145 research outputs found
Introduction
How do our interpretations link up to our causal claims? How does attention to causality refine our interpretations? In one sense, it is strange that we find ourselves asking these ques-tions. After all, interpretive researchers routinely find them-selves using causal language, and scholars oriented toward establishing causal claims also spend much time interpreting actorsâ motivations and beliefs. If political scientists ipso facto do both, whatâs the big deal
Heating in the Accreted Neutron Star Ocean: Implications for Superburst Ignition
We perform a self-consistent calculation of the thermal structure in the
crust of a superbursting neutron star. In particular, we follow the
nucleosynthetic evolution of an accreted fluid element from its deposition into
the atmosphere down to a depth where the electron Fermi energy is 20 MeV. We
include temperature-dependent continuum electron capture rates and realistic
sources of heat loss by thermal neutrino emission from the crust and core. We
show that, in contrast to previous calculations, electron captures to excited
states and subsequent gamma-emission significantly reduce the local heat loss
due to weak-interaction neutrinos. Depending on the initial composition these
reactions release up to a factor of 10 times more heat at densities < 10^{11}
g/cc than obtained previously. This heating reduces the ignition depth of
superbursts. In particular, it reduces the discrepancy noted by Cumming et al.
between the temperatures needed for unstable 12C ignition on timescales
consistent with observations and the reduction in crust temperature from Cooper
pair neutrino emission.Comment: 10 pages, 11 figures, the Astrophysical Journal, in press (scheduled
for v. 662). Revised from v1 in response to referee's comment
Recommended from our members
Ultralow-threshold, continuous-wave upconverting lasing from subwavelength plasmons.
Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb3+/Er3+-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70âWâcm-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light
Intercellular adhesion molecule-1 expression in human endometrium: implications for long term progestin only contraception
BACKGROUND: Neutrophils infiltrate the endometrium pre-menstrually and after long-term progestin only-contraceptive (LTPOC) treatment. Trafficking of neutrophils involves endothelial cell-expressed intercellular adhesion molecule (ICAM-1). Previous studies observed that ICAM-1 was immunolocalized to the endothelium of endometrial specimens across the menstrual cycle, but disagreed as to whether extra-endothelial cell types express ICAM-1 and whether ICAM-1 expression varies across the menstrual cycle. METHODS: Endometrial biopsies were obtained from women across the menstrual cycle and from those on LTPOC treatment (either Mirena or Norplant). The biopsies were formalin-fixed and paraffin-embedded with subsequent immunohistochemical staining for ICAM-1. RESULTS: The current study found prominent ICAM-1 staining in the endometrial endothelium that was of equivalent intensity in different blood vessel types irrespective of the steroidal or inflammatory endometrial milieu across the menstrual cycle and during LTPOC therapy. Unlike the endothelial cells, the glands were negative and the stromal cells were weakly positive for ICAM immunostaining. CONCLUSION: The results of the current study suggest that altered expression of ICAM-1 by endothelial cells does not account for the influx of neutrophils into the premenstrual and LTPOC-derived endometrium. Such neutrophil infiltration may depend on altered expression of neutrophil chemoattractants
Russia in a changing climate
Climate change will shape the future of Russia, and vice versa, regardless of who rules in the Kremlin. The world\u27s largest country is warming faster than Earth as a whole, occupies more than half the Arctic Ocean coastline, and is waging a carbon-intensive war while increasingly isolated from the international community and its efforts to reduce greenhouse gas emissions. Officially, the Russian government argues that, as a major exporter of hydrocarbons, Russia benefits from maintaining global reliance on fossil fuels and from climate change itself, because warming may increase the extent and quality of its arable land, open a new year-round Arctic sea route, and make its harsh climate more livable. Drawing on the collective expertise of a large group of Russia-focused social scientists and a comprehensive literature review, we challenge this narrative. We find that Russia suffers from a variety of impacts due to climate change and is poorly prepared to adapt to these impacts. The literature review reveals that the fates of Russia\u27s hydrocarbon-dependent economy, centralized political system, and climate-impacted population are intertwined and that research is needed on this evolving interrelationship, as global temperatures rise and the international economy decarbonizes in response. This article is categorized under: Policy and Governance \u3e National Climate Change Policy Trans-disciplinary Perspectives \u3e National Reviews Trans-disciplinary Perspectives \u3e Regional Reviews. © 2023 The Authors. WIREs Climate Change published by Wiley Periodicals LLC
Possible Resonances in the 12C + 12C Fusion Rate and Superburst Ignition
Observationally inferred superburst ignition depths are shallower than models
predict. We address this discrepancy by reexamining the superburst trigger
mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic
electron captures trigger superbursts. We find that all electron capture
reactions are thermally stable in accreting neutron star oceans and thus are
not a viable trigger mechanism. Fusion reactions other than 12C + 12C are
infeasible as well since the possible reactants either deplete at much
shallower depths or have prohibitively large Coulomb barriers. Thus we confirm
the proposal of Cumming & Bildsten and Strohmayer & Brown that 12C + 12C
triggers superbursts. We then examine the 12C + 12C fusion rate. The reaction
cross-section is experimentally unknown at astrophysically relevant energies,
but resonances exist in the 12C + 12C system throughout the entire measured
energy range. Thus it is likely, and in fact has been predicted, that a
resonance exists near the Gamow peak energy ~ 1.5 MeV. For such a hypothetical
1.5 MeV resonance, we derive both a fiducial value and upper limit to the
resonance strength and find that such a resonance could decrease the
theoretically predicted superburst ignition depth by up to a factor of 4; in
this case, observationally inferred superburst ignition depths would accord
with model predictions for a range of plausible neutron star parameters. Said
differently, such a resonance would decrease the temperature required for
unstable 12C ignition at a column depth 10^12 g/cm^2 from 6 x 10^8 K to 5 x
10^8 K. Determining the existence of a strong resonance in the Gamow window
requires measurements of the 12C + 12C cross-section down to a center-of-mass
energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.Comment: 13 pages, 4 figures; minor improvements, results and conclusions
unchanged, accepted to Ap
Dense matter with eXTP
In this White Paper we present the potential of the Enhanced X-ray Timing and
Polarimetry (eXTP) mission for determining the nature of dense matter; neutron
star cores host an extreme density regime which cannot be replicated in a
terrestrial laboratory. The tightest statistical constraints on the dense
matter equation of state will come from pulse profile modelling of
accretion-powered pulsars, burst oscillation sources, and rotation-powered
pulsars. Additional constraints will derive from spin measurements, burst
spectra, and properties of the accretion flows in the vicinity of the neutron
star. Under development by an international Consortium led by the Institute of
High Energy Physics of the Chinese Academy of Science, the eXTP mission is
expected to be launched in the mid 2020s.Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019
- âŠ