Nature article: Supernovae Mystery Solved – University of Copenhagen

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30 June 2010

Nature article: Supernovae Mystery Solved

An international collaboration, including researchers at the Dark Cosmology Centre in Copenhagen, has solved a puzzle regarding Type Ia supernovae. These supernovae form a homogeneous class of stellar explosions and are used as "standard candles" to study the acceleration of the Universe. However, it has long been known that they demonstrate diversity in their spectral evolution, questioning their usefulness as standard candles. In an article published in this week's volume of the journal Nature, the research group argues that supernovae Ia explode asymmetrically, and that the different spectral appearance is merely a consequence of the random directions from which a SN is viewed. This is good news for the use of supernovae in cosmology.

Type Ia supernovae have played a key role in cosmology, and in the discovery of Dark Energy, because they can be used to measure distances across the Universe. Although they are not perfect standard candles they can be standardized in an empirical way: the brighter supernovae decline slower while the dimmer faster. It is widely believed today that the progenitor to such a supernova is a White Dwarf in a binary system that is accreting material from its companion star until it reaches 1.4 solar masses and explodes.

Detailed investigations have revealed, however, that the nature of SNe Ia is more complicated. Supernovae that look like twins concerning their luminosity evolution, can demonstrate a quite different behavior in the speed with which their spectral features evolve (the so-called velocity gradient). This fact has been raising concerns on whether these events can indeed be used as cosmological standard candles.

An international team of scientists, led by Keiichi Maeda in Tokyo and including DARK research staff Max Stritzinger, Jesper Sollerman and Giorgos Leloudas, has now shown that the velocity gradient is closely related to the asymmetric nature of these supernovae. The asymmetry was measured using the Doppler shift of a spectral emission line that traces the inner parts of the supernova explosion. 

Supernovae with a high velocity gradient demonstrate a redshift in this line, while supernovae with a low velocity gradient preferentially show a blueshift. This behavior can be explained by an asymmetric explosion model where ignition takes place off-centre (See figure 1). The different viewing angles from which an observer can look at the explosion result in the different spectral signatures.

While there had already been hints that supernovae Ia might explode asymmetrically, this is the first time that this has been shown by observationally studying supernovae at late times, when the inner regions of the explosions are revealed to our eyes.

Except from offering us a new insight on how these stars explode and solving the problem of spectral diversity, these results are good news for the use of supernovae as standard candles: if we just observe enough supernovae the differences from viewing angles will average out.

Links:

Article in Nature
Niels Bohr Institute's press release (Danish)
Niels Bohr Institute's press release (English)
Institute for the Physics and Mathematics of the Universe