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Type Ia supernovae (SNe Ia), which make up about one quarter of all
supernovae, are believed to come from exploding white dwarf stars,
though how the white dwarf reaches the critical conditions to make it
explode is still unclear.
More than 95% of stars will end their lives as white dwarfs (including
our Sun when it runs out of fuel), but only a small fraction of these
will actually explode. A lonely white dwarf star is stable - it won't
spontaneously erupt. However, if there is a source of matter nearby -
e.g. another star - the white dwarf can steal mass from this
companion, with explosive consequences. Thus, astronomers have been
trying to find out what types of double star systems including at
least one white dwarf can lead to the formation of Type Ia
supernovae.
Since white dwarfs are rather faint when they are not exploding,
observations alone cannot solve this well-known 'progenitor
problem'. Therefore testing of theoretical models has become a
critical step in understanding the origin of SNe Ia.
The biggest mystery shrouding SNe Ia is this: what type of star is
'donating' mass to the white dwarf? Is the companion a normal
(Sun-like) star tranquilly passing matter to the white dwarf, thereby
slowly pushing it closer and closer to the critical limit, or is it
another white dwarf star that violently smashes into the more massive
one, immediately causing an explosion?
Using a detailed model for the evolution of double stars,
state-of-the-art hydrodynamic explosion models and a sophisticated
method for predicting how the energy from the explosion is turned into
observable light (spectra), MPA researchers and collaborators
determined that white dwarfs which smash violently into each other
give rise to a range of brightnesses that matches the range in
brightness that is actually observed for Type Ia supernovae. Even more
encouraging, the model brightness distribution peaks at about the same
value as the one from observations (see figure 2). Any model scenario
that is claimed to account for a large fraction of SNe Ia must be able
to explain observational trends. Not only does the violent merger
model do very well in terms of reproducing the brightness distribution
of real SNe Ia, it also produces the right number of events as a
function of time (the 'delay time distribution', see figure 3).
In this particular model the peak brightness of the explosion is
directly related to the mass of the more massive (primary) white dwarf.
To get a typical explosion, however, most primary white dwarfs have to
grow in mass before they explode. The team has identified an
evolutionary pathway which serves to 'beef up' the mass of the primary
well before the merger occurs. However, it has yet to be confirmed
whether white dwarfs can really be 'beefed up' by their companions
sufficiently in large enough numbers.
While the MPA researchers are excited about their result, they remain
slightly cautious. It is still unclear if this formation scenario of
pre-merging white dwarfs is realized in nature as efficiently as the
binary evolution model indicates. Some further work and (probably)
future observations are needed to confirm the various aspects of the
model.
If it turns out that such an 'evolutionary channel' that leads to more
massive primary white dwarfs readily contributes to making white dwarf
pairs, then it is likely that violent white dwarf mergers are driving
the underlying brightness distribution of SNe Ia. If not, then some
other explosion scenario could be dominating the SN Ia scene.
Ashley Ruiter, Stuart Sim, Ruediger Pakmor, Markus Kromer, Ivo
Seitenzahl, Stefan Taubenberger
Original publication
Ruiter, A. J.; Sim, S. A.; Pakmor, R.; Kromer, M.; Seitenzahl, I. R.;
Belczynski, K.; Fink, M.; Herzog, M.; Hillebrandt, W.; Roepke, F. K.;
Taubenberger, S.
"On the brightness distribution of Type Ia supernovae from violent
white dwarf mergers",
submitted to MNRAS.The draft is available on astro-ph:
http://adsabs.harvard.edu/abs/2012arXiv1209.0645R
Further references
Li et al. 2012
Maoz et al. 2012 (MMB12)
Graur and Maoz 2012 (GM12)
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