767 research outputs found
Neutron-powered precursors of kilonovae
The merger of binary neutron stars (NSs) ejects a small quantity of neutron
rich matter, the radioactive decay of which powers a day to week long thermal
transient known as a kilonova. Most of the ejecta remains sufficiently dense
during its expansion that all neutrons are captured into nuclei during the
r-process. However, recent general relativistic merger simulations by Bauswein
and collaborators show that a small fraction of the ejected mass (a few per
cent, or ~1e-4 Msun) expands sufficiently rapidly for most neutrons to avoid
capture. This matter originates from the shocked-heated interface between the
merging NSs. Here we show that the beta-decay of these free neutrons in the
outermost ejecta powers a `precursor' to the main kilonova emission, which
peaks on a timescale of a few hours following merger at U-band magnitude ~22
(for an assumed distance of 200 Mpc). The high luminosity and blue colors of
the neutron precursor render it a potentially important counterpart to the
gravitational wave source, that may encode valuable information on the
properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS
equation of state. Future work is necessary to assess the robustness of the
fast moving ejecta and the survival of free neutrons in the face of neutrino
absorptions, although the precursor properties are robust to a moderate amount
of leptonization. Our results provide additional motivation for short latency
gravitational wave triggers and rapid follow-up searches with sensitive ground
based telescopes.Comment: 6 pages, 5 figures, accepted to MNRAS main journa
Monte Carlo Neutrino Transport Through Remnant Disks from Neutron Star Mergers
We present Sedonu, a new open source, steady-state, special relativistic
Monte Carlo (MC) neutrino transport code, available at
bitbucket.org/srichers/sedonu. The code calculates the energy- and
angle-dependent neutrino distribution function on fluid backgrounds of any
number of spatial dimensions, calculates the rates of change of fluid internal
energy and electron fraction, and solves for the equilibrium fluid temperature
and electron fraction. We apply this method to snapshots from two-dimensional
simulations of accretion disks left behind by binary neutron star mergers,
varying the input physics and comparing to the results obtained with a leakage
scheme for the case of a central black hole and a central hypermassive neutron
star. Neutrinos are guided away from the densest regions of the disk and escape
preferentially around 45 degrees from the equatorial plane. Neutrino heating is
strengthened by MC transport a few scale heights above the disk midplane near
the innermost stable circular orbit, potentially leading to a stronger
neutrino-driven wind. Neutrino cooling in the dense midplane of the disk is
stronger when using MC transport, leading to a globally higher cooling rate by
a factor of a few and a larger leptonization rate by an order of magnitude. We
calculate neutrino pair annihilation rates and estimate that an energy of
2.8e46 erg is deposited within 45 degrees of the symmetry axis over 300 ms when
a central BH is present. Similarly, 1.9e48 erg is deposited over 3 s when an
HMNS sits at the center, but neither estimate is likely to be sufficient to
drive a GRB jet.Comment: 23 pages, 16 figures, Accepted to The Astrophysical Journa
Modeling the Diversity of Type Ia Supernova Explosions
Type Ia supernovae (SNe Ia) are a prime tool in observational cosmology. A
relation between their peak luminosities and the shapes of their light curves
allows to infer their intrinsic luminosities and to use them as distance
indicators. This relation has been established empirically. However, a
theoretical understanding is necessary in order to get a handle on the
systematics in SN Ia cosmology. Here, a model reproducing the observed
diversity of normal SNe Ia is presented. The challenge in the numerical
implementation arises from the vast range of scales involved in the physical
mechanism. Simulating the supernova on scales of the exploding white dwarf
requires specific models of the microphysics involved in the thermonuclear
combustion process. Such techniques are discussed and results of simulations
are presented.Comment: 6 pages, ASTRONUM-2009 "Numerical Modeling of Space Plasma Flows",
Chamonix, France, July 2009, to appear in ASP Conf. Pro
Nebular models of sub-chandrasekhar mass type ia supernovae: Clues to the origin of ca-rich transients
We use non-local thermal equilibrium radiative transport modeling to examine observational signatures of sub- Chandrasekhar mass double detonation explosions in the nebular phase. Results range from spectra that look like typical and subluminous Type Ia supernovae (SNe) for higher mass progenitors to spectra that look like Ca-rich transients for lower mass progenitors. This ignition mechanism produces an inherent relationship between emission features and the progenitor mass as the ratio of the nebular [Ca II]/[Fe III] emission lines increases with decreasing white dwarf mass. Examining the [Ca II]/[Fe III] nebular line ratio in a sample of observed SNe we find further evidence for the two distinct classes of SNe Ia identified in Polin et al. by their relationship between Si II velocity and B-band magnitude, both at time of peak brightness. This suggests that SNe Ia arise from more than one progenitor channel, and provides an empirical method for classifying events based on their physical origin. Furthermore, we provide insight to the mysterious origin of Ca-rich transients. Low-mass double detonation models with only a small mass fraction of Ca (1%) produce nebular spectra that cool primarily through forbidden [Ca II] emission
Signatures of hypermassive neutron star lifetimes on r-process nucleosynthesis in the disk ejecta from neutron star mergers
We investigate the nucleosynthesis of heavy elements in the winds ejected by
accretion disks formed in neutron star mergers. We compute the element
formation in disk outflows from hypermassive neutron star (HMNS) remnants of
variable lifetime, including the effect of angular momentum transport in the
disk evolution. We employ long-term axisymmetric hydrodynamic disk simulations
to model the ejecta, and compute r-process nucleosynthesis with tracer
particles using a nuclear reaction network containing species. We
find that the previously known strong correlation between HMNS lifetime,
ejected mass, and average electron fraction in the outflow is directly related
to the amount of neutrino irradiation on the disk, which dominates mass
ejection at early times in the form of a neutrino-driven wind. Production of
lanthanides and actinides saturates at short HMNS lifetimes ( ms),
with additional ejecta contributing to a blue optical kilonova component for
longer-lived HMNSs. We find good agreement between the abundances from the disk
outflow alone and the solar r-process distribution only for short HMNS
lifetimes ( ms). For longer lifetimes, the rare-earth and third
r-process peaks are significantly under-produced compared to the solar pattern,
requiring additional contributions from the dynamical ejecta. The
nucleosynthesis signature from a spinning black hole (BH) can only overlap with
that from a HMNS of moderate lifetime ( ms). Finally, we show that
angular momentum transport not only contributes with a late-time outflow
component, but that it also enhances the neutrino-driven component by moving
material to shallower regions of the gravitational potential, in addition to
providing additional heating.Comment: 18 pages, 11 figures, published version with small change
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