1,203 research outputs found
Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts
We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts
are cosmological and they are beamed in the form of a conical jet with a large
bulk Lorentz factor . In such a conic beam, the relativistic ejecta
may have a spatial variation in the bulk Lorentz factor and the density
distribution of gamma-ray emitting jet material. An apparent luminosity
function arises because the axis of the cone is randomly oriented with respect
to the observer's line of sight. The width and the shape of the luminosity
function are determined by the ratio of the beam opening angle of the conical
jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and
the jet material density is uniform on the photon emitting jet surface. We
calculate effects of spatial variation of the Lorentz factor and the spatial
density fluctuations within the cone on the luminosity function and the
statistics of gamma-ray bursts. In particular, we focus on the redshift
distribution of the observed gamma-ray bursts. The maximum distance to and the
average redshift of the gamma-ray bursts are strongly affected by the
beaming-induced luminosity function. The bursts with the angle-dependent
Lorentz factor which peaks at the center of the cone have substantially higher
average gamma-ray burst redshifts. When both the jet material density and the
Lorentz factor are inhomogeneous in the conical beam, the average redshift of
the bursts could be 5 times higher than that of the case in which relativistic
jet is completely homogeneous and structureless. Even the simplest models for
the gamma-ray burst jets and their apparent luminosity distributions have a
significant effect on the redshift distribution of the gamma-ray bursts.Comment: 15 pages, 4 figures, submitted to ApJ
"Irregular" Verbs in Korean Revisited
A number of Korean verbs 1 do not follow the general phonological rules in their conjugation. However, the patternedness of their "irregularity" has long been noted by most grammarians. Thus even the earliest analyses set up different "classes" of "irregular" verbs (Choy 1937- 1971, Martin 1954, He 1965- 1972, C-W Kim 1967)2. Furthermore it was well known that many of the irregularities were due to some earlier historical processes. More recently in applying the generative theory, many linguists (C-W Kim 1970, Chag-yun Kim 1971, Lee 1973, Cook 1973) have come to believe that most, if not all, of these "anomalous" verbs are not really exceptions to some fixed rules but that they behave differently because they have different underlying forms. Thus superficially identical forms of "regular" and "irregular" verbs are thought to be a direct result of certain phonological rules which neutralize them in a well-defined environment. This paper purports to review some of the "regular" solutions thus far given and to present my own claim on the underlying representations of the "irregular" verbs and the phonlogical rules required to derive their phonetic representations. Choy(l937-71) gives twelve classes of verbs which show anomaly either in the shape of their stems or in the affixes that are attached to them. I shall group them into five sections in each of which related processes will be discussed
Constraints on the kinematics of the 44Ti ejecta of Cassiopeia A from INTEGRAL/SPI
The medium-lived 44Ti isotope is synthesised by explosive Si-burning in
core-collapse supernovae. It is extremely sensitive to the dynamics of the
explosion and therefore can be used to indirectly probe the explosion scenario.
The young supernova remnant Cassiopeia A is to date the only source of
gamma-ray lines from 44Ti decay. The emission flux has been measured by
CGRO/COMPTEL, BeppoSAX/PDS and INTEGRAL/IBIS. The high-resolution spectrometer
SPI on-board the INTEGRAL satellite can provide spectrometric information about
the emission. The line profiles reflect the kinematics of the 44Ti in
Cassiopeia A and can thus place constraints on its nucleosynthesis and
potentially on the associated explosion process. Using 4 years of data from
INTEGRAL/SPI, we have searched for the gamma-ray signatures from the decay of
the 44Ti isotope. The overwhelming instrumental background noise required an
accurate modelling and a solid assessment of the systematic errors in the
analysis. Due to the strong variability of the instrumental background noise,
it has not been possible to extract the two lines at 67.9 and 78.4keV.
Regarding the high-energy line at 1157.0keV, no significant signal is seen in
the 1140-1170keV band, thereby suggesting that the line signal from Cassiopeia
A is broadened by the Doppler effect. From our spectrum, we derive a ~ 500km/s
lower limit at 2sigma on the expansion velocity of the 44Ti ejecta. Our result
does not allow us to constrain the location of 44Ti since the velocities
involved throughout the remnant, derived from optical and X-ray studies, are
all far above our lower limit.Comment: 7 pages, 8 figures, published in A&
Explosive nucleosynthesis in the neutrino-driven aspherical supernova explosion of a non-rotating 15 star with solar metallicity
We investigate explosive nucleosynthesis in a non-rotating 15 star
with solar metallicity that explodes by a neutrino-heating supernova (SN)
mechanism aided by both standing accretion shock instability (SASI) and
convection. To trigger explosions in our two-dimensional hydrodynamic
simulations, we approximate the neutrino transport with a simple light-bulb
scheme and systematically change the neutrino fluxes emitted from the
protoneutron star. By a post-processing calculation, we evaluate abundances and
masses of the SN ejecta for nuclei with the mass number employing a
large nuclear reaction network. Aspherical abundance distributions, which are
observed in nearby core-collapse SN remnants, are obtained for the non-rotating
spherically-symmetric progenitor, due to the growth of low-mode SASI. Abundance
pattern of the supernova ejecta is similar to that of the solar system for
models whose masses ranges (0.4-0.5) \Ms of the ejecta from the inner region
(\le 10,000\km) of the precollapse core. For the models, the explosion
energies and the \nuc{Ni}{56} masses are and
(0.05-0.06) \Ms, respectively; their estimated baryonic masses of the neutron
star are comparable to the ones observed in neutron-star binaries. These
findings may have little uncertainty because most of the ejecta is composed by
matter that is heated via the shock wave and has relatively definite
abundances. The abundance ratios for Ne, Mg, Si and Fe observed in Cygnus loop
are well reproduced with the SN ejecta from an inner region of the 15\Ms
progenitor.Comment: 15 pages, 1 table, 17 figures, accepted for publication in
Astrophyscal Journa
The Reddening Towards Cassiopeia A's Supernova: Constraining the 56Ni Yield
We present new reddening measurements towards the young supernova remnant
Cassiopeia A, using two techniques not previously applied to this object. Our
observations of the near-infrared [Fe II] 1.257 micron and 1.644 micron lines
show the extinction to be highly variable across the remnant, increasing
towards the west and the south, consistent with previous radio and X-ray
observations. While the absolute value of AV as determined by the [Fe II] lines
is uncertain due to conflicting calculations and observations of their
intrinsic flux ratio, parts of the remnant without previous optical
measurements show comparatively higher reddening. We find AV = 6.2 +/- 0.6 from
the broadband shape of the infrared synchrotron emission of a knot within 13"
of the expansion center. Given this reddening, the apparent faintness of the
nascent supernova, and iron mass constraints from X-ray observations, we
estimate an ejected mass of 56Ni of 0.058 - 0.16 MSun. Taken with gamma-ray
observations of the 44Ti decay chain, this nickel mass is broadly consistent
with the solar 44Ca/56Fe ratio.Comment: 8 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Modeling the growth of stylolites in sedimentary rocks
[1] Stylolites are ubiquitous pressure solution seams found in sedimentary rocks. Their morphology is shown to follow two self-affine regimes. Analyzing the scaling properties of their height over their average direction shows that (1) at small scale, they are self-affine surfaces with a Hurst exponent around 1, and (2) at large scale, they follow another self-affine scaling with Hurst exponent around 0.5. In the present paper, we show theoretically the influence of the main principal stress and the local geometry of the stylolitic interface on the dissolution reaction rate. We compute how it is affected by the deviation between the principal stress axis and the local interface between the rock and the soft material in the stylolite. The free energy entering in the dissolution reaction kinetics is expressed from the surface energy term and via integration from the stress perturbations due to these local misalignments. The resulting model shows the interface evolution at different stress conditions. In the stylolitic case, i.e., when the main principal stress is normal to the interface, two different stabilizing terms dominate at small and large scales which are linked respectively to the surface energy and to the elastic interactions. Integrating the presence of small-scale heterogeneities related to the rock properties of the grains in the model leads to the formulation of a Langevin equation predicting the dynamic evolution of the surface. This equation leads to saturated surfaces obeying the two observed scaling laws. Analytical and numerical analysis of this surface evolution model shows that the crossover length separating both scaling regimes depends directly on the applied far-field stress magnitude. This method gives the basis for the development of a paleostress magnitude marker. We apply the computation of this marker, i.e., the morphological analysis, on a stylolite found in the Dogger limestone layer located in the neighborhood of the ANDRA Underground Research Laboratory at Bure (eastern France). The results are consistent with the two scaling regimes expected, and the practical determination of the major principal paleostress, from the estimation of a crossover length, is illustrated on this example
Exoplanet Diversity in the Era of Space-based Direct Imaging Missions
This whitepaper discusses the diversity of exoplanets that could be detected
by future observations, so that comparative exoplanetology can be performed in
the upcoming era of large space-based flagship missions. The primary focus will
be on characterizing Earth-like worlds around Sun-like stars. However, we will
also be able to characterize companion planets in the system simultaneously.
This will not only provide a contextual picture with regards to our Solar
system, but also presents a unique opportunity to observe size dependent
planetary atmospheres at different orbital distances. We propose a preliminary
scheme based on chemical behavior of gases and condensates in a planet's
atmosphere that classifies them with respect to planetary radius and incident
stellar flux.Comment: A white paper submitted to the National Academy of Sciences Exoplanet
Science Strateg
Four distinct trajectories of tau deposition identified in Alzheimerâs disease
Alzheimerâs Disease Neuroimaging Initiative.Alzheimerâs disease (AD) is characterized by the spread of tau pathology throughout the cerebral cortex. This spreading pattern was thought to be fairly consistent across individuals, although recent work has demonstrated substantial variability in the population with AD. Using tau-positron emission tomography scans from 1,612 individuals, we identified 4 distinct spatiotemporal trajectories of tau pathology, ranging in prevalence from 18 to 33%. We replicated previously described limbic-predominant and medial temporal lobe-sparing patterns, while also discovering posterior and lateral temporal patterns resembling atypical clinical variants of AD. These âsubtypesâ were stable during longitudinal follow-up and were replicated in a separate sample using a different radiotracer. The subtypes presented with distinct demographic and cognitive profiles and differing longitudinal outcomes. Additionally, network diffusion models implied that pathology originates and spreads through distinct corticolimbic networks in the different subtypes. Together, our results suggest that variation in tau pathology is common and systematic, perhaps warranting a re-examination of the notion of âtypical ADâ and a revisiting of tau pathological staging.J.W.V. acknowledges support from the government of Canada through a tri-council Vanier Canada Graduate Doctoral fellowship from the McGill Centre for Integrative Neuroscience and the Healthy Brains, Healthy Lives initiative, and from the National Institutes of Health (NIH) (no. T32MH019112). A.L.Y. is supported by a Medical Research Council Skills Development Fellowship (MR/T027800/1). N.P.O. is a UK Research and Innovation Future Leaders Fellow (no. MR/S03546X/1). N.P.O. and D.C.A. acknowledge support from the UK National Institute for Health Research University College London Hospitals Biomedical Research Centre, and D.C.A. acknowledges support from the Engineering and Physical Sciences Research Council grant no. EP/M020533/1. M.J.G. is supported by the Miguel Servet program (no. CP19/00031) and a research grant (no. PI20/00613) of the Instituto de Salud Carlos III-Fondo Europeo de Desarrollo Regional. R.L.J. acknowledges support from the NIH (no. K99AG065501). This project received funding from the European Unionâs Horizon 2020 research and innovation programme under grant no. 666992. The BioFINDER studies are supported by the Swedish Research Council (no. 2016-00906), the Knut and Alice Wallenberg Foundation (no. 2017-0383), the Marianne and Marcus Wallenberg Foundation (no. 2015.0125), the Strategic Research Area MultiPark (Multidisciplinary Research in Parkinsonâs disease) at Lund University, the Swedish Alzheimerâs Foundation (no. AF-939932), the Swedish Brain Foundation (no. FO2019-0326), the Swedish Parkinson Foundation (no. 1280/20), the SkĂ„ne University Hospital Foundation (no. 2020-O000028), Regionalt Forskningsstöd (no. 2020-0314) and the Swedish Federal Government under the ALF agreement (no. 2018-Projekt0279). The Tau PET study in Gangnam Severance Hospital was supported by a grant from the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (nos. NRF2018R1D1A1B07049386 and NRF2020R1F1A1076154) and a grant from the Korea Health Technology R&D Project through the Korea Health Industry Development Institute funded by the Ministry of Health and Welfare, Republic of Korea (grant no. HI18C1159). We also thank B. L. Miller, H. J. Rosen, M. Gorno Tempini and W. Jagust for supporting the UCSF tau-PET studies, which were funded through the following sources: National Institute on Aging (NIA) no. R01 AG045611 (G.D.R.), no. P50 AG23501 (B.L.M., H.J.R., G.D.R.), no. P01 AG019724 (B.L.M., H.J.R., G.D.R.). The precursor of 18F-flortaucipir was provided by AVID Radiopharmaceuticals. The precursor of 18F-flutemetamol was sponsored by GE Healthcare. The precursor of 18F-RO948 was provided by Roche. Data collection and sharing for this project were funded by ADNI (NIH grant no. U01 AG024904) and Department of Defense ADNI (award no. W81XWH-12-2-0012). ADNI is funded by the NIA, the National Institute of Biomedical Imaging and Bioengineering and through generous contributions from the following: AbbVie, Alzheimerâs Association; Alzheimerâs Drug Discovery Foundation; Araclon Biotech; Bioclinica; Biogen; Bristol Myers Squibb; CereSpir; Cogstate; Eisai; Elan Pharmaceuticals; Eli Lilly and Company; EUROIMMUN; F. Hoffmann-La Roche and its affiliated company Genentech; Fujirebio; GE Healthcare; IXICO; Janssen Alzheimer Immunotherapy Research Development; Johnson & Johnson Pharmaceutical Research Development; Lumosity; Lundbeck; Merck; Meso Scale Diagnostics; NeuroRx Research; Neurotrack Technologies; Novartis Pharmaceuticals Corporation; Pfizer; Piramal Imaging; Servier; Takeda Pharmaceutical Company; and Transition Therapeutics. The Canadian Institutes of Health Research is providing funds to support ADNI clinical sites in Canada. Private sector contributions are facilitated by the Foundation for the National Institutes of Health (www.fnih.org). The grantee organization is the Northern California Institute for Research and Education, and the study is coordinated by the Alzheimerâs Therapeutic Research Institute at the University of Southern California. ADNI data are disseminated by the Laboratory for Neuro Imaging at the University of Southern California
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