23 research outputs found
The role of dredge-up in double white dwarf mergers
We present the results of an investigation of the dredge-up and mixing during
the merger of two white dwarfs with different chemical compositions by
conducting hydrodynamic simulations of binary mergers for three representative
mass ratios. In all the simulations, the total mass of the two white dwarfs is
. Mergers involving a CO and a He white dwarf have
been suggested as a possible formation channel for R Coronae Borealis type
stars, and we are interested in testing if such mergers lead to conditions and
outcomes in agreement with observations. Even if the conditions during the
merger and subsequent nucleosynthesis favor the production of , the merger must avoid dredging up large amounts of , or
else it will be difficult to produce sufficient to explain
the oxygen ratio observed to be of order unity. We performed a total of 9
simulations using two different grid-based hydrodynamics codes using fixed and
adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that
in most of the simulations, of is
indeed dredged up during the merger. However, in SPH simulations where the
accretor is a hybrid He/CO white dwarf with a layer of
helium on top, we find that no is being dredged up, while in
the simulation of has been
brought up, making a WD binary consisting of a hybrid CO/He WD and a companion
He WD an excellent candidate for the progenitor of RCB stars.Comment: Accepted for publication in Ap
Using Spatial Distributions to Constrain Progenitors of Supernovae and Gamma Ray Bursts
We carry out a comprehensive theoretical examination of the relationship
between the spatial distribution of optical transients and the properties of
their progenitor stars. By constructing analytic models of star-forming
galaxies and the evolution of stellar populations within them, we are able to
place constraints on candidate progenitors for core-collapse supernovae (SNe),
long-duration gamma ray bursts, and supernovae Ia. In particular we first
construct models of spiral galaxies that reproduce observations of
core-collapse SNe, and we use these models to constrain the minimum mass for
SNe Ic progenitors to approximately 25 solar masses. Secondly, we lay out the
parameters of a dwarf irregular galaxy model, which we use to show that the
progenitors of long-duration gamma-ray bursts are likely to have masses above
approximately 43 solar masses. Finally, we introduce a new method for
constraining the time scale associated with SNe Ia and apply it to our spiral
galaxy models to show how observations can better be analyzed to discriminate
between the leading progenitor models for these objects.Comment: 18 pages, 19 figures, ApJ, in pres
Remnants of Binary White Dwarf Mergers
We carry out a comprehensive smooth particle hydrodynamics simulation survey
of double-degenerate white dwarf binary mergers of varying mass combinations in
order to establish correspondence between initial conditions and remnant
configurations. We find that all but one of our simulation remnants share
general properties such as a cold, degenerate core surrounded by a hot disk,
while our least massive pair of stars forms only a hot disk. We characterize
our remnant configurations by the core mass, the rotational velocity of the
core, and the half-mass radius of the disk. We also find that some of our
simulations with very massive constituent stars exhibit helium detonations on
the surface of the primary star before complete disruption of the secondary.
However, these helium detonations are insufficiently energetic to ignite
carbon, and so do not lead to prompt carbon detonations.Comment: 17 pages, 11 figures, Accepted to Ap
Prompt Ia Supernovae Are Significantly Delayed
The time delay between the formation of a population of stars and the onset
of type Ia supernovae (SNe Ia) sets important limits on the masses and nature
of SN Ia progenitors. Here we use a new observational technique to measure this
time delay by comparing the spatial distributions of SNe Ia to their local
environments. Previous work attempted such analyses encompassing the entire
host of each SN Ia, yielding inconclusive results. Our approach confines the
analysis only to the relevant portions of the hosts, allowing us to show that
even so-called "prompt" SNe Ia that trace star-formation on cosmic timescales
exhibit a significant delay time of 200-500 million years. This implies that
either the majority of Ia companion stars have main-sequence masses less than 3
solar masses, or that most SNe Ia arise from double-white dwarf binaries. Our
results are also consistent with a SNe Ia rate that traces the white dwarf
formation rate, scaled by a fixed efficiency factor.Comment: 6 pages, 6 figures, ApJ, in pres
On Type Ia Supernovae From The Collisions of Two White Dwarfs
We explore collisions between two white dwarfs as a pathway for making Type
Ia Supernovae (SNIa). White dwarf number densities in globular clusters allow
10-100 redshift <1 collisions per year, and observations by (Chomiuk et al.
2008) of globular clusters in the nearby S0 galaxy NGC 7457 have detected what
is likely to be a SNIa remnant. We carry out simulations of the collision
between two 0.6 solar mass white dwarfs at various impact parameters and mass
resolutions. For impact parameters less than half the radius of the white
dwarf, we find such collisions produce approximately 0.4 solar masses of Ni56,
making such events potential candidates for underluminous SNIa or a new class
of transients between Novae and SNIa.Comment: 4 pages, 4 figures, 1 tabl