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Search Results: 1 - 10 of 405633 matches for " Gregory M. Loeb "
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Eavesdropping on Plant Volatiles by a Specialist Moth: Significance of Ratio and Concentration
Dong H. Cha,Charles E. Linn Jr.,Peter E. A. Teal,Aijun Zhang,Wendell L. Roelofs,Gregory M. Loeb
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0017033
Abstract: We investigated the role that the ratio and concentration of ubiquitous plant volatiles play in providing host specificity for the diet specialist grape berry moth Paralobesia viteana (Clemens) in the process of locating its primary host plant Vitis sp. In the first flight tunnel experiment, using a previously identified attractive blend with seven common but essential components (“optimized blend”), we found that doubling the amount of six compounds singly [(E)- & (Z)-linalool oxides, nonanal, decanal, β-caryophyllene, or germacrene-D], while keeping the concentration of other compounds constant, significantly reduced female attraction (average 76% full and 59% partial upwind flight reduction) to the synthetic blends. However, doubling (E)-4,8-dimethyl 1,3,7-nonatriene had no effect on female response. In the second experiment, we manipulated the volatile profile more naturally by exposing clonal grapevines to Japanese beetle feeding. In the flight tunnel, foliar damage significantly reduced female landing on grape shoots by 72% and full upwind flight by 24%. The reduction was associated with two changes: (1) more than a two-fold increase in total amount of the seven essential volatile compounds, and (2) changes in their relative ratios. Compared to the optimized blend, synthetic blends mimicking the volatile ratio emitted by damaged grapevines resulted in an average of 87% and 32% reduction in full and partial upwind orientation, respectively, and the level of reduction was similar at both high and low doses. Taken together, these results demonstrate that the specificity of a ubiquitous volatile blend is determined, in part, by the ratio of key volatile compounds for this diet specialist. However, P. viteana was also able to accommodate significant variation in the ratio of some compounds as well as the concentration of the overall mixture. Such plasticity may be critical for phytophagous insects to successfully eavesdrop on variable host plant volatile signals.
Measuring the Redshift of Reionization with a Modest Array of Low-Frequency Dipoles
Jonathan M. Bittner,Abraham Loeb
Physics , 2010, DOI: 10.1088/1475-7516/2011/04/038
Abstract: The designs of the first generation of cosmological 21-cm observatories are split between single dipole experiments which integrate over a large patch of sky in order to find the global (spectral) signature of reionization, and interferometers with arcminute-scale angular resolution whose goal is to measure the 3D power spectrum of ionized regions during reionization. We examine whether intermediate scale instruments with complete Fourier (uv) coverage are capable of placing new constraints on reionization. We find that even without using a full power spectrum analysis, the global redshift of reionization, z_reion, can in principle be measured from the variance in the 21-cm signal among multiple beams as a function of frequency at a roughly 1 degree angular scale. At this scale, the beam-to-beam variance in the differential brightness temperature peaks when the average neutral fraction was around 50%, providing a convenient flag of z_reion. We choose a low angular resolution of order 1 degree to exploit the physical size of the ionized regions and maximize the signal-to-noise ratio. Thermal noise, foregrounds, and instrumental effects should also be manageable at this angular scale, as long as the uv coverage is complete within the compact core required for low-resolution imaging. For example, we find that z_reion can potentially be detected to within a redshift uncertainty of less than around 1 in around 500 hours of integration on the existing MWA prototype (with only 32x16 dipoles), operating at an angular resolution of around 1 degree and a spectral resolution of 2.4 MHz.
On Poynting-Flux-Driven Bubbles and Shocks Around Merging Neutron Star Binaries
M. V. Medvedev,A. Loeb
Physics , 2012, DOI: 10.1093/mnras/stt366
Abstract: Merging binaries of compact relativistic objects (neutron stars and black holes) are thought to be progenitors of short gamma-ray bursts and sources of gravitational waves, hence their study is of great importance for astrophysics. Because of the strong magnetic field of one or both binary members and high orbital frequencies, these binaries are strong sources of energy in the form of Poynting flux (e.g., magnetic-field-dominated outflows, relativistic leptonic winds, electromagnetic and plasma waves). The steady injection of energy by the binary forms a bubble (or a cavity) filled with matter with the relativistic equation of state, which pushes on the surrounding plasma and can drive a shock wave in it. Unlike the Sedov-von Neumann-Taylor blast wave solution for a point-like explosion, the shock wave here is continuously driven by the ever-increasing pressure inside the bubble. We calculate from the first principles the dynamics and evolution of the bubble and the shock surrounding it and predict that such systems can be observed as radio sources a few hours before and after the merger. At much later times, the shock is expected to settle onto the Sedov-von Neumann-Taylor solution, thus resembling an explosion.
Dynamics of Astrophysical Bubbles and Bubble-Driven Shocks: Basic Theory, Analytical Solutions and Observational Signatures
M. V. Medvedev,A. Loeb
Physics , 2012, DOI: 10.1088/0004-637X/768/2/113
Abstract: Bubbles in the interstellar medium are produced by astrophysical sources, which continuously or explosively deposit large amount of energy into the ambient medium. These expanding bubbles can drive shocks in front of them, which dynamics is markedly different from the widely used Sedov-von Neumann-Taylor blast wave solution. Here we present the theory of a bubble-driven shock and show how its properties and evolution are determined by the temporal history of the source energy output, generally referred to as the source luminosity law, $L(t)$. In particular, we find the analytical solutions for a driven shock in two cases: the self-similar scaling $L\propto (t/t_s)^p$ law (with $p$ and $t_s$ being constants) and the finite activity time case, $L\propto (1-t/t_s)^{-p}$. The latter with $p>0$ describes a finite-time-singular behavior, which is relevant to a wide variety of systems with explosive-type energy release. For both luminosity laws, we derived the conditions needed for the driven shock to exist and predict the shock observational signatures. Our results can be relevant to stellar systems with strong winds, merging neutron star/magnetar/black hole systems, and massive stars evolving to supernovae explosions.
The imprint of the relative velocity between baryons and dark matter on the 21-cm signal from reionization
Jonathan M. Bittner,Abraham Loeb
Physics , 2011,
Abstract: The post-recombination streaming of baryons through dark matter keeps baryons out of low mass (<10^6 solar masses) halos coherently on scales of a few comoving Mpc. It has been argued that this will have a large impact on the 21-cm signal before and after reionization, as it raises the minimum mass required to form ionizing sources. Using a semi-numerical code, we show that the impact of the baryon streaming effect on the 21-cm signal during reionization (redshifts z approximately 7-20) depends strongly on the cooling scenario assumed for star formation, and the corresponding virial temperature or mass at which stars form. For the canonical case of atomic hydrogen cooling at 10^4 Kelvin, the minimum mass for star formation is well above the mass of halos that are affected by the baryon streaming and there are no major changes to existing predictions. For the case of molecular hydrogen cooling, we find that reionization is delayed by a change in redshift of approximately 2 and that more relative power is found in large modes at a given ionization fraction. However, the delay in reionization is degenerate with astrophysical assumptions, such as the production rate of UV photons by stars.
Complex and CR-structures on compact Lie groups associated to Abelian actions
J. -J. Loeb,M. Manjarin,M. Nicolau
Mathematics , 2006,
Abstract: It was shown by Samelson and Wang that each compact Lie group K of even dimension admits left-invariant complex structures. When K has odd dimension it admits a left-invariant CR-structure of maximal dimension. This has been proved recently by Charbonnel and Khalgui who have also given a complete algebraic description of these structures. In this article we present an alternative and more geometric construction of this type of invariant structures on a compact Lie group K when it is semisimple. We prove that each left-invariant complex structure, or each CR-structure of maximal dimension with a transverse CR-action by R, is induced by a holomorphic C^l action on a quasi-projective manifold X naturally associated to K. We then show that X admits more general Abelian actions, also inducing complex or CR structures on K which are generically non-invariant.
Early Cosmic Formation of Massive Black Holes
M. Umemura,A. Loeb,E. L. Turner
Physics , 1993, DOI: 10.1086/173499
Abstract: The evolution of nonlinear density fluctuations around the Jeans mass shortly after cosmological recombination is analyzed using a 3D hydrodynamics/dark--matter code. The Cosmic Background Radiation (CBR) exerts Compton friction on free electrons due to peculiar velocities. The dynamics therefore depends strongly on the gas ionization history. Under a variety of ionization conditions and in systems with or without non-baryonic components, the baryons lose angular momentum efficiently and collapse to form a compact optically--thick object which would probably quickly evolve into a massive black hole. Attention is concentrated on elucidating some of the novel physical effects in early cosmological collapses, but ways in which more realistic calculations might be made and in which the scenario could be incorporated into a more complete cosmogonic model are discussed.
Recoiled star clusters in the Milky Way halo: N-body simulations and a candidate search through SDSS
Ryan M. O'Leary,Abraham Loeb
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20078.x
Abstract: During the formation of the Milky Way, > 100 central black holes (BHs) may have been ejected from their small host galaxies as a result of asymmetric gravitational wave emission. We previously showed that many of these BHs are surrounded by a compact cluster of stars that remained bound to the BH during the ejection process. In this paper, we perform long term N-body simulations of these star clusters to determine the distribution of stars in these clusters today. These numerical simulations, reconciled with our Fokker-Planck simulations, show that stellar density profile follows a power-law with slope ~ -2.15, and show that large angle scattering and tidal disruptions remove 20 - 90% of the stars by ~10^10 yr. We then analyze the photometric and spectroscopic properties of recoiled clusters accounting for the small number of stars in the clusters. We use our results to perform a systematic search for candidates in the Sloan Digital Sky Survey. We find no spectroscopic candidates, in agreement with our expectations from the completeness of the survey. Using generic photometric models of present day clusters we identify ~100 recoiling cluster candidates. Follow-up spectroscopy would be able to determine the nature of these candidates.
The combinatorics of Mancala-type games: Ayo, Tchoukaitlon, and 1/pi
Duane M. Broline,Daniel E. Loeb
Mathematics , 1995,
Abstract: Certain endgame considerations in the two-player Nigerian Mancala-type game Ayo can be identified with the problem of finding winning positions in the solitaire game Tchoukaitlon. The periodicity of the pit occupancies in $s$ stone winning positions is determined. Given $n$ pits, the number of stones in a winning position is found to be asymptotically bounded by $n^{2}/\pi$.
A Rare Case of Thymic Carcinoma  [PDF]
Farshad Anvari, Arnold M. Schwartz, Gregory Trachiotis
Surgical Science (SS) , 2011, DOI: 10.4236/ss.2011.23030
Abstract: Thymic carcinomas are unusual tumors of the thymus gland. Basaloid carcinoma, an unusal epithelioid varient of a thymic carcinoma, is a rare histopathological subtype, and is not well charecterized in the literature. We present the anatomical and histogical features of a basoloid thymic carcinoma, and discuss current diagnosis and imaging strategies, as well as the operative and oncologic care of this type of thymmic carcinoma. Basaloid carcinomas of the thymus after complete surgical resection and adjuvant therapy genarally have a favorable long-term prognosis.
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