2,531 research outputs found
Environment assisted electron capture
Electron capture by {\it isolated} atoms and ions proceeds by
photorecombination. In this process a species captures a free electron by
emitting a photon which carries away the excess energy. It is shown here that
in the presence of an {\it environment} a competing non-radiative electron
capture process can take place due to long range electron correlation. In this
interatomic (intermolecular) process the excess energy is transferred to
neighboring species. The asymptotic expression for the cross section of this
process is derived. We demonstrate by explicit examples that under realizable
conditions the cross section of this interatomic process can clearly dominate
that of photorecombination
Double Neutron Star Systems and Natal Neutron Star Kicks
We study the four double neutron star systems found in the Galactic disk in
terms of the orbital characteristics of their immediate progenitors and the
natal kicks imparted to neutron stars. Analysis of the effect of the second
supernova explosion on the orbital dynamics, combined with recent results from
simulations of rapid accretion onto neutron stars lead us to conclude that the
observed systems could not have been formed had the explosion been symmetric.
Their formation becomes possible if kicks are imparted to the radio-pulsar
companions at birth. We identify the constraints imposed on the immediate
progenitors of the observed double neutron stars and calculate the ranges
within which their binary characteristics (orbital separations and masses of
the exploding stars) are restricted. We also study the dependence of these
limits on the magnitude of the kick velocity and the time elapsed since the
second explosion. For each of the double neutron stars, we derive a minimum
kick magnitude required for their formation, and for the two systems in close
orbits we find it to exceed 200km/s. Lower limits are also set to the
center-of-mass velocities of double neutron stars, and we find them to be
consistent with the current proper motion observations.Comment: 25 pages, 6 figs (9 parts), 4 tables, AASTeX, Accepted in Ap
Using binary stars to bound the mass of the graviton
Interacting white dwarf binary star systems, including helium cataclysmic
variable (HeCV) systems, are expected to be strong sources of gravitational
radiation, and should be detectable by proposed space-based laser
interferometer gravitational wave observatories such as LISA. Several HeCV star
systems are presently known and can be studied optically, which will allow
electromagnetic and gravitational wave observations to be correlated.
Comparisons of the phases of a gravitational wave signal and the orbital light
curve from an interacting binary white dwarf star system can be used to bound
the mass of the graviton. Observations of typical HeCV systems by LISA could
potentially yield an upper bound on the inverse mass of the graviton as strong
as km (
eV), more than two orders of magnitude better than present solar system derived
bounds.Comment: 21 pages plus 4 figures; ReVTe
Collateral and Debt Maturity Choice. A Signaling Model
This paper derives optimal loan policies under asymmetric information where banks offer loan contracts of long and short duration, backed or unbacked with collateral. The main novelty of the paper is that it analyzes a setting in which high quality firms use collateral as a complementary device along with debt maturity to signal their superiority. The least-cost signaling equilibrium depends on the relative costs of the signaling devices, the difference in firm quality and the proportion of good firms in the market. Model simulations suggest a non-monotonic relationship between firm quality and debt maturity, in which high quality firms have both long-term secured debt and short-term secured or non-secured debt.
Can rates of ocean primary production and biological carbon export be related through their probability distributions?
© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Global Biogeochemical Cycles 32 (2018): 954-970, doi:10.1029/2017GB005797.We describe the basis of a theory for interpreting measurements of two key biogeochemical fluxesâprimary production by phytoplankton (p, ÎŒg C · Lâ1 · dayâ1) and biological carbon export from the surface ocean by sinking particles (f, mg C · mâ2 · dayâ1)âin terms of their probability distributions. Given that p and f are mechanistically linked but variable and effectively measured on different scales, we hypothesize that a quantitative relationship emerges between collections of the two measurements. Motivated by the many subprocesses driving production and export, we take as a null model that largeâscale distributions of p and f are lognormal. We then show that compilations of p and f measurements are consistent with this hypothesis. The compilation of p measurements is extensive enough to subregion by biome, basin, depth, or season; these subsets are also well described by lognormals, whose logâmoments sort predictably. Informed by the lognormality of both p and f we infer a statistical scaling relationship between the two quantities and derive a linear relationship between the logâmoments of their distributions. We find agreement between two independent estimates of the slope and intercept of this line and show that the distribution of f measurements is consistent with predictions made from the moments of the p distribution. These results illustrate the utility of a distributional approach to biogeochemical fluxes. We close by describing potential uses and challenges for the further development of such an approach.National Science Foundation Grant Number: OCE-1315201;
Simons Foundation Grant Numbers: 329108, 553242;
National Aeronautics and Space Administration Grant Numbers: NNX16AR47G, NNX16AR49
Modeling resilience and sustainability in ancient agricultural systems
The reasons why people adopt unsustainable agricultural practices, and the ultimate environmental implications of those practices, remain incompletely understood in the present world. Archaeology, however, offers unique datasets on coincident cultural and ecological change, and their social and environmental effects. This article applies concepts derived from ecological resilience thinking to assess the sustainability of agricultural practices as a result of long-term interactions between political, economic, and environmental systems. Using the urban center of Gordion, in central Turkey, as a case study, it is possible to identify mismatched social and ecological processes on temporal, spatial, and organizational scales, which help to resolve thresholds of resilience. Results of this analysis implicate temporal and spatial mismatches as a cause for local environmental degradation, and increasing extralocal economic pressures as an ultimate cause for the adoption of unsustainable land-use practices. This analysis suggests that a research approach that integrates environmental archaeology with a resilience perspective has considerable potential for explicating regional patterns of agricultural change and environmental degradation in the past
Pulsar Kicks and Spin Tilts in the Close Double Neutron Stars PSR J0737-3039, PSR B1534+12 and PSR B1913+16
In view of the recent measurement of the scintillation velocity for PSR
J0737-3039, we examine the complete set of constraints imposed on the pulsar B
natal kicks (magnitude and orientation) and predict the most favorable pulsar
kick velocity and spin tilt for both isotropic and polar kicks. Our analysis
takes into account both currently unknown parameters: the orientation of the
orbital plane on the sky (Omega) and the radial component of the systemic
velocity (V_r). Assuming that the system's peculiar velocity is entirely due to
the second supernova explosion, we find that the system may have crossed the
Galactic plane multiple times since the birth of the second neutron star and
that the post-supernova peculiar velocity could have been as high as 1200km/s.
We also confirm the absolute lower and upper limits on the physical parameters
derived in our earlier study. For specific combinations of the two unknown
parameters Omega and V_r, however, we find much tighter constraints on the
pre-supernova binary configuration and natal kicks imparted to pulsar B, as
well as on the age of system. Once Omega is measured in the coming year, it
will be straightforward to use the results presented here to further constrain
the natal kicks and the spin-tilt predictions. We complete our comprehensive
study and derive similar constraints and spin-tilt predictions for PSR
B1534+12, where the only free parameter is V_r. Lastly, for PSR B1913+16, we
update the progenitor and kick constraints using the measured pulsar spin tilt
and allowing for Roche-lobe overflow from the progenitor of the pulsar
companion.Comment: Replaced Fig. 16 with corrected version. See ApJ 616, p. 414 for
high-resolution figures and notes added in proo
From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
The 10th recorded outburst of the recurrent eclipsing nova USco was observed
simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9
after outburst. Two full passages of the companion in front of the nova ejecta
were observed, witnessing the reformation of the accretion disk. On day 22.9,
we observed smooth eclipses in UV and optical but deep dips in the X-ray light
curve which disappeared by day 34.9, then yielding clean eclipses in all bands.
X-ray dips can be caused by clumpy absorbing material that intersects the line
of sight while moving along highly elliptical trajectories. Cold material from
the companion could explain the absence of dips in UV and optical light. The
disappearance of X-ray dips before day 34.9 implies significant progress in the
formation of the disk. The X-ray spectra contain photospheric continuum
emission plus strong emission lines, but no clear absorption lines. Both
continuum and emission lines in the X-ray spectra indicate a temperature
increase from day 22.9 to day 34.9. We find clear evidence in the spectra and
light curves for Thompson scattering of the photospheric emission from the
white dwarf. Photospheric absorption lines can be smeared out during scattering
in a plasma of fast electrons. We also find spectral signatures of resonant
line scattering that lead to the observation of the strong emission lines.
Their dominance could be a general phenomenon in high-inclination systems such
as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
N-Methylimidazole Promotes The Reaction Of Homophthalic Anhydride With Imines
The addition of N-methylimidazole (NMI) to the reaction of homophthalic anhydride with imines such as pyridine-3-carboxaldehyde-N-trifluoroethylimine (9) reduces the amount of elimination byproduct and improves the yield of the formal cycloadduct, tetrahydroisoquinolonic carboxylate 10. Carboxanilides of such compounds are of interest as potential antimalarial agents. A mechanism that rationalizes the role of NMI is proposed, and a gram-scale procedure for the synthesis and resolution of 10 is also described
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