Supernova SN 2012fr left a chemical fingerprint which has been analysed by a team of researchers led by Childress from the ANU Research School of Astronomy and Astrophysics and which also includes Nobel Laureate Professor Brian Schmidt. Their data shows unprecedented, and quite unusual, layering in the material that was burnt and ejected in the explosion, especially silicon and iron. Two distinct layers of silicon were found: one thick, outer layer that had faded by the time the supernova reached its peak brightness on 12 November 2012 (16 days after the initial explosion), and one deeper layer that hardly changed for several weeks after the explosion.

As it turns out, SN 2012fr is not just another supernova but a really interesting case.

Since it was discovered within a day of explosion, we were able to study it in greater detail than almost any supernova ever discovered” Childress said. “Because we know the distance to its host galaxy (NGC 1365), this supernova actually lets us better calibrate all Type Ia Supernova observations to measure distances in the Universe, using what we call the ‘standard candle’ technique”. Despite its unusual layers, SN 2012fr appears to still be classified as a so-called ‘normal’ Type Ia Supernova, which Professor Schmidt used in his Nobel Prize winning work to discover Dark Energy, and it also presents a key link in our cosmic distance ladder. “Our analyses of SN 2012fr will increase the precision of which we can measure distances outside of our own galaxy, as well as improve our understanding of these explosive events and our ability to use them in the hunt for Dark Energy, the source of the accelerated expansion of the Universe”, said Childress.

ANU: Key link found in Cosmic Distance Ladder
arXiv: Spectroscopic Observations of SN 2012fr: A Luminous Normal Type Ia Supernova with Early High Velocity Features and Late Velocity Plateau