433 research outputs found
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
Proton-Rich Nuclear Statistical Equilibrium
Proton-rich material in a state of nuclear statistical equilibrium (NSE) is
one of the least studied regimes of nucleosynthesis. One reason for this is
that after hydrogen burning, stellar evolution proceeds at conditions of equal
number of neutrons and protons or at a slight degree of neutron-richness.
Proton-rich nucleosynthesis in stars tends to occur only when hydrogen-rich
material that accretes onto a white dwarf or neutron star explodes, or when
neutrino interactions in the winds from a nascent proto-neutron star or
collapsar-disk drive the matter proton-rich prior to or during the
nucleosynthesis. In this paper we solve the NSE equations for a range of
proton-rich thermodynamic conditions. We show that cold proton-rich NSE is
qualitatively different from neutron-rich NSE. Instead of being dominated by
the Fe-peak nuclei with the largest binding energy per nucleon that have a
proton to nucleon ratio close to the prescribed electron fraction, NSE for
proton-rich material near freeze-out temperature is mainly composed of Ni56 and
free protons. Previous results of nuclear reaction network calculations rely on
this non-intuitive high proton abundance, which this paper will explain. We
show how the differences and especially the large fraction of free protons
arises from the minimization of the free energy as a result of a delicate
competition between the entropy and the nuclear binding energy.Comment: 4 pages, 7 figure
The light curve of SN 1987A revisited: constraining production masses of radioactive nuclides
We revisit the evidence for the contribution of the long-lived radioactive
nuclides 44Ti, 55Fe, 56Co, 57Co, and 60Co to the UVOIR light curve of SN 1987A.
We show that the V-band luminosity constitutes a roughly constant fraction of
the bolometric luminosity between 900 and 1900 days, and we obtain an
approximate bolometric light curve out to 4334 days by scaling the late time
V-band data by a constant factor where no bolometric light curve data is
available. Considering the five most relevant decay chains starting at 44Ti,
55Co, 56Ni, 57Ni, and 60Co, we perform a least squares fit to the constructed
composite bolometric light curve. For the nickel isotopes, we obtain best fit
values of M(56Ni) = (7.1 +- 0.3) x 10^{-2} Msun and M(57Ni) = (4.1 +- 1.8) x
10^{-3} Msun. Our best fit 44Ti mass is M(44Ti) = (0.55 +- 0.17) x 10^{-4}
Msun, which is in disagreement with the much higher (3.1 +- 0.8) x 10^{-4} Msun
recently derived from INTEGRAL observations. The associated uncertainties far
exceed the best fit values for 55Co and 60Co and, as a result, we only give
upper limits on the production masses of M(55Co) < 7.2 x 10^{-3} Msun and
M(60Co) < 1.7 x 10^{-4} Msun. Furthermore, we find that the leptonic channels
in the decay of 57Co (internal conversion and Auger electrons) are a
significant contribution and constitute up to 15.5% of the total luminosity.
Consideration of the kinetic energy of these electrons is essential in lowering
our best fit nickel isotope production ratio to [57Ni/56Ni]=2.5+-1.1, which is
still somewhat high but is in agreement with gamma-ray observations and model
predictions.Comment: 7 pages, 6 pages, 2 table
On Carbon Burning in Super Asymptotic Giant Branch Stars
We explore the detailed and broad properties of carbon burning in Super
Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models.
The location of first carbon ignition, quenching location of the carbon burning
flames and flashes, angular frequency of the carbon core, and carbon core mass
are studied as a function of the ZAMS mass, initial rotation rate, and mixing
parameters such as convective overshoot, semiconvection, thermohaline and
angular momentum transport. In general terms, we find these properties of
carbon burning in SAGB models are not a strong function of the initial rotation
profile, but are a sensitive function of the overshoot parameter. We
quasi-analytically derive an approximate ignition density, g cm, to predict the location of first carbon ignition
in models that ignite carbon off-center. We also find that overshoot moves the
ZAMS mass boundaries where off-center carbon ignition occurs at a nearly
uniform rate of / 1.6
. For zero overshoot, =0.0, our models in the ZAMS mass
range 8.9 to 11 show off-center carbon ignition. For
canonical amounts of overshooting, =0.016, the off-center carbon
ignition range shifts to 7.2 to 8.8 . Only systems with
and ZAMS mass 7.2-8.0 show
carbon burning is quenched a significant distance from the center. These
results suggest a careful assessment of overshoot modeling approximations on
claims that carbon burning quenches an appreciable distance from the center of
the carbon core.Comment: Accepted ApJ; 23 pages, 21 figures, 5 table
The Three Dimensional Evolution to Core Collapse of a Massive Star
We present the first three dimensional (3D) simulation of the final minutes
of iron core growth in a massive star, up to and including the point of core
gravitational instability and collapse. We self-consistently capture the
development of strong convection driven by violent Si burning in the shell
surrounding the iron core. This convective burning builds the iron core to its
critical (Chandrasekhar) mass and collapse ensues, driven by electron capture
and photodisintegration. The non-spherical structure and motion (turbulent
fluctuations) generated by 3D convection is substantial at the point of
collapse. We examine the impact of such physically-realistic 3D initial
conditions on the core-collapse supernova mechanism using 3D simulations
including multispecies neutrino leakage. We conclude that non-spherical
progenitor structure should not be ignored, and has a significant and favorable
impact on the likelihood for neutrino-driven explosions.Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters. Movies
may be viewed at http://flash.uchicago.edu/~smc/progen3
Neutrinos from beta processes in a presupernova: probing the isotopic evolution of a massive star
We present a new calculation of the neutrino flux received at Earth from a
massive star in the hours of evolution prior to its explosion as a
supernova (presupernova). Using the stellar evolution code MESA, the neutrino
emissivity in each flavor is calculated at many radial zones and time steps. In
addition to thermal processes, neutrino production via beta processes is
modeled in detail, using a network of 204 isotopes. We find that the total
produced flux has a high energy spectrum tail, at
MeV, which is mostly due to decay and electron capture on isotopes with . In a tentative window of observability of MeV and hours pre-collapse, the contribution of beta processes to the flux
is at the level of . For a star at kpc distance, a 17 kt
liquid scintillator detector would typically observe several tens of events
from a presupernova, of which up to due to beta processes. These
processes dominate the signal at a liquid argon detector, thus greatly
enhancing its sensitivity to a presupernova.Comment: 14 pages, 5 figure
The Production of Ti44 and Co60 in Supernova
The production of the radioactive isotopes Ti and Co in all
types of supernovae is examined and compared to observational constraints
including Galactic --ray surveys, measurements of the diffuse 511 keV
radiation, --ray observations of Cas A, the late time light curve of SN
1987A, and isotopic anomalies found in silicon carbide grains in meteorites.
The (revised) line flux from Ti decay in the Cas A supernova remnant
reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper
bound expected from our models. The necessary concurrent ejection of Ni
would also imply that Cas A was a brighter supernova than previously thought
unless extinction in the intervening matter was very large. Thus, if confirmed,
the reported amount of Ti in Cas A provides very interesting constraints
on both the supernova environment and its mechanism. The abundances of
Ti and Co ejected by Type II supernovae are such that
gamma-radiation from Ti decay SN 1987A could be detected by a future
generation of gamma-ray telescopes and that the decay of Co might
provide an interesting contribution to the late time light curve of SN 1987A
and other Type II supernovae. To produce the solar Ca abundance and
satisfy all the observational constraints, nature may prefer at least the
occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia
supernovae. Depending on the escape fraction of positrons due to Co made
in all kinds of Type Ia supernovae, a significant fraction of the steady state
diffuse 511 keV emission may arise from the annihilation of positrons produced
during the decay of Ti to Ca. The Ca and Ti isotopic anomalies in
pre-solar grains confirm the production of Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in
press Ap
Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations
Rigorous statistical methods for estimating thermonuclear reaction rates and
nucleosynthesis are becoming increasingly established in nuclear astrophysics.
The main challenge being faced is that experimental reaction rates are highly
complex quantities derived from a multitude of different measured nuclear
parameters (e.g., astrophysical S-factors, resonance energies and strengths,
particle and gamma-ray partial widths). We discuss the application of the Monte
Carlo method to two distinct, but related, questions. First, given a set of
measured nuclear parameters, how can one best estimate the resulting
thermonuclear reaction rates and associated uncertainties? Second, given a set
of appropriate reaction rates, how can one best estimate the abundances from
nucleosynthesis (i.e., reaction network) calculations? The techniques described
here provide probability density functions that can be used to derive
statistically meaningful reaction rates and final abundances for any desired
coverage probability. Examples are given for applications to s-process neutron
sources, core-collapse supernovae, classical novae, and big bang
nucleosynthesis.Comment: Accepted for publication in J. Phys. G Focus issue "Enhancing the
interaction between nuclear experiment and theory through information and
statistics
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