UCAC4 is an updated version of UCAC3 with about the same number of stars also covering all-sky. Bugs were fixed, Schmidt plate survey data were avoided, and precise five-band photometry was added for ...about half the stars. Astrograph observations have been supplemented for bright stars by FK6, Hipparcos, and Tycho-2 data to compile a UCAC4 star catalog complete from the brightest stars to about magnitude R = 16. Epoch 1998-2004 positions are obtained from observations with the 20 cm aperture USNO Astrograph's "red lens," equipped with a 4k by 4k CCD. Mean positions and PMs are derived by combining these observations with over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. These data are supplemented by 2MASS near-IR photometry for about 110 million stars and five-band (B, V, g. r, i) APASS data for over 51 million stars.
A unified modelling framework for all unresolved terms in the filtered progress variable transport equation in large-eddy simulations of turbulent premixed flames is proposed, using convolutional ...neural networks. A direct numerical simulation database of a turbulent premixed stoichiometric methane/air jet flame is used in order to train convolutional neural networks to predict both the filtered progress variable source term and the unresolved scalar transport terms. A single variable readily available from the large-eddy simulation is required in order to calculate all inputs to networks, namely the Favre-filtered progress variable c˜. In the context of flame tabulated chemistry (premixed flamelet), the trained networks are shown to produce quantitatively good predictions of all unresolved terms in an a priori study, despite their different nature and irrespective of variations in filter size, without having to resort to solving any additional transport equations. The framework proposed in this study thus opens perspectives for the application of deep learning to the modelling of the non-linear aerothermochemistry equations which involve unresolved source and transport terms.
Blazars exhibit flares with a doubling time-scale of the order of minutes. Such rapid flares are theoretically challenging and several models have been put forward to explain the fast variability. In ...this paper, we continue the discussion concerning the effects of non-linear, time-dependent synchrotron self-Compton (SSC) cooling. In previous papers, we were able to show that the non-linearity, introduced by a time-dependent electron injection, has severe consequences for both the spectral energy distribution (SED) and the monochromatic synchrotron light curve. The non-linear cooling introduces novel breaks in the SED, which are usually explained by complicated underlying electron distributions, while the much faster cooling of the SSC process causes significant differences in the synchrotron light curves. In this paper, we calculate the inverse Compton light curves, taking into account both the SSC and the external Compton process. The light curves are calculated from the monochromatic intensities by introducing the retardation due to the finite size of the emission region and the geometry of the source. Even though some of the obvious effects of the SSC cooling are washed out by the retardation, there are still several observational constraints which could help to discriminate between the non-linear and the usual linear models, such as different flux states, temporal shapes or faster variability of the light curves at different energies.
The third US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC3, was released at the IAU General Assembly on 2009 August 10. It is the first all-sky release in this series and contains just over ...100 million objects, about 95 million of them with proper motions, covering about R = 8-16 mag. Current epoch positions are obtained from the observations with the 20 cm aperture USNO Astrograph's 'red lens,' equipped with a 4k X 4k CCD. Proper motions are derived by combining these observations with over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. For most of the faint stars in the southern hemisphere, the Yale/San Juan first epoch plates from the Southern Proper Motion (SPM) program (YSJ1) form the basis for proper motions. These data are supplemented by all-sky Schmidt plate survey astrometry and photometry obtained from the SuperCOSMOS project, as well as 2MASS near-IR photometry. Major differences of UCAC3 data as compared with UCAC2 include a completely new raw data reduction with improved control over systematic errors in positions, significantly improved photometry, slightly deeper limiting magnitude, coverage of the north pole region, greater completeness by inclusion of double stars, and weak detections. This of course leads to a catalog which is not as 'clean' as UCAC2 and problem areas are outlined for the user in this paper. The positional accuracy of stars in UCAC3 is about 15-100 mas per coordinate, depending on magnitude, while the errors in proper motions range from 1 to 10 mas yr--1 depending on magnitude and observing history, with a significant improvement over UCAC2 achieved due to the re-reduced SPM data and inclusion of more astrograph plate data unavailable at the time of UCAC2.
The validity of the commonly used flame marker for heat release rate (HRR) visualization, namely the rate of the reaction OH+CH2O⇔HCO+H2O is re-examined. This is done both for methane–air and ...multi-component fuel–air mixtures for lean and stoichiometric conditions. Two different methods are used to identify HRR correlations, and it is found that HRR correlations vary strongly with stoichiometry. For the methane mixture there exist alternative HRR markers, while for the multi-component fuel flame the above correlation is found to be inadequate. Alternative markers for the HRR visualization are thus proposed and their performance under turbulent conditions is evaluated using DNS data.
In this paper, the optical depths in blazars due to photo-pair production is calculated for a time-dependent, non-linear injection model. Several target photon fields are taken into account, namely ...the internal synchrotron, synchrotron self-Compton and external Compton radiation, as well as a constant external soft photon field. By applying the optical depths to theoretical blazar spectra, only the constant external photon field turns out to significantly influence the radiation at high energies. The impact of the internal time-dependent radiation fields is either minor or requires extreme parameter settings. Additionally, the synchrotron self-absorption turn-over energy for low synchrotron energies is calculated, which is inherently time dependent. It would be challenging to use it to constrain free parameters, since precise knowledge of the observation time relative to the injection time is needed. In conclusion, optical depth does not significantly influence the non-linear, time-dependent injection and cooling model.
Between 1997 and 2004 several observing runs were conducted, mainly with the CTIO 0.9 m, to image International Celestial Reference Frame (ICRF) counterparts (mostly QSOs) in order to determine ...accurate optical positions. Contemporary to these deepCCDimages, the same fields were observed with theU.S.Naval Observatory astrograph in the same bandpass. They provide accurate positions on the Hipparcos/Tycho-2 system for stars in the 10-16 mag range used as reference stars for the deep CCD imaging data. Here we present final optical position results of 413 sources based on reference stars obtained by dedicated astrograph observations that were reduced following two different procedures. These optical positions are compared to radio very long baseline interferometry positions. The current optical system is not perfectly aligned to the ICRF radio system with rigid body rotation angles of 3-5 mas (= 3sigma level) found between them for all three axes. Furthermore, statistically, the optical-radio position differences are found to exceed the total, combined, known errors in the observations. Systematic errors in the optical reference star positions and physical offsets between the centers of optical and radio emissions are both identified as likely causes. A detrimental, astrophysical, random noise component is postulated to be on about the 10 mas level. If confirmed by future observations, this could severely limit the Gaia to ICRF reference frame alignment accuracy to an error of about 0.5 mas per coordinate axis with the current number of sources envisioned to provide the link. A list of 36 ICRF sources without the detection of an optical counterpart to a limiting magnitude of about R = 22 is provided as well.
Parahydrogen‐Based Hyperpolarization for Biomedicine Hövener, Jan‐Bernd; Pravdivtsev, Andrey N.; Kidd, Bryce ...
Angewandte Chemie International Edition,
August 27, 2018, Letnik:
57, Številka:
35
Journal Article
Recenzirano
Odprti dostop
Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is ...rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2), which is inherently stable and long‐lived. When brought into contact with another molecule, this “spin order on demand” allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2‐based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.
Spin doctoring: Considerable progress has been made in the past decade in the area of parahydrogen‐based hyperpolarization techniques for biomedical applications. This Review provides a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agent, and applications.
ABSTRACT
The processes operating in blazar jets are still an open question. Modelling the radiation emanating from an extended part of the jet allows one to capture these processes on all scales. ...Kinetic codes solving the Fokker–Planck equation along the jet flow are well suited to this task, as they can efficiently derive the radiation and particle spectra without the need for computationally demanding plasma physical simulations. Here, we present a new extended hadro-leptonic jet code – ExHaLe-jet– which considers simultaneously the processes of relativistic protons and electrons. Within a pre-set geometry and bulk flow, the particle evolution is derived self-consistently. Highly relativistic secondary electrons (and positrons) are created through γ–γ pair production, Bethe–Heitler pair production, and pion/muon decay. These secondaries are entrained in the jet flow decreasing the ratio of protons to electrons with distance from the jet base. For particle–photon interactions, we consider all internal and many external photon fields, such as the accretion disc, broad-line region, and the dusty torus. The external fields turn out to be the most important source for particle–photon interactions governing the resulting photon and neutrino spectra. In this paper, we present the code and an initial parameter study, while in follow-up works we present extensions of the code and more specific applications.
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