Date:
Thu, 09/01/201414:00-15:00
"Helium shell detonations from accreting white dwarfs"
There are many double white dwarf (WD) binaries in our galaxy that contain a M < 0.2 M_sun helium WD and a more massive WD that is either made of carbon/oxygen or oxygen/neon. These binaries are often in such short orbital periods that gravitational wave losses will drive the He WD into contact, transferring helium to the more massive WD. The unstable burning of the accreted helium on the more massive WD can become dynamical.
It remains an open question as to whether convectively burning helium shells transition to a deflagration or detonation as the convective velocities approach the sound speed. Using a physically motivated one-dimensional model and two-dimensional direct numerical simulations of the detonation structure, we find the minimum helium layer thickness that sustains a lateral detonation and show that it depends on the density of the helium and the composition (specifically C12 and O16 abundances) within the layer. We explore the properties of the successful detonations in these thin helium layers, finding detonation speeds slower than the Chapman-Jouget (CJ) speed for complete helium burning of v_CJ = 1.5e9 cm/s. Though gravitationally unbound, the ashes still have substantial amounts of unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as Ca40, Ti44, Cr48, and Fe52. It is rare for these thin shells to generate large amounts of Ni56.
We close by discussing how these unbound helium burning ashes can create faint and fast '.Ia' supernovae as well as events with virtually no radioactivity, and speculate on how the slower helium detonation velocities may impact the potential off-center ignition of a carbon detonation that could cause a Type Ia supernova in the double detonation scenario. We also explore how the presence of carbon \& oxygen within the layer (eg. from the composition of accreted material, production during the convective burning, or mixing with the underlying WD) modifies the nucleosynthesis, detonation speed, and minimum thickness for a steady detonation, revealing a new branch of solutions that can propagate in very thin helium layers when O16 is present at a minimum mass fraction of ~0.07. Driven by energy release from rapid alpha-captures on O16 and subsequent elements, these much slower detonations only create ashes up to Si28 in the outer detonated He shell, yet are still fast enough to create an off-center focused convergent spot in the underlying WD that may cause a secondary detonation.
There are many double white dwarf (WD) binaries in our galaxy that contain a M < 0.2 M_sun helium WD and a more massive WD that is either made of carbon/oxygen or oxygen/neon. These binaries are often in such short orbital periods that gravitational wave losses will drive the He WD into contact, transferring helium to the more massive WD. The unstable burning of the accreted helium on the more massive WD can become dynamical.
It remains an open question as to whether convectively burning helium shells transition to a deflagration or detonation as the convective velocities approach the sound speed. Using a physically motivated one-dimensional model and two-dimensional direct numerical simulations of the detonation structure, we find the minimum helium layer thickness that sustains a lateral detonation and show that it depends on the density of the helium and the composition (specifically C12 and O16 abundances) within the layer. We explore the properties of the successful detonations in these thin helium layers, finding detonation speeds slower than the Chapman-Jouget (CJ) speed for complete helium burning of v_CJ = 1.5e9 cm/s. Though gravitationally unbound, the ashes still have substantial amounts of unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as Ca40, Ti44, Cr48, and Fe52. It is rare for these thin shells to generate large amounts of Ni56.
We close by discussing how these unbound helium burning ashes can create faint and fast '.Ia' supernovae as well as events with virtually no radioactivity, and speculate on how the slower helium detonation velocities may impact the potential off-center ignition of a carbon detonation that could cause a Type Ia supernova in the double detonation scenario. We also explore how the presence of carbon \& oxygen within the layer (eg. from the composition of accreted material, production during the convective burning, or mixing with the underlying WD) modifies the nucleosynthesis, detonation speed, and minimum thickness for a steady detonation, revealing a new branch of solutions that can propagate in very thin helium layers when O16 is present at a minimum mass fraction of ~0.07. Driven by energy release from rapid alpha-captures on O16 and subsequent elements, these much slower detonations only create ashes up to Si28 in the outer detonated He shell, yet are still fast enough to create an off-center focused convergent spot in the underlying WD that may cause a secondary detonation.