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Fig. 1:
Band-limited light curves as obtained from our radiative
transfer calculations for the U-, B-, V- and R-band filters. Lines of
different colour correspond to models of different White Dwarf mass. The
different symbols show photometric data of three observed Type Ia
supernovae.
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Fig. 2:
Synthetic spectrum at 3 days before maximum brightness as
obtained from our radiative transfer calculations (thick black line).
For comparison the blue line shows the observed spectrum of the Type Ia
supernova SN 2004eo at the corresponding epoch. The colour coding below
the spectrum indicates the contribution of each chemical element to the
emission in the corresponding wavelength bin.
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Stellar explosions give rise to the temporary appearance of bright new
stars in the sky. Among these so-called supernovae is a particular class
of objects that is characterized by the absence of hydrogen but strong
evidence of silicon in their spectra. These "Type Ia" supernovae are of
particular interest to modern astrophysics. Their importance stems from
the fact that their absolute brightness can be obtained from
observations using an empirically derived relationship between their
light curve shape and brightness. Since objects of known absolute
brightness can be used to measure distances, this makes Type Ia
supernovae the most important beacons to measure the expansion rate of
the Universe. Despite their importance, however, we still lack a good
understanding of the physical origin of these explosions or of the
astrophysical systems in which they occur.
In the widely-accepted standard model, Type Ia supernovae result from
thermonuclear explosions of a White Dwarf star composed of carbon and
oxygen. Isolated White Dwarfs, which represent the final stages of
relatively low-mass stars like our Sun, are eternally stable. Having
exhausted their nuclear energy supply, these stars simply cool down and
fade over billions of years. Many stars, however, are part of a binary
system. In such a case it is possible for the two stars to interact with
each other and some such interaction is most likely the reason why
explosions of White Dwarfs occur. Several evolutionary scenarios for
binary star systems that could lead to Type Ia supernova explosions have
been proposed but, despite considerable effort, we still do not know
which ones are realized in nature.
In the currently most favoured scenario the White Dwarf accretes
hydrogen-rich material from a companion star until it grows to a critical mass
(the Chandrasekhar mass). At that point, the temperatures and densities
in the core of the White Dwarf grow sufficiently high that thermonuclear
reactions start and the White Dwarf explodes. Although this scenario is
capable to reproduce the observed diversity of Type Ia supernovae
(Press Release February 2007),
there are severe problems in explaining the observed rate of
Type Ia supernovae within this scenario as was recently revealed by MPA researchers
(Press Release February 2010).
The mergers of two White Dwarfs in a close binary were suggested as an
alternative to trigger Type Ia supernovae. However, it has long been
unclear whether these systems could really produce thermonuclear
explosions. Only recently a team of MPA researchers has shown
(Press Release January 2010)
that this is indeed possible. However, they concluded that this
progenitor channel will produce most likely a peculiar sub-class of the
Type Ia supernovae.
A third option, which has received relatively little attention so far,
is that of a White Dwarf which accretes helium-rich material from a
companion star. This leads to the build-up of a helium layer on the
White Dwarf, which may eventually become unstable and ignite, ultimately
leading to an explosion of the underlying White Dwarf. In contrast to
the other scenarios outlined above, this would happen when the mass of
the White Dwarf is lower than the Chandrasekhar limit. Since low-mass
White Dwarfs are more common than White Dwarfs that grow to the
Chandrasekhar limit, this model may do much better than the standard
model in accounting for the observed rate of Type Ia supernovae.
However, it had been largely discarded since the presence of a thick
helium-rich layer leads to a composition of the explosion ejecta which differs
from that seen in observations. New calculations by Bildsten and Shen
(University of California, Santa Barbara) of the properties of the
helium shell, however, have indicated that ignition might occur for much
thinner shells than previously thought. This opens the door for
considering whether this explosion might resemble a Type Ia supernova if
the helium shell is thin enough.
In a recent paper (published in the Astrophysical Journal Letters) a
team of MPA scientists re-investigated this sub-Chandrasekhar-mass
scenario, considering an idealized case where the influence of the layer
of accreted helium is negligible. For this they hydrodynamically
simulated artificial explosions of a set of "naked"
sub-Chandrasekhar-mass carbon-oxygen White Dwarfs. Using radiative transfer simulations
they derived synthetic observables for their explosion models and
compared them to observations of Type Ia supernovae. ”The light
curves and spectra we obtain from these simulations are in astonishingly
good agreement with observed properties of Type Ia supernovae,“
says Stuart Sim, the principal investigator of this study. Thus, the MPA
scientists conclude that ”explosions of sub-Chandrasekhar-mass
White Dwarfs might be a viable model of Type Ia supernovae if the
optical display is dominated not by the products of burning in the
helium-shell but by the ejecta produced in the explosion of the underlying
White Dwarf.“
Friedrich Röpke, a co-investigator of the study, mentions another
appealing property of sub-Chandrasekhar-mass explosions: ”Since
the mass of the exploding White Dwarf is not fixed to the Chandrasekhar
limit, the initial mass of the exploding carbon-oxygen White Dwarf
provides a simple physical parameter to account for the range of
observed brightnesses of Type Ia supernovae.“ Thus, this scenario
has the potential to give a simple physical explanation for the range of
properties of Type Ia supernovae. The race is now on to answer the
critical question: can real explosions simultaneously meet conditions
for igniting the helium shell whilst not giving rise to an outer ejecta
layer which is incompatible with observations?
Markus Kromer
Original publications:
S.A. Sim, F.K. Röpke, W. Hillebrandt, M. Kromer, R. Pakmor,
M. Fink, A.J. Ruiter, I.R. Seitenzahl,
"Detonations in Sub-Chandrasekhar-mass C+O White Dwarfs",
The Astrophysical Journal Letters 714 (2010), L52-L57
M. Fink, F. K. Roepke, W. Hillebrandt, I. R. Seitenzahl, S. A. Sim, M. Kromer,
"Double-detonation sub-Chandrasekhar supernovae: can minimum helium
shell masses detonate the core?",
Astronomy & Astrophysics 514 (2010), id.A53
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