320 research outputs found
Nucleosynthesis Calculations for the Ejecta of Neutron Star Coalescences
We present the results of fully dynamical r-process network calculations for
the ejecta of neutron star mergers (NSMs). The late stages of the inspiral and
the final violent coalescence of a neutron star binary have been calculated in
detail using a 3D hydrodynamics code (Newtonian gravity plus backreaction
forces emerging from the emission of gravitational waves) and a realistic
nuclear equation of state. The found trajectories for the ejecta serve as input
for dynamical r-process calculations where all relevant nuclear reactions
(including beta-decays depositing nuclear energy in the expanding material) are
followed. We find that all the ejected material undergoes r-process. For an
initial Ye close to 0.1 the abundance distributions reproduce very accurately
the solar r-process pattern for nuclei with A above 130. For lighter nuclei
strongly underabundant (as compared to solar) distributions are encountered. We
show that this behaviour is consistent with the latest observations of very
old, metal-poor stars, despite simplistic arguments that have recently been
raised against the possibility of NSM as possible sources of Galactic r-process
material.Comment: 5 pages, 2 figures, proceedings of Nuclei in the Cosmos 2000, to be
published in Nucl. Phys. A; minor correctio
On the astrophysical robustness of neutron star merger r-process
In this study we explore the nucleosynthesis in the dynamic ejecta of compact
binary mergers. We are particularly interested in the question how sensitive
the resulting abundance patterns are to the parameters of the merging system.
Therefore, we systematically investigate combinations of neutron star masses in
the range from 1.0 to 2.0 \Msun and, for completeness, we compare the results
with those from two simulations of a neutron star black hole merger. The ejecta
masses vary by a factor of five for the studied systems, but all amounts are
(within the uncertainties of the merger rates) compatible with being a major
source of cosmic r-process. The ejecta undergo a robust r-process
nucleosynthesis which produces all the elements from the second to the third
peak in close-to-solar ratios. Most strikingly, this r-process is extremely
robust, all 23 investigated binary systems yield practically identical
abundance patterns. This is mainly the result of the ejecta being extremely
neutron rich (\ye ) and the r-process path meandering along the
neutron drip line so that the abundances are determined entirely by nuclear
rather than by astrophysical properties. This robustness together with the ease
with which both the second and third peak are reproduced make compact binary
mergers the prime candidate for the source of the observed unique heavy
r-process component.Comment: accepted for publication in MNRA
Neutrino signatures and the neutrino-driven wind in Binary Neutron Star Mergers
We present VULCAN/2D multi-group flux-limited-diffusion radiation
hydrodynamics simulations of binary neutron star (BNS) mergers, using the Shen
equation of state, covering ~100 ms, and starting from azimuthal-averaged 2D
slices obtained from 3D SPH simulations of Rosswog & Price for 1.4 Msun
(baryonic) neutron stars with no initial spins, co-rotating spins, and
counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a
multi-angle neutrino-transport solver. We find polar-enhanced neutrino
luminosities, dominated by and ``'' neutrinos at peak,
although emission may be stronger at late times. We obtain typical peak
neutrino energies for , , and ``'' of ~12, ~16,
and ~22 MeV. The super-massive neutron star (SMNS) formed from the merger has a
cooling timescale of ~1 s. Charge-current neutrino reactions lead to the
formation of a thermally-driven bipolar wind with ~10
Msun/s, baryon-loading the polar regions, and preventing any production of a
GRB prior to black-hole formation. The large budget of rotational free energy
suggests magneto-rotational effects could produce a much greater polar mass
loss. We estimate that ~10 Msun of material with electron fraction in
the range 0.1-0.2 become unbound during this SMNS phase as a result of neutrino
heating. We present a new formalism to compute the
annihilation rate based on moments of the neutrino specific intensity computed
with our multi-angle solver. Cumulative annihilation rates, which decay as
, decrease over our 100 ms window from a few 10 to ~10
erg/s, equivalent to a few 10 to ~10 pairs per second.Comment: 23 pages, 20 figures, 2 tables, submitted to ApJ, high resolution
version of the paper available at http://hermes.as.arizona.edu/~luc/ms.pd
Neutrino Signatures and the Neutrino-Driven Wind in Binary Neutron Star Mergers
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering âł 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4Mâ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by ¯νe and âνΟâ neutrinos at the peak, although νe emission may be stronger at late times. We obtain typical peak neutrino energies for νe, ¯νe, and âνΟâ of âź12, âź16, and âź22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of âž 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with (M¡) âź 10^â3 Mâ s^â1 and baryon-loading in the polar regions, preventing any production of a Îł-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that âž 10^â4 Mâ of material with an electron fraction in the range 0.1â0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the νi ¯νi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as âźt^â1.8, decrease over our 100 ms window from a few Ă1050 to âź 1049 erg sâ1, equivalent to a few Ă10^54 to âź10^53 eâe+ pairs per second
The runaway instability in general relativistic accretion disks
When an accretion disk falls prey to the runaway instability, a large portion
of its mass is devoured by the black hole within a few dynamical times. Despite
decades of effort, it is still unclear under what conditions such an
instability can occur. The technically most advanced relativistic simulations
to date were unable to find a clear sign for the onset of the instability. In
this work, we present three-dimensional relativistic hydrodynamics simulations
of accretion disks around black holes in dynamical space-time. We focus on the
configurations that are expected to be particularly prone to the development of
this instability. We demonstrate, for the first time, that the fully
self-consistent general relativistic evolution does indeed produce a runaway
instability.Comment: 5 pages, 3 figures, minor corrections to match published version in
MNRAS, +link to animatio
A "kilonova" associated with short-duration gamma-ray burst 130603B
Short-duration gamma-ray bursts (SGRBs) are intense flashes of cosmic
gamma-rays, lasting less than ~2 s, whose origin is one of the great unsolved
questions of astrophysics today. While the favoured hypothesis for their
production, a relativistic jet created by the merger of two compact stellar
objects (specifically, two neutron stars, NS-NS, or a neutron star and a black
hole, NS-BH), is supported by indirect evidence such as their host galaxy
properties, unambiguous confirmation of the model is still lacking. Mergers of
this kind are also expected to create significant quantities of neutron-rich
radioactive species, whose decay should result in a faint transient in the days
following the burst, a so-called "kilonova". Indeed, it is speculated that this
mechanism may be the predominant source of stable r-process elements in the
Universe. Recent calculations suggest much of the kilonova energy should appear
in the near-infrared (nIR) due to the high optical opacity created by these
heavy r-process elements. Here we report strong evidence for such an event
accompanying SGRB 130603B. If this simplest interpretation of the data is
correct, it provides (i) support for the compact object merger hypothesis of
SGRBs, (ii) confirmation that such mergers are likely sites of significant
r-process production and (iii) quite possibly an alternative, un-beamed
electromagnetic signature of the most promising sources for direct detection of
gravitational waves.Comment: preprint of paper appearing in Nature (3 Aug 2013
The first direct double neutron star merger detection: implications for cosmic nucleosynthesis
The astrophysical r-process site where about half of the elements heavier
than iron are produced has been a puzzle for several decades. Here we discuss
the role of neutron star mergers (NSMs) in the light of the first direct
detection of such an event in both gravitational (GW) and electromagnetic (EM)
waves. We analyse bolometric and NIR lightcurves of the first detected double
neutron star merger and compare them to nuclear reaction network-based
macronova models. The slope of the bolometric lightcurve is consistent with the
radioactive decay of neutron star ejecta with (but not
larger), which provides strong evidence for an r-process origin of the
electromagnetic emission. This rules out in particular "nickel winds" as major
source of the emission. We find that the NIR lightcurves can be well fitted
either with or without lanthanide-rich ejecta. Our limits on the ejecta mass
together with estimated rates directly confirm earlier purely theoretical or
indirect observational conclusions that double neutron star mergers are indeed
a major site of cosmic nucleosynthesis. If the ejecta mass was {\em typical},
NSMs can easily produce {\em all} of the estimated Galactic r-process matter,
and --depending on the real rate-- potentially even more. This could be a hint
that the event ejected a particularly large amount of mass, maybe due to a
substantial difference between the component masses. This would be compatible
with the mass limits obtained from the GW-observation. The recent observations
suggests that NSMs are responsible for a broad range of r-process nuclei and
that they are at least a major, but likely the dominant r-process site in the
Universe.Comment: 11 pages, 8 figures; accepted for A \&
Supernovae versus Neutron Star Mergers as the Major r-Process Sources
I show that recent observations of r-process abundances in metal-poor stars
are difficult to explain if neutron star mergers (NSMs) are the major r-process
sources. In contrast, such observations and meteoritic data on Hf182 and I129
in the early solar system support a self-consistent picture of r-process
enrichment by supernovae (SNe). While further theoretical studies of r-process
production and enrichment are needed for both SNe and NSMs, I emphasize two
possible direct observational tests of the SN r-process model: gamma rays from
decay of r-process nuclei in SN remnants and surface contamination of the
companion by SN r-process ejecta in binaries.Comment: 5 pages, to appear in ApJ
Neutrinos, Weak Interactions, and r-process Nucleosynthesis
Two of the key issues in understanding the neutron-to-proton ratio in a
core-collapse supernova are discussed. One of these is the behavior of the
neutrino-nucleon cross sections as supernova energies. The other issue is the
many-body properties of the neutrino gas near the core when both one- and
two-body interaction terms are included.Comment: To be published in the Proceedings of "International Symposium on
Structure of Exotic Nuclei and Nuclear Forces (SENUF 06)", March 2006, Tokyo,
Japa
Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052
First results are reported of a new abundance study of neutron-capture
elements in the ultra-metal-poor (UMP; [Fe/H] = -3.1) halo field giant star CS
22892-052. Using new high resolution, high signal-to-noise spectra, abundances
of more than 30 neutron-capture elements (Z>30) have been determined. Six
elements in the 40<Z<56 domain (Nb, Ru, Rh, Pd, Ag and Cd) have been detected
for the first time in a UMP star. Abundances are also derived for three of the
heaviest stable elements (Os, Ir, and Pb). A second transition of thorium,
Th{4086}, confirms the abundance deduced from the standard Th{4019} line, and
an upper limit to the abundance of uranium is established from the absence of
the U{3859} line. As found in previous studies, the abundances of the heavier
(Z>=56) stable neutron-capture elements in CS 22892-052 match well the scaled
solar system r-process abundance distribution. From the observed Th abundance,
an average age of ~= 16 +/- 4 Gyr is derived for cs22892-052, consistent with
the lower age limit of ~= 11 Gyr derived from the upper limit on the U
abundance. The concordance of scaled solar r-process and CS 22892-052
abundances breaks down for the lighter neutron-capture elements, supporting
previous suggestions that different r-process production sites are responsible
for lighter and heavier neutron-capture elements.Comment: To be published in the Astrophysical Journal Letter
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