Effects Of Finite Resolution Research Articles

Formation of Giant Planets by Gas Disk Gravitational Instability on Wide Orbits around Protostars with Varied Masses. II. Quadrupled Spatial Resolution and Beta Cooling

Exoplanet demographics are sufficiently advanced to provide important constraints on theories of planet formation. While core and pebble accretion are preferred for rocky and icy planets, there appears to be a need for gas disk gravitational instability (GDGI) to play a role in the formation of M-dwarf gas giants and those orbiting at large distances. Here we present GDGI models that go beyond those presented by Boss (2011) dealing with the formation of wide-orbit gas giants. The new models use quadrupled spatial resolution, in both the radial and azimuthal directions, to reduce the effects of finite spatial resolution. The new models also employ the β cooling approximation, instead of the diffusion approximation used by Boss (2011), in order to push the models further in time. As in Boss (2011), the central protostars have masses of 0.1, 0.5, 1.0, 1.5, or 2.0 M ⊙, surrounded by disks with masses ranging from 0.019 M ⊙ to 0.21 M ⊙. For each case, two models are computed, one with an initial minimum Toomre Q stability value ranging from 1.1 to 1.7, and one with a higher initial disk temperature, resulting in the initial minimum Q ranging from 2.2 to 3.4. These new models continue to show that GDGI can explain the formation of gas giants at distances of ∼30 to ∼50 au on eccentric orbits (e less than ∼0.2), though the number formed drops to 0 as the protostar mass decreases to 0.1 M ⊙.

CARS.jl: Efficient fitting of dual‐pump multi‐species CARS spectra using compressed libraries

AbstractIn this publication, we demonstrate the extension of our loss‐less library approach previously limited to fitting ro‐vibrational N2 CARS spectra for thermometry purposes to accurate and efficient fitting of dual‐pump CARS spectra with multiple species. The fundamental reasoning behind the compression approach is to make use of finite resolution effects arising from the laser linewidths, which are typically larger than the transitional linewidths. For accurate reconstruction at runtime of the fit, in addition to the squared modulus and real component, also the imaginary component as well as intra‐region species cross terms have to be compressed and tabulated for dual‐pump multi‐species spectra. The resulting libraries do not require to tabulate combinations of mole fractions, leading to a near linear growth of the library with respect to the number of species. The accuracy and computational efficiency of the method is benchmarked using a reference implementation provided alongside this publication. Library generation times, mainly depending on the underlying spectral model, are typically in the order of seconds to a few minutes. The simulation of a single dual‐pump spectrum containing three species is sped up by a factor of >5000 without significantly affecting the accuracy of reconstruction. In a simulated experiment with Stokes‐noise imposed data, the time required to it temperature, mole fraction of three resonant species and wavenumber shift on a single spectrum required 90 ms.

Reviving stochasticity: uncertainty in SMBH binary eccentricity is unavoidable

ABSTRACT We study supermassive black hole (SMBH) binary eccentricity of equal-mass galaxy mergers in N-body simulations with the ketju code, which combines the gadget-4 fast multipole gravity solver with accurate regularized integration and post-Newtonian corrections around SMBHs. In simulations with realistic, high-eccentricity galactic merger orbits, the hard binary eccentricity is found to be a non-linear function of the deflection angle in the SMBH orbit during the final, nearly radial close encounter between the SMBHs before they form a bound binary. This mapping between the deflection angle and the binary eccentricity has no apparent resolution dependence in our simulations spanning the resolution range of 1 × 105 to 8 × 106 particles per galaxy. The mapping is also captured using a simple model with an analytical potential, indicating that it is driven by the interplay between a smooth asymmetric stellar background potential and dynamical friction acting on the SMBHs. Due to the non-linearity of this mapping, in eccentric major merger configurations, small, parsec-scale variations in the merger orbit can result in binary eccentricities varying in nearly the full possible range between e = 0 and e = 1. In idealized simulations, such variations are caused by finite resolution effects, and convergence of the binary eccentricity can be achieved with increasing resolution. However, in real galaxies, other mechanisms such as nuclear gas and substructure that perturb the merger orbit are likely to be significant enough for the binary eccentricity to be effectively random. Our results indicate that the distribution of these effectively random eccentricities can be studied using even moderate resolution simulations.

Measurements of observables sensitive to colour reconnection in t{bar{t}} events with the ATLAS detector at sqrt{s} = 13 TeV

A measurement of observables sensitive to effects of colour reconnection in top-quark pair-production events is presented using 139 hbox {fb}^{-1} of 13 TeV proton–proton collision data collected by the ATLAS detector at the LHC. Events are selected by requiring exactly one isolated electron and one isolated muon with opposite charge and two or three jets, where exactly two jets are required to be b-tagged. For the selected events, measurements are presented for the charged-particle multiplicity, the scalar sum of the transverse momenta of the charged particles, and the same scalar sum in bins of charged-particle multiplicity. These observables are unfolded to the stable-particle level, thereby correcting for migration effects due to finite detector resolution, acceptance and efficiency effects. The particle-level measurements are compared with different colour reconnection models in Monte Carlo generators. These measurements disfavour some of the colour reconnection models and provide inputs to future optimisation of the parameters in Monte Carlo generators.

Impact of the turbulence wavenumber spectrum and probing beam geometry on Doppler reflectometry perpendicular velocity measurements

The perpendicular propagation velocity of turbulent density fluctuations is an important parameter in fusion plasmas, since sheared plasma flows are crucial for reducing turbulence, and thus an essential input parameter for turbulent transport simulations. In the recent past various fusion devices have observed poloidal asymmetry in this velocity using Doppler reflectometry (DR) and correlation reflectometry. In this work, the phase screen model is used to analytically explain and quantify the combined effect of finite wavenumber resolution due to plasma curvature and probing beam geometry in a realistic turbulence wavenumber spectrum, leading to a reduced dominantly back-scattered wavenumber and a further underestimation of the perpendicular propagation velocity determined by DR. The full-wave code IPF-FD3D, which simulates microwave propagation and scattering, is used as a synthetic DR to study the effects of this diagnostic effect in a circular geometry using various isotropic synthetic turbulence wavenumber spectra. Angular scans from the midplane and variations in the position of the probing antenna are shown to estimate the impact of the diagnostic effect on the poloidal asymmetries.

Tidal tracks and artificial disruption of cold dark matter haloes

ABSTRACT We describe a simple extension to existing models for the tidal heating of dark matter subhaloes which takes into account second-order terms in the impulse approximation for tidal heating. We show that this revised model can accurately match the tidal tracks along which subhaloes evolve as measured in high-resolution N-body simulations. We further demonstrate that, when a constant density core is introduced into a subhalo, this model is able to quantitatively reproduce the evolution and artificial disruption of N-body subhaloes arising from finite resolution effects. Combining these results we confirm prior work indicating that artificial disruption in N-body simulations can result in a factor two underestimate of the subhalo mass function in the inner regions of host haloes, and a 10–20 per cent reduction over the entire virial volume.

Effects of Spectral Resolution on Simple Magnetic Field Diagnostics of the Mg ii H and K Lines

We study the effects of finite spectral resolution on the magnetic field values retrieved through the weak-field approximation (WFA) from the cores of the Mg ii h and k lines. The retrieval of the line-of-sight component of the magnetic field, B LOS, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere is accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution effects, which more prominently affect the outer lobes of Stokes V, are brought into the line core through spectral smearing, degrading the accuracy of the WFA and resulting in an inference bias, which is more pronounced as the resolution becomes poorer. When applied to a diverse set of spectra emerging from a sunspot simulation, we find good accuracy in the retrieved B LOS when comparing it to the model value at the height where the optical depth in the line core is unity. The accuracy is preserved up to field strengths of B ∼ 1500 G. Limited spectral resolution results in a small bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. Reduced spectral resolution degrades the accuracy of the inferences, because spectral mixing results in the line effectively probing deeper layers of the atmosphere.

Digital imaging of a random walk by computer simulation: Using a simple model to interpret the effects of finite spatio-temporal resolution

We discuss computer simulations of a particle that hops forward and backward randomly on a one-dimensional lattice. To track the motion of the particle, we use a data acquisition protocol that mimics a virtual digital optical imaging system. Plots of the data extracted from these images yield a pattern that is similar to the patterns observed in real imaging of a moving particle in a living cell. To explore the physical origin of the observed patterns, we use different spatio-temporal resolutions and show that the characteristics of the observed patterns are determined by the spatial and temporal resolutions of the digital imaging device. The simulations help not only to determine the origin of these patterns in the imaging data, but also to avoid misinterpreting the imaging data in terms of a special property of the particle or its dynamics.

Receiver Energy Efficiency and Transmission Latency Analysis of Outage-Constrained Differential PSK With a Finite Resolution ADC

We consider a power constrained downlink communication scenario where energy efficiency, reliability, and latency take precedence over rate, as in some Internet of Things (IoT) applications. To reduce its receiver power consumption and complexity, we assume that the IoT device has a single RF chain and investigate the finite-resolution analog-to-digital converter (ADC) operation with differential Phase Shift Keying (PSK) modulation. A lower ADC resolution leads to an exponential decrease in power consumption, while adopting differential PSK enables the use of a low-cost detector with no channel state information (CSI) or carrier phase recovery circuitry. Our main goal in this article is to compare, both analytically and numerically using representative IoT system design parameters, the receiver energy efficiency of the proposed differential PSK system with a coherent PSK system that uses estimated CSI under reliability and latency constraints in Rayleigh slow-fading and ADC quantization distortion conditions. To mitigate the ADC finite-resolution effects without requiring CSI at the receiver or transmitter, we propose and analyze a PSK differentially-modulated Alamouti space-time block transmission scheme with only two RF chains at the transmitter while still restricting the IoT device to have a single RF chain. Our results demonstrate that in the considered low-rate, and outage and latency-constrained scenario with stringent power consumption requirements, differentially-modulated Alamouti transmissions to a single RF-chain IoT device with a low-resolution ADC is an attractive choice in terms of receiver energy efficiency, reliability, and transmission latency.

Modeling the diffusive dynamics of critical fluctuations near the QCD critical point

The experimental search for the QCD critical point by means of relativistic heavy-ion collisions necessitates the development of dynamical models of fluctuations. In this work we study the fluctuations of the net-baryon density near the critical point. Due to net-baryon number conservation the correct dynamics is given by the fluid dynamical diffusion equation, which we extend by a white noise stochastic term to include intrinsic fluctuations. We quantify finite resolution and finite size effects by comparing our numerical results to analytic expectations for the structure factor and the equal-time correlation function. In small systems the net-baryon number conservation turns out to be quantitatively and qualitatively important, as it introduces anticorrelations at larger distances. Including nonlinear coupling terms in the form of a Ginzburg-Landau free energy functional we observe non-Gaussian fluctuations quantified by the excess kurtosis. We study the dynamical properties of the system close to equilibrium, for a sudden quench in temperature and a Hubble-like temperature evolution. In the real-time dynamical systems we find the important dynamical effects of critical slowing down, weakening of the extremal value and retardation of the fluctuation signal. In this work we establish a set of general tests, which should be met by any model propagating fluctuations, including upcoming $3+1$ dimensional fluctuating fluid dynamics.

Numerical Methods for Simulating Star Formation

We review the numerical techniques for ideal and non-ideal magneto-hydrodynamics (MHD) used in the context of star formation simulations. We outline the specific challenges offered by modelling star forming environments, which are dominated by supersonic turbulence in a radiative, self-gravitating fluid. These conditions are rather unique in physics and engineering and pose particularly severe restrictions on the robustness and accuracy of numerical codes. One striking aspect is the formation of collapsing fluid elements leading to the formation of singularities that represent point-like objects, namely the proto-stars. Although a few studies have attempted to resolve the formation of the first and second Larson's cores, resolution limitations force us to use sink particle techniques, with sub-grid models to compute the accretion rates of mass, momentum and energy, as well as their ejection rate due to radiation and jets from the proto-stars. We discuss the most popular discretisation techniques used in the community, namely smoothed particle hydrodynamics, finite difference and finite volume methods, stressing the importance to maintain a divergence free magnetic field. We discuss how to estimate the truncation error of a given numerical scheme, and its importance in setting the magnitude of the numerical diffusion. This can have a strong impact on the outcome of these simulations, where viscosity and resistivity are implemented at the grid scale. We then present various numerical techniques to model non-ideal MHD effects (Ohmic and ambipolar diffusion, and Hall effect). These physical ingredients are posing strong challenges in term of resolution and time stepping. For the latter, several strategies are discussed to overcome the limitations due to ridiculously small time steps. An important aspect of star formation simulations is the radiation field. We discuss the current state-of-the-art, with a variety of techniques offering pros and cons in different conditions. Finally, we present more advanced strategies to mitigate the adverse effect of finite numerical resolution, which are very popular in the context of supersonic, self-gravitating fluids, namely adaptive mesh refinement, moving meshes, SPH and high-order methods. Advances in these directions are likely to trigger immense progresses in the future of our field.

Effects of the instrument response function and the gate width in time-domain diffuse correlation spectroscopy: model and validations.

Time-domain diffuse correlation spectroscopy (TD-DCS) is an emerging noninvasive optical technique with the potential to resolve blood flow (BF) and optical coefficients (reduced scattering and absorption) in depth. Here, we study the effects of finite temporal resolution and gate width in a realistic TD-DCS experiment. We provide a model for retrieving the BF from gated intensity autocorrelations based on the instrument response function, which allows for the use of broad time gates. This, in turn, enables a higher signal-to-noise ratio that is critical for in vivo applications. In numerical simulations, the use of the proposed model reduces the error in the estimated late gate BF from 34% to 3%. Simulations are also performed for a wide set of optical properties and source-detector separations. In a homogeneous phantom experiment, the discrepancy between later gates BF index and ungated BF index is reduced from 37% to 2%. This work not only provides a tool for data analysis but also physical insights, which can be useful for studying and optimizing the system performance.

Polarimetric imaging of circumstellar disks

Context. Polarimetric images of circumstellar environments, even when corrected with adaptive optics, have a limited angular resolution. Finite resolution greatly affects polarimetric images because of the canceling of adjacent polarization signals with opposite signs. In radio astronomy this effect is called beam depolarization and is well known. However, radio techniques to mitigate beam depolarization are not directly applicable to optical images as a consequence of the inherent lack of phase information at optical wavelengths. Aims. We explore the effects of a finite point-spread function (PSF) on polarimetric images and the application of Richardson-Lucy deconvolution to polarimetric images. Methods. We simulated polarimetric images of highly simplified, circumstellar disk models and convolved these with simulated and actual SPHERE/ZIMPOL PSFs. We attempted to deconvolve simulated images in orthogonal linear polarizations and polarized intensity images. Results. The most significant effect of finite angular resolution is the loss of polarimetric signal close to the central star where large polarization signals of opposite signs average out. The finite angular resolution can also introduce polarized light in areas beyond the original, polarized signal such as outside of disks. These effects are particularly severe for disks that are not rotationally symmetric. The deconvolution of polarimetric images is far from trivial. Richardson-Lucy deconvolution applied to images in opposite linear polarization states, which are subsequently subtracted from each other, cannot recover the signal close to the star. Sources that lack rotational symmetry cannot be recovered with this deconvolution approach.

Model for Concerted Power Stroke Generation in Single Myosin V and F1-ATPase Imaging Trajectories

The idea of a concerted mechanism of molecular group transfer and large conformational change in power stroke generation serves as the basis for an elasto-chemical theory of the rate constants that dictate the stepping kinetics in single F1-ATPase motors. The theory was employed to calculate the rate and equilibrium constant dependence on the rotor angle in experiments with magnetic tweezers. Using independent biochemical and single-molecule imaging data, the model was used to correctly predict the rates of binding and release of fluorescently labeled ATP without any adjustable parameters. This initial success of the model prompted its further development to other systems and experiments. In particular, the idea of group transfer elastically coupled to conformational change monitored by the probe is applied and extended to (1) calculate power stroke rate constants in two different force-spectroscopy experiments on single and double-headed myosin V; to (2) extract rate constants for hydrolysis and synthesis from controlled rotation data on F1-ATPase; and to (3) account for “dynamical effects” due to the internal and probe-related friction in the single-molecule imaging experiments. An important aspect of the analysis is the statistical modeling of biasing effects of finite time resolution in the photon-counting trajectories and the finite response time of the imaging probes.

A simultaneous understanding of jet and hadron suppression

In the context of the hybrid strong/weak coupling model for jet quenching, we perform a global fit to hadron and jet data in the most central bins both at RHIC and LHC. The qualitative and quantitative success of the analysis is attributed to the fact that the model correctly captures the fact that wider jets lose, on average, more energy than the narrower ones, to which high energy hadrons belong. We show how one can understand the relative jet and hadron suppression by analyzing the jet fragmentation functions, and also discuss the role of plasma finite resolution effects.

Intensity contrast of solar plage as a function of magnetic flux at high spatial resolution

Magnetic elements have an intensity contrast that depends on the type of region they are located in (for example quiet Sun, or active region plage). Observed values also depend on the spatial resolution of the data. Here we investigate the contrast-magnetic field dependence in active region plage observed near disk center with SUNRISE during its second flight in 2013. The wavelengths under study range from the visible at 525 nm to the near ultraviolet (NUV) at 300 nm and 397 nm. We use quasi-simultaneous spectropolarimetric and photometric data from the Imaging Magnetograph eXperiment (IMaX) and the Sunrise Filter Imager (SuFI), respectively. We find that in all wavelength bands, the contrast exhibits a qualitatively similar dependence on the line-of-sight magnetic field, BLOS, as found in the quiet Sun, with the exception of the continuum at 525 nm. There, the contrast of plage magnetic elements peaks for intermediate values of BLOS and decreases at higher field strengths. By comparison, the contrast of magnetic elements in the quiet Sun saturates at its maximum value at large BLOS. We find that the explanation of the turnover in contrast in terms of the effect of finite spatial resolution of the data is incorrect with the evidence provided by the high-spatial resolution SUNRISE data, as the plage magnetic elements are larger than the quiet Sun magnetic elements and are well-resolved. The turnover comes from the fact that the core pixels of these larger magnetic elements are darker than the quiet Sun. We find that plages reach lower contrast than the quiet Sun at disk center at wavelength bands formed deep in the photosphere, such as the visible continuum and the 300 nm band. This difference decreases with formation height and disappears in the Ca II H core, in agreement with empirical models of magnetic element atmospheres.

Hunting for heavy winos in the Galactic Center

Observing gamma rays using ground-based atmospheric Cherenkov telescopes provides one of the only probes of heavy weakly interacting dark matter. A canonical target is the thermal wino, for which the strongest limits come from searches for photon lines from annihilations in the Galactic Center. Irreducible finite energy resolution effects motivate refining the prediction for a wino signal beyond the photon line approximation; recently, modern effective field theory techniques have been utilized to obtain a precise calculation of the full photon energy spectrum from wino annihilation. In this paper, we investigate the implications for a realistic mock H.E.S.S.-like line search. We emphasize the impact of including the non-trivial spectral shape, and we carefully treat the region of interest, presenting results for choices between $1^{\circ}$ and $4^{\circ}$ from the Galactic Center. Projected limits for wino masses from $1$-$70$ TeV are interpreted as a constraint on the wino annihilation rate, or alternatively as the minimum core size required such that the wino is not excluded. If there is a thermal wino, H.E.S.S. will be able to probe cores of several kpc, which would begin to cause tension between this dark matter candidate and astrophysical observations/simulations.

Effects of finite spatial and temporal resolution in direct numerical simulations of incompressible isotropic turbulence

A study combining spectral filtering and numerical simulations at enhanced spatial and/or temporal resolution is used to clarify the proper scaling of dissipation and enstrophy in forced incompressible isotropic turbulence.

Effects of finite hot-wire spatial resolution on turbulence statistics and velocity spectra in a round turbulent free jet

The effect of finite hot-wire spatial resolution on turbulence statistics and velocity spectra in a round turbulent free jet is investigated. To quantify spatial resolution effects, measurements were taken using a nano-scale thermal anemometry probe (NSTAP) and compared to results from conventional hot-wires with sensing lengths of $$l=0.5$$ and 1 mm. The NSTAP has a sensing length significantly smaller than the Kolmogorov length scale $$\eta$$ for the present experimental conditions, whereas the sensing lengths for the conventional probes are larger than $$\eta$$ . The spatial resolution is found to have a significant impact on the dissipation both on and off the jet centreline with the NSTAP results exceeding those obtained from the conventional probes. The resolution effects along the jet centreline are adequately predicted using a Wyngaard-type spectral technique (Wyngaard in J Sci Instr 1(2):1105-1108,1968), but additional attenuation on the measured turbulence quantities are observed off the centreline. The magnitude of this attenuation is a function of both the ratio of wire length to Kolmogorov length scale and the magnitude of the shear. The effect of spatial resolution is noted to have an impact on the power-law decay parameters for the turbulent kinetic energy that is computed. The effect of spatial filtering on the streamwise dissipation energy spectra is also considered. Empirical functions are proposed to estimate the effect of finite resolution, which take into account the mean shear.

A Theory for Rate Constants in Rotation Trajectories of F1-ATPase

I present an elastic chemo-mechanical theory to treat single molecule imaging and “stalling” experiments in the F1-ATPase enzyme. Using a molecular group transfer approach the theory couples chemical reactions in the stator and the physics of torsional elasticity in the rotor. In the theory we predicted and compared with experiment the rate and equilibrium constant dependence of steps such as ATP binding as a function of the rotor angle.[PNAS, 112, 14230 (2015)] Using independent experimental data from biochemical ensemble and single-molecule imaging experiments, the model correctly predicts the controlled rotation data on fluorescent ATP without any adjustable parameters. We took into account the biasing effect of finite experimental time resolution in the single fluorescence trajectories and treated these data by developing computational statistical methods.[PNAS, 113 (48), 12029 (2016)] A theory-based method for the extraction of rate constants for hydrolysis and synthesis from controlled rotation data was also provided for angular range where no such data is currently available [PNAS, 114, 7272 (2016)] The framework is generic and we plan to apply it to other biomolecular motors.