300 research outputs found
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
Galactic Evolution of Silicon Isotopes: Applications to Presolar SiC Grains from Meteorites
We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium (ISM) at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are 29Si and 30Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of different mass move the silicon isotopic composition? (4) Why do present calcu-lations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley & Weaver super-nova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains
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 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
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
Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains From Meteorites
We calculate and discuss the chemical evolution of the isotopic silicon
abundances in the interstellar medium at distances and times appropriate to the
birth of the solar system. This has several objectives, some of which are
related to anomalous silicon isotope ratios within presolar grains extracted
from meteorites; namely: (1) What is the relative importance for silicon
isotopic compositions in the bulk ISM of Type II supernovae, Type Ia
supernovae, and AGB stars? (2) Are Si and Si primary or secondary
nucleosynthesis products? (3) In what isotopic direction in a three-isotope
plot do core-collapse supernovae of different mass move the silicon isotopic
composition? (4) Why do present calculations not reproduce the solar ratios for
silicon isotopes, and what does that impose upon studies of anomalous Si
isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution
features recorded in the anomalous SiC grains? Our answers are formulated on
the basis of the Woosley \& Weaver (1995) supernova yield survey.
Renormalization with the calculated interstellar medium silicon isotopic
composition and solar composition is as an important and recurring concept of
this paper. Possible interpretations of the silicon isotope anomalies measured
in single SiC grains extracted from carbonaceous meteorites are then presented.
The calculations suggest that the temporal evolution of the isotopic silicon
abundances in the interstellar medium may be recorded in these grains.Comment: Accepted ApJ. 41 pages including 13 figures and 2 tables. uuencoded
gzipped postscrip
Placing the Sun in Galactic Chemical Evolution: Mainstream SiC Particles
We examine the consequences and implications of the possibility that the
best-fit =4/3 line of the silicon isotopic ratios measured in mainstream SiC
grains is identical or parallel to to the mean ISM evolution line of the
silicon isotopes. Even though the mean ISM evolution proceeds along a line of
unity slope when deviations are expressed in terms of the native representation
(the mean ISM), the evolution line can become a slope 4/3 line in the solar
representation, provided that the solar composition is displaced from the mean
ISM evolution. During the course of this analysis, we introduce new methods for
relating the solar composition to that of the mean ISM at the time of solar
birth. These new developments offer a unique view on the meaning of the
mainstream SiC particles, and affords a new way of quantitatively answering the
question whether the sun has a special composition relative to the mean ISM at
solar birth. If the correlation slope of the silicon isotopes in the mean ISM
could be decisively established, then its value would quantify the difference
between the solar and mean ISM silicon abundances. Our formalism details the
transformations between the two representations, and applies not only to
Si and Si, but to any two purely secondary isotopes of any
element (O, Ne, Mg, and perhaps S). Both the advantages and disadvantages of
this technique are critically reviewed.Comment: 19 pages including 4 figures, ApJ in pres
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