Type Ia supernovae (SNe Ia) are spectacular explosions in white dwarf
stars and play an essential role in astrophysics in general and in
cosmological studies in particular. However, many puzzles about the
nature and the inherent physical mechanisms in SNe Ia are still waiting
to be answered. Robotic surveys of the next decade will provide an
unprecedented wealth of observed Type Ia supernovae, detected shortly
after explosion. Researchers at MPA examine here whether different
explosion models are expected to leave clear imprints in such early
observations that could be used in future photometric surveys to help
shedding light on the progenitors and explosion mechanism of SNe Ia.
Most likely, you are reading this article using a device whose
existence relies on the silicon chip, such as a PC, laptop or mobile
phone. Together with a number of other chemical elements such as iron, a
significant fraction of the silicon in our Universe today has been
forged from lighter elements in the thermonuclear fires raging in
cataclysmic events known as "Type Ia supernovae" (SNe Ia). These violent
explosions mark the brilliant death of a low mass star. During their
evolution, SNe Ia can become incredibly bright – to the point at which
they outshine their host galaxies (see for example SN 1994D shown in
Figure 1).
This is one of the properties that make SNe Ia ideal for cosmological
studies in which they are frequently used as distance indicators
mapping out the recent expansion history of the Universe. Specifically,
SNe Ia were instrumental in establishing our current cosmological
picture which involves a dark energy component responsible for the
accelerated expansion. This discovery was recognized by the Nobel prize
committee in 2011. However, despite their astrophysical and cosmological
significance, astrophysicists are still in the dark about many aspects
concerning SNe Ia.
It is broadly accepted that the supernova marks a thermonuclear explosion in a white dwarf made up of mainly carbon and oxygen that has been part of a binary system. White dwarfs are compact objects which are stabilized by electron degeneracy pressure. They are the evolutionary end state of low mass stars after their nuclear fuel has been exhausted. However, it is still heavily debated what the nature of the companion is, whether it is a sun-like or giant star or another white dwarf.
Moreover, the details of how the thermonuclear explosion is triggered
and how it proceeds are still under active investigation. In
particular, it is not clear if the burning front propagates as a
supersonic detonation, as a subsonic turbulent deflagration, or whether a
mixture of both modes is realized and the burning starts subsonically
and then transitions into a detonation (delayed detonation model).
Related to the previous questions, it is still unclear at which mass the white dwarf explodes, in particular whether the supernova sets in at the theoretical mass limit for systems stabilized by electron degeneracy pressure (about 1.4 times the mass of our sun), or below it. This limit is referred to as "Chandrasekhar mass" and consequently one distinguishes Chandrasekhar mass and sub-Chandrasekhar mass models. In the latter case, the explosion can for example be triggered by a merger with another white dwarf.
Finally, it still has to be firmly established whether one scenario
is exclusively responsible for SNe Ia or whether a mixture of the
different explosion and progenitor possibilities is realised in nature.
Researchers at MPA performed a theoretical study, developing predictions for the early optical appearance for a number of common explosion models for standard SNe Ia. They focussed specifically on identifying clear signatures in the early light curve, i.e. the time evolution of the emission in a particular passband. Such a signature would make it possible to clearly identify specific explosion scenarios from early photometric observations.
The reason for the interest and focus on early observables is
two-fold: currently, the tightest constraints on the nature of SN Ia
progenitors come from the earliest data points shortly after explosion.
Moreover, upcoming high-cadence surveys and upgrades of existing
transient search programmes will drastically increase the number of SNe
Ia detected shortly after explosion.
For the main part of the study, the scientists selected two
Chandrasekhar mass explosion models, namely the well-known carbon
deflagration model W7 and the delayed detonation model N100. In
addition, they focussed on three sub-Chandrasekhar models, in particular
a merger of two white dwarfs, a double detonation in a carbon-oxygen
white dwarf with a helium shell and a pure detonation in a white dwarf
core. Using the radiation hydrodynamical code Stella, they
followed the supernova ejecta evolution in all these models and
calculated colour light curves in various pass bands (see Figure 2).
While for most scenarios, the light curves of the various models
evolve similarly, the double detonation model shows a steep rise and a
pronounced first shoulder due to radioactive material located close to
the ejecta surface. This material has been synthesized in the first
detonation in the Helium shell. Unfortunately, this signature is very
similar to the traces left by the interaction between ejecta and a
companion star or ejecta and circumstellar material, which have been
investigated by other groups, rendering it a challenge to establish a
clear link between such a feature in the early observables and the
physical properties of the explosion scenario.
Investigating the early light curves in more detail, the researchers
found that none of the standard models follow a power-law rise. However,
such a behaviour, namely that the emitted luminosity increases
proportional to some power of the time since explosion, is often assumed
when reconstructing the explosion date from observational data. The
scientists demonstrate that this can lead to errors of several days in
determining the explosion date without degrading the fidelity of the
fits. Potentially, this has severe consequences for estimating the size
and nature of the exploding object from early data, which requires a
precise determination of the time of explosion.
In summary, the researchers demonstrated that it is very challenging
to identify specific explosion scenarios based on early photometric data
alone. The additional availability of early spectroscopic information
may help to break some of the degeneracy. Unlike typically assumed, they
predict an early non-power law rise for all of the investigated
standard explosion models. This can lead to serious difficulties in
dating the explosion and deriving constraints about the nature of the
exploding object.
Authors
Noebauer, Ulrich
Postdoc
Authors
Noebauer, Ulrich
Postdoc
Phone: 2297
Email: ulnoe@mpa-garching.mpg.de
Links: personal homepage (the institute is not responsible for the contents of personal homepages)
Taubenberger, Stefan
Postdoc
Phone: ESO
Email: tauben@mpa-garching.mpg.de
Hillebrandt, Wolfgang
Director emeritus
Phone: 2200
Email: wfh@mpa-garching.mpg.de
Original Publication