15,367 research outputs found

    Quark-model study of few-baryon systems

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    We review the application of non-relativistic constituent quark models to study one, two and three non-strange baryon systems. We present results for the baryon spectra, potentials and observables of the NN, NΔ\Delta, ΔΔ\Delta\Delta and NN(1440)^*(1440) systems, and also for the binding energies of three non-strange baryon systems. We make emphasis on observable effects related to quark antisymmetry and its interplay with quark dynamics.Comment: 82 pages, 36 figures, 18 tables. Accepted for publication in Reports on Progress in Physic

    Random graphs with few disjoint cycles

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    The classical Erd\H{o}s-P\'{o}sa theorem states that for each positive integer k there is an f(k) such that, in each graph G which does not have k+1 disjoint cycles, there is a blocker of size at most f(k); that is, a set B of at most f(k) vertices such that G-B has no cycles. We show that, amongst all such graphs on vertex set {1,..,n}, all but an exponentially small proportion have a blocker of size k. We also give further properties of a random graph sampled uniformly from this class; concerning uniqueness of the blocker, connectivity, chromatic number and clique number. A key step in the proof of the main theorem is to show that there must be a blocker as in the Erd\H{o}s-P\'{o}sa theorem with the extra `redundancy' property that B-v is still a blocker for all but at most k vertices v in B

    Testing hydrodynamics schemes in galaxy disc simulations

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    We examine how three fundamentally different numerical hydrodynamics codes follow the evolution of an isothermal galactic disc with an external spiral potential. We compare an adaptive mesh refinement code (RAMSES), a smoothed particle hydrodynamics code (SPHNG), and a volume-discretised meshless code (GIZMO). Using standard refinement criteria, we find that RAMSES produces a disc that is less vertically concentrated and does not reach such high densities as the SPHNG or gizmo runs. The gas surface density in the spiral arms increases at a lower rate for the RAMSES simulations compared to the other codes. There is also a greater degree of substructure in the SPHNG and GIZMOruns and secondary spiral arms are more pronounced. By resolving the Jeans’ length with a greater number of grid cells we achieve more similar results to the Lagrangian codes used in this study. Other alterations to the refinement scheme (adding extra levels of refinement and refining based on local density gradients) are less successful in reducing the disparity between RAMSES and SPHNG/GIZMO. Although more similar, SPHNG displays different density distributions and vertical mass profiles to all modes of gizmo (including the smoothed particle hydrodynamics version). This suggests differences also arise which are not intrinsic to the particular method but rather due to its implementation. The discrepancies between codes (in particular, the densities reached in the spiral arms) could potentially result in differences in the locations and timescales for gravitational collapse, and therefore impact star formation activity in more complex galaxy disc simulations

    Study of railway curve squeal in the time domain using a high-frequency vehicle/track interaction model

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    [EN] Railway curve squeal is an intense tonal and annoying type of noise commonly attributed to self-excited vibrations during curving. The mechanisms for its generation remain unclear and it is still a subject of discussion among researchers. Most of them have considered the falling behaviour of the friction coefficient with the slip velocity essential for reenergising the system. Recently, some authors have found that squeal can also appear even for constant friction coefficient through the wheel modal coupling between the normal and tangential directions caused by the wheel/rail contact. This paper particularly evaluates whether the latter mechanism is sufficient to find squeal in curving conditions. The introduction of flexibility in the railway subsystems is required to widen the domain to the high-frequency range in which squeal occurs. One single flexible and rotatory wheelset is considered and suitable forces are prescribed at the primary suspension seats in the current investigation. The rails are modelled through the Moving Element Method (MEM), permitting to extend the range of validity of beam models usually utilised in the literature. This work extends the formulation to rails supported by a viscoelastic Winkler bedding. Both wheelset and track models are coupled by means of a non-linear and unsteady wheel/rail contact model based on Kalker¿s Variational Theory. Simulation results for different track curvatures and friction coefficients are presented and discussed, showing tonal peaks in the tangential contact forces of the inner wheel. These results can be associated with squeal according to the characterisation of this phenomenon, indicating that squeal can be found in curving conditions using advanced dynamic interaction models even with constant friction coefficient.The authors gratefully acknowledge the financial support of Spanish Ministry of Economy, Industry and Competitiveness and the European Regional Development Fund (project TRA2017-84701-R), as well as Generalitat Valenciana (project Prometeo/2016/007) and European Commission through the project "RUN2Rail - Innovative RUNning gear soluTiOns for new dependable, sustainable, intelligent and comfortable RAIL vehicles" (Horizon 2020 Shift2Rail JU call 2017, grant number 777564).Giner Navarro, J.; Martínez Casas, J.; Denia, FD.; Baeza González, LM. (2018). Study of railway curve squeal in the time domain using a high-frequency vehicle/track interaction model. Journal of Sound and Vibration. 431:177-191. https://doi.org/10.1016/j.jsv.2018.06.004S17719143

    Criticality of mostly informative samples: A Bayesian model selection approach

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    We discuss a Bayesian model selection approach to high dimensional data in the deep under sampling regime. The data is based on a representation of the possible discrete states ss, as defined by the observer, and it consists of MM observations of the state. This approach shows that, for a given sample size MM, not all states observed in the sample can be distinguished. Rather, only a partition of the sampled states ss can be resolved. Such partition defines an {\em emergent} classification qsq_s of the states that becomes finer and finer as the sample size increases, through a process of {\em symmetry breaking} between states. This allows us to distinguish between the resolutionresolution of a given representation of the observer defined states ss, which is given by the entropy of ss, and its relevancerelevance which is defined by the entropy of the partition qsq_s. Relevance has a non-monotonic dependence on resolution, for a given sample size. In addition, we characterise most relevant samples and we show that they exhibit power law frequency distributions, generally taken as signatures of "criticality". This suggests that "criticality" reflects the relevance of a given representation of the states of a complex system, and does not necessarily require a specific mechanism of self-organisation to a critical point.Comment: 31 pages, 7 figure

    The Giant Radio Galaxy MRC B0319-454: Circumnuclear Structure of the Host Galaxy ESO248-G10

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    We present optical and near-infrared images and spectra of ESO248-G10, the host of giant radio galaxy MRC B0319-454. From near-infrared colours, the active nucleus is shown to be reddened by hot dust emission or dust extinction. Star forming regions are identified beyond a radius of 5" (8.1 kpc) where hot gas is prevalent. The optical spectrum shows gas rotation at speeds of up to 350km/s out to >=32kpc along the radio axis. A model is proposed in which the giant elliptical is triaxial with the radio axis along the short axis, and the figure rotation is around the long axis. From the model, the angles of the principal axes are psi=34 degrees, theta=65 degrees, and phi=19 degrees with axis ratios of B/A=0.75 and C/A=0.69. A second velocity feature from 5" to 15" to the north-east of the nucleus is proposed to be a merging gas-rich galaxy inducing star formation while settling into an orbit about the figure rotation axis.Comment: Accepted for publication in The Astrophysical Journal. 22 pages, 12 figure

    NLTE Spectra of Kilonovae

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    The electromagnetic transient following a binary neutron star merger is known as a kilonova (KN). Owing to rapid expansion velocities and small ejecta masses, KNe rapidly transition into the Non-Local Thermodynamic Equilibrium (NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5 to 20 days after merger using the \texttt{SUMO} spectral synthesis code. We study three homogeneous composition, 1D multi-zone models with characteristic electron fractions of Ye0.35,0.25Y_e \sim 0.35, 0.25 and 0.150.15. We find that emission features in the spectra tend to emerge in windows of reduced line blocking, as the ejecta are still only partially transparent even at 20 days. For the Ye0.35Y_e \sim 0.35 (lanthanide-free) ejecta, we find that the neutral and singly ionised species of Rb, Sr, Y and Zr dominate the spectra, all with good potential for identification. We directly test and confirm an impact of Sr on the 10000 angstrom spectral region in lanthanide-free ejecta, but also see that its signatures may be complex. We suggest the Rb I 5p1\rm{5p^{1}}- 5s1\rm{5s^{1}} 7900 angstrom transition as a candidate for the λ0\lambda_0 \sim 7500--7900 angstrom P-Cygni feature in AT2017gfo. For the Ye0.25Y_e \sim 0.25 and 0.150.15 compositions, lanthanides are dominant in the spectral formation, in particular Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably that scattering and fluorescence play important roles even up to 20 days after merger, implying that the KN ejecta are not yet optically thin at this time.Comment: 20 pages (29 with appendices), 17 figures, resubmitted to MNRAS after referee repor
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