545 research outputs found
A line-binned treatment of opacities for the spectra and light curves from neutron star mergers
The electromagnetic observations of GW170817 were able to dramatically
increase our understanding of neutron star mergers beyond what we learned from
gravitational waves alone. These observations provided insight on all aspects
of the merger from the nature of the gamma-ray burst to the characteristics of
the ejected material. The ejecta of neutron star mergers are expected to
produce such electromagnetic transients, called kilonovae or macronovae.
Characteristics of the ejecta include large velocity gradients, relative to
supernovae, and the presence of heavy -process elements, which pose
significant challenges to the accurate calculation of radiative opacities and
radiation transport. For example, these opacities include a dense forest of
bound-bound features arising from near-neutral lanthanide and actinide
elements. Here we investigate the use of fine-structure, line-binned opacities
that preserve the integral of the opacity over frequency. Advantages of this
area-preserving approach over the traditional expansion-opacity formalism
include the ability to pre-calculate opacity tables that are independent of the
type of hydrodynamic expansion and that eliminate the computational expense of
calculating opacities within radiation-transport simulations. Tabular opacities
are generated for all 14 lanthanides as well as a representative actinide
element, uranium. We demonstrate that spectral simulations produced with the
line-binned opacities agree well with results produced with the more accurate
continuous Monte Carlo Sobolev approach, as well as with the commonly used
expansion-opacity formalism. Additional investigations illustrate the
convergence of opacity with respect to the number of included lines, and
elucidate sensitivities to different atomic physics approximations, such as
fully and semi-relativistic approaches.Comment: 27 pages, 22 figures. arXiv admin note: text overlap with
arXiv:1702.0299
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 \&
Composition Effects on Kilonova Spectra and Light Curves: I
The merger of neutron star binaries is believed to eject a wide range of
heavy elements into the universe. By observing the emission from this ejecta,
scientists can probe the ejecta properties (mass, velocity and composition
distributions). The emission (a.k.a. kilonova) is powered by the radioactive
decay of the heavy isotopes produced in the merger and this emission is
reprocessed by atomic opacities to optical and infra-red wavelengths.
Understanding the ejecta properties requires calculating the dependence of this
emission on these opacities. The strong lines in the optical and infra-red in
lanthanide opacities have been shown to significantly alter the light-curves
and spectra in these wavelength bands, arguing that the emission in these
wavelengths can probe the composition of this ejecta. Here we study variations
in the kilonova emission by varying individual lanthanide (and the actinide
uranium) concentrations in the ejecta. The broad forest of lanthanide lines
makes it difficult to determine the exact fraction of individual lanthanides.
Nd is an exception. Its opacities above 1 micron are higher than other
lanthanides and observations of kilonovae can potentially probe increased
abundances of Nd. Similarly, at early times when the ejecta is still hot (first
day), the U opacity is strong in the 0.2-1 micron wavelength range and kilonova
observations may also be able to constrain these abundances
Large droplet impact on water layers
The impact of large droplets onto an otherwise undisturbed layer of water is considered. The work, which is motivated primarily with regard to aircraft icing, is to try and help understand the role of splashing on the formation of ice on a wing, in particular for large droplets where splash appears, to have a significant effect. Analytical and numerical approaches are used to investigate a single droplet impact onto a water layer. The flow for small times after impact is determined analytically, for both direct and oblique impacts. The impact is also examined numerically using the volume of fluid (VOF) method. At small times there are promising comparisons between the numerical results, the analytical solution and experimental work capturing the ejector sheet. At larger times there is qualitative agreement with experiments and related simulations. Various cases are considered, varying the droplet size to layer depth ratio, including surface roughness, droplet distortion and air effects. The amount of fluid splashed by such an impact is examined and is found to increase with droplet size and to be significantly influenced by surface roughness. The makeup of the splash is also considered, tracking the incoming fluid, and the splash is found to consist mostly of fluid originating in the layer
Halted-Pendulum Relaxation: Application to White Dwarf Binary Initial Data
Studying compact star binaries and their mergers is integral to modern
astrophysics. In particular, binary white dwarfs are associated with Type Ia
supernovae, used as standard candles to measure the expansion of the Universe.
Today, compact-star mergers are typically studied via state-of-the-art
computational fluid dynamics codes. One such numerical techniques, Smoothed
Particle Hydrodynamics (SPH), is frequently chosen for its excellent mass,
energy, and momentum conservation. Furthermore, the natural treatment of vacuum
and the ability to represent highly irregular morphologies make SPH an
excellent tool for the numerical study of compact-star binaries and mergers.
However, for many scenarios, including binary systems, the outcome simulations
are only as accurate as the initial conditions. For SPH, it is essential to
ensure that particles are distributed semi-regularly, correctly representing
the initial density profile. Additionally, particle noise in the form of
high-frequency local motion and low-frequency global dynamics must be damped
out. Damping the latter can be as computationally intensive as the actual
simulation. Here, we discuss a new and straightforward relaxation method,
Halted-Pendulum Relaxation (HPR), to remove the global oscillation modes of SPH
particle configurations. In combination with effective external potentials
representing gravitational and orbital forces, we show that HPR has an
excellent performance in efficiently relaxing SPH particles to the desired
density distribution and removing global oscillation modes. We compare the
method to frequently used relaxation approaches such as gravitational glass,
increased artificial viscosity, and Weighted Voronoi Tesselations, and test it
on a white dwarf binary model at its Roche lobe overflow limit
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