Draft version September 14, 2016
Preprint typeset using LATEX style AASTeX6 v. 1.0
MERGERS OF DOUBLE DEGENERATE CARBON-OXYGEN WHITE DWARFS WITH THIN HELIUM
LAYERS: A MECHANISM FOR TYPE IA SUPERNOVAE FROM LOW-MASS WHITE DWARFS
Rahul Kashyap,
Advisor: Robert Fisher
Department of Physics, University of Massachusetts Dartmouth,MA, USA
1. ABSTRACT
Type Ia supernovae (SNe Ia) are one of the most important tools in modern cosmology (1). The observables of these
events have characteristic behaviors unique to SNe Ia. This allows us to calibrate SNe Ia to measure the distances
and redshifts of their host galaxies. The empirical relation(2) used for this purpose has been verified for nearby SNe
Ia using independent means. However, the fundamental reasons of their observables have not been understood fully in
computational study thus far. Moreover, we don‘t know how many and what kinds of system produce them primarily.
In case of multiple progenitors, their relative contributions top SNe Ia population are also unknown. It has been known
for a long time that SNe Ia form from exploding white dwarfs. But, an isolated white dwarf is a stable object so,
binary interaction is a theoretical requirement if such a carbon-oxygen (C/O) white dwarf has to explode in a realistic
situation.Two main channels have been promising in numerical modelling in the literature – Single degenerate and
Double Degenerate systems, depending on whether system has one or two white dwarfs. Single degenerate systems
presents a natural channel of detonation i.e. by reaching the white dwarf mass to its critical limit but, fails to explain
many observational details of SNe Ia population. Double-degenerate systems explain the population statistics of SNe
Ia quite well but, they are very hard to reach to the condition of detonation except some recent proposed mechanisms
e.g. (3). The range of observed SNe Ia energies by including a thin helium layer in the merging white dwarfs. A thin
He layer on a white dwarf is a natural outcome of the stellar evolution and it‘s easy to achieve detonation condition in
He layer during final phase of mergers. The He-detonation, in turn, can cause carbon-detonation through the focussing
and convergence of shock detonation wave in the degenerate material(4; 5; 6; 7). This hypothesis, although interesting,
has not been shown to work in a numerical simulation thus far except in one dimensional models where the assumption
of geometry of the problem can be attributed to the cause of detonation.
In this work, I will explore a different effect of He-assisted detonation (referred to as double-detonation in literature)
through which carbon detonation can be achieved. I will investigate the modelling of nuclear reactions in the numerical
simulation of a uniform medium using astrophysical simulation software, FLASH. I will compare the results nucle-
osynthetic yield and nuclear energy yields against a general reaction network code, (TORCH). I will use the version of
TORCH code edited by Dean Townsley. The code will be used to find out the feasibility of carbon detonation purely
because of energy injection due to Helium burning. For this purpose, I will work on including 47-isotope network in
FLASH. The 47-isotope network will be applied to a binary white dwarf (initialized using SPH data) with thin Helium
layer to see the effect of its coupling with hydrodynamics. I will further investigate the effect of size of nuclear reaction
networks on our results and determine what minimum size produces significant difference compared to FLASH.
2. MAJOR PROPOSED MILESTONES
• 5 Oct: I will present the mathematical methods and basic algorithm for implementing 47-isotope nuclear reaction
network. I will identify key isotopes, reactions and routines. The deliverables will be a list of key isotopes and
main reactions.
• 26 Oct: I will deliver the codes which will set up reaction rates, Jacobian and stiff ode solver written to be
integrated in FLASH.
• 23 Nov: I will present the 47-isotope network module (Aprox47) in FLASH. Moreover, I will present the evolution
of some of the global quantities such as total momentum, total internal energy, cumulative nuclear energy of
uniform medium simulation. Nuclear energy, nucleosynthetic yield and maximum tempearture will be compared
between torch code (using 47 isotopes) and the nuclear reaction networks from FLASH (using 47 isotopes).
2• 14 Dec: I simulate a binary WD system with thin Helium layer using the Aprox47 module in FLASH. The binary
white dwarf will be intialized using SPH data.
Software: FLASH, TORCH , yt
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