Continuum Region Research Articles

The Oxyometer: A Novel Instrument Concept for Characterizing Exoplanet Atmospheres

With the Transiting Exoplanet Survey Satellite (TESS) and ground-based surveys searching for rocky exoplanets around cooler, nearby stars, the number of Earth-sized exoplanets that are well suited for atmospheric follow-up studies will increase significantly. For atmospheric characterization, the James Webb Space Telescope will only be able to target a small fraction of the most interesting systems, and the usefulness of ground-based observatories will remain limited by a range of effects related to Earth’s atmosphere. Here, we explore a new method for ground-based exoplanet atmospheric characterization that relies on simultaneous, differential, ultra-narrow-band photometry. The instrument uses a narrow-band interference filter and an optical design that enables simultaneous observing over two 0.3 nm wide bands spaced 1 nm apart. We consider the capabilities of this instrument in the case where one band is centered on an oxygen-free continuum region while the other band overlaps the 760 nm oxygen bandhead in the transmission spectrum of the exoplanet, which can be accessible from Earth in systems with large negative line-of-sight velocities. We find that M9 and M4 dwarfs that meet this radial velocity requirement will be the easiest targets but must be nearby (<8 pc) and will require the largest upcoming Extremely Large Telescopes. The oxyometer instrument design is simple and versatile and could be adapted to enable the study of a wide range of atmospheric species. We demonstrate this by building a prototype oxyometer and present its design and a detection of a 50 ppm simulated transit signal in the laboratory. We also present data from an on-sky test of a prototype oxyometer, demonstrating the ease of use of the compact instrument design.

A hybrid quasicontinuum method

SummaryQuasicontinuum (QC) is a partitioned‐domain multiscale method where the full‐atomistic description is retained in highly deformable subdomains while the energy in the remaining region is approximated using a continuum constitutive relation called the Cauchy‐Born (CB) rule. While the CB rule can capture the full nonlinearity of the strain energy, its computational cost is one or two orders of magnitude higher than the linear elasticity (LE) constitutive relation. In this paper, we propose an extension to the QC method, referred to as “hybrid QC,” where both the CB and LE relations are combined to increase the efficiency of the original QC method. This extension is summed up to a two‐step process. First, the continuum region is further divided into subregions with CB being restricted to subregions adjoining the atomistic region while LE is adopted in the remaining subregions. Second, a corrective scheme is applied to the LE subregion to recover the higher‐order accuracy. The hybrid QC method is validated through two examples: (i) a crystal containing a dipole of Lomer dislocations and (ii) a nanoindentation system. The simulation results show that the proposed hybrid QC method is numerically more efficient than the original QC method while maintaining virtually the same accuracy.

Hybrid grains

We propose a technique to simulate granular materials that exploits the dual strengths of discrete and continuum treatments. Discrete element simulations provide unmatched levels of detail and generality, but prove excessively costly when applied to large scale systems. Continuum approaches are computationally tractable, but limited in applicability due to built-in modeling assumptions; e.g., models suitable for granular flows typically fail to capture clogging, bouncing and ballistic motion. In our hybrid approach, an oracle dynamically partitions the domain into continuum regions where safe, and discrete regions where necessary. The domains overlap along transition zones, where a Lagrangian dynamics mass-splitting coupling principle enforces agreement between the two simulation states. Enrichment and homogenization operations allow the partitions to evolve over time. This approach accurately and efficiently simulates scenarios that previously required an entirely discrete treatment.

Landau damping of Alfvénic modes in stellarators

It is found that the presence of the so-called non-axisymmetric resonances of wave–particle interaction in stellarators (which are associated with the lack of axial symmetry of the magnetic configuration, Kolesnichenko et al 2002 Phys. Plasmas 9 517) may have a strong stabilizing influence through Landau mechanism on the toroidicity-induced Alfvén Eigenmodes (TAE) and isomon modes (Alfvénic modes with equal poloidal and toroidal mode numbers and frequencies in the continuum region) destabilized by the energetic ions. These resonances involve largest harmonics of the equilibrium magnetic field of stellarators and lead to absorption of the mode energy by thermal ions in medium pressure plasma, in which case the effect is large. On the other hand, at the high pressure attributed to, e.g., a Helias reactor, thermal ions can interact also with high frequency Alfvén gap modes (helicity-induced Alfvén Eigenmodes and mirror-induced Alfvén Eigenmodes), leading to a considerable damping of these modes. Only resonances with passing particles are considered. The developed theory is applied to various modes in the Wendelstein 7-X stellarator and a Helias reactor, and to two TAE modes in the LHD helical device.

Cubic Fokker-Planck-DSMC hybrid method for diatomic rarefied gas flow through a slit and an orifice

Fokker-Planck kinetic models have been devised as an approximation of the Boltzmann collision operator. Cubic Fokker-Planck-DSMC hybrid method is employed to simulate the diatomic gas flow through a thin slit and a thin orifice. Pressure driven nitrogen expansion gas flows with two different pressure ratios are investigated at Knudsen number 0.001. The DSMC method is physically accurate for all flow regime; however it is computationally expensive in high density or near continuum regions. The Fokker-Planck-DSMC hybrid scheme employs DSMC in rarefied regions and Fokker-Planck method in near continuum flow regions for an efficient and accurate solution. Numerical procedures of the cubic Fokker-Planck method are implemented within the framework of an existing DSMC-solver, SPARTA. The Fokker-Planck-DSMC hybrid solution reproduces pure DSMC solution with improved computational efficiency up to a factor of five for vacuum flow through a thin orifice. In addition, breakdown of translational equilibrium is investigated. Domain criterion of FP-DSMC is safely smaller than Bird's breakdown criterion.

Cubic Fokker–Planck method for rarefied monatomic gas flow through a slit and an orifice

The flow through a thin slit and a thin orifice is studied with the Direct Simulation Monte Carlo (DSMC), cubic Fokker–Planck (FP), and a coupled FP-DSMC hybrid method. Pressure driven monatomic gas flows through a slit and an orifice with various values of degree of rarefaction and pressure ratio are computed. The DSMC method is physically accurate for all flow regimes; however it is computationally expensive in high density, near continuum regions. An alternative stochastic particle scheme, the cubic FP kinetic model has addressed this issue by approximating the particle collisions involved in the Boltzmann collision integral with continuous stochastic processes. The ability of the cubic FP method to reproduce breakdown of translational equilibrium is discussed. In addition, a coupled FP-DSMC hybrid scheme is employed aiming at an efficient and accurate solution. The FP-DSMC hybrid scheme employs DSMC in rarefied regions and FP method in near continuum flow regions. Numerical procedures of the cubic FP method are implemented within the framework of an existing DSMC-solver, SPARTA. The FP-DSMC hybrid solution reproduces pure DSMC solution with improved computational efficiency up to a factor of eight for vacuum flow through a thin slit.

Resolving the origin of hydrogen-line emission in YSOs with near-infrared interferometry

AbstractThe origin of the Brγ-line emission in Herbig Ae/Be stars is still an open question and might be related e.g., to a disc wind or the stellar magnetosphere. The study of the continuum and Brγ-emitting region of Herbig Ae/Be stars with high-spectral and high-spatial resolution gives great insights into the sub-au scale hydrogen gas distribution.We observed the Herbig Be star MWC 120 with the VLTI/AMBER instrument in different spectral channels across the Brγ line with a spectral resolution of R~1500. Using radiative transfer modeling we found a radius of the line emitting region of ~0.4 au that is only two times smaller than the K-band continuum region. This is consistent with a disc wind scenario rather than an origin of magnetospheric emission.We present near-infrared AMBER (R~12000) observations of the Herbig B[e] star MWC297 in the Brγ-line. We found that the near-infrared continuum emission is ~3.6 times more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged.

New correlations for slip flow and heat transfer over a micro spherical particle in gaseous fluid

In this work, a numerical model was established to simulate the gas flow and heat transfer over a micro spherical particle in the slip regime, in which the gas rarefaction effects (slip phenomena and gas compressibility) and temperature-dependent properties were considered. The Navier-Stokes equations and energy equation were adopted to govern the gas flow and heat transfer in the continuum region, for which the velocity slip and temperature jump boundary conditions were implemented at the gas-particle interface to predict the slip phenomena. The influences of the gas compressibility and temperature-dependent properties on the gas flow and heat transfer characteristics were investigated based on the numerical predictions. It shows that the drag force and heat transfer rate on the particle surface decrease due to the gas velocity slip and temperature jump, respectively. The drag force acting on the particle surface increases with the increase of particle temperature, which is caused by the increasing viscosity of the gas around the particle. The heat transfer coefficient decreases as the particle temperature increases, which is caused by the increasing thermal resistance due to the increase of temperature jump. Finally, the novel correlations for drag coefficient and Nusselt number of the rarefied gas flow over a micro spherical particle were proposed, which consider the influences of the gas rarefaction effects and temperature-dependent properties.

Consistent Iron Abundances Derived from Neutral and Singly Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-poor Stars∗

Abstract Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high-resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] ≈ −2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate archival UV spectra for three other warm dwarfs with [Fe/H] ≈ −3.3, −2.2, and −1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color–temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of Fe i and Fe ii lines with wavelengths from 2290 to 6430 Å. The [Fe/H] ratios derived separately from Fe i and Fe ii lines agree in all six stars, with [Fe ii/H]–[Fe i/H] ranging from +0.00 ± 0.07 to −0.12 ± 0.09 dex, when strong lines and Fe i lines with lower excitation potential &lt;1.2 eV are excluded. This constrains the extent of any deviations from LTE that may occur within this parameter range. While our result confirms non-LTE calculations for some warm, metal-poor dwarfs, it may not be generalizable to more metal-poor dwarfs, where deviations from LTE are predicted to be larger. We also investigate trends of systematically lower abundances derived from Fe i lines in the Balmer continuum region (≈3100–3700 Å), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.

Modelling of CO2 absorption via hollow fiber membrane contactors: Comparison of pore gas diffusivity models

This paper discusses the modelling of CO2 absorption in water as an absorbent via a hollow fiber membrane contactor to predict the performance of the absorption process under various operational conditions. The governing equations are two dimensional. Since, the gas diffusion along the pores is responsible for the mass transfer across the membrane thickness, i.e. both the skin layer and the membrane bulk, how to estimate the effective gas diffusivity has a vital degree of importance. When small pores combined with low pressures are dealt with, a challenging issue appears, i.e. the effective gas diffusivity is a contributive phenomenon, in which, several mechanisms are cooperated. Selecting an inappropriate model, inevitably leads to unrealistic values for the tuning parameters. Conventionally, the Bosanquet equation has been widely used for modelling the gas diffusivity through the membrane pores. In the current paper, it is intended to prove that the Bosanquet model is a limiting form of Wakao et al. model, when we are far enough from the continuum region. The modelling results are compared with six sets of different membranes from the open literature including high and low pressures. The results show that, ignoring the viscous flow contribution in Dg,eff (the Bosanquet gas diffusivity model) causes underestimation of the absorption flux. Based on the observations, there is a meaningful consistency between the model and the experimental data once the contribution of the viscous flow in Deff (the Wakao et al. gas diffusivity model) is considered.

The circumstellar environment of HD 50138 revealed by VLTI/AMBER at high angular resolution

Context. HD 50138 is a Herbig B[e] star with a circumstellar disc detected at infrared and millimeter wavelength. Its brightness makes it a good candidate for near-infrared interferometry observations. Aims. We aim to resolve, spatially and spectrally, the continuum and hydrogen emission lines in the 2.12–2.47 micron region, to shed light on the immediate circumstellar environment of the star. Methods. VLTI/AMBER K-band observations provide spectra, visibilities, differential phases, and closure phases along three long baselines for the continuum, and H I emission in Brγ and five high-n Pfund lines. By computing the pure line visibilities, we derive the angular size of the different line-emitting regions. A simple local thermodynamic equilibrium (LTE) model was created to constrain the physical conditions of H I emitting region. Results. The continuum region cannot be reproduced by a geometrical two-dimensional (2D) elongated Gaussian fitting model. We estimate the size of the region to be 1 au. We find the detected hydrogen lines (Brγ and Pfund lines) come from a more compact region of size 0.4 au. The Brγ line exhibits an S-shaped differential phase, indicative of rotation. The continuum and Brγ line closure phases show offsets of ~ −25 ± 5° and 20 ± 10° respectively. This is evidence of an asymmetry in their origin, but with opposing directions. We find that we cannot converge on constraints for the HI physical parameters without a more detailed model. Conclusions. Our analysis reveals that HD 50138 hosts a complex circumstellar environment. Its continuum emission cannot be reproduced by a simple disc brightness distribution. Similarly, several components must be evoked to reproduce the interferometric observables within the Brγ line. Combining the spectroscopic and interferometric data of the Brγ and Pfund lines favours an origin in a wind region with a large opening angle. Finally, although we cannot exclude the possibility that HD 50138 is a young star our results point to an evolved source.

Theoretical study of high-order harmonic generation from the hydrogen molecular ion with a dichromatic spatially inhomogeneous field

We theoretically investigate the enhancement of high-order harmonic generation by numerically solving the non-Born–Oppenheimer time-dependent Schrödinger equation from the hydrogen molecular ion in a dichromatic inhomogeneous laser field. An ultrabroad supercontinuum up to 300 orders spectral width is generated. It is found that not only the inhomogeneity, but also the dichromatic field contributes to the significant extension of the harmonic cutoff compared with a monochromatic inhomogeneous laser field. Meanwhile, the long quantum paths can be suppressed and short ones can be enhanced by selecting optimized inhomogeneous parameter [Formula: see text], intensity and carrier envelope phase of the dichromatic inhomogeneous laser field. Furthermore, by superposing a properly selected range of the harmonic spectrum in the continuum region, an isolated 29-as pulse is generated. Both the classical theory and quantum time–frequency analysis are adopted to explain the physical mechanism.

The contribution of faint AGNs to the ionizing background at z ~ 4

Context. Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in observational cosmology. Bright quasi-stellar objects (QSOs) are known to ionize their surrounding neighborhood, but they are too few to ensure the required HI ionizing background. A significant contribution by faint active galactic nuclei (AGNs), however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei at z &gt; 4. Aims. This work is part of a long-term project aimed at measuring the Lyman Continuum escape fraction for a large sample of AGNs at z ~ 4 down to an absolute magnitude of M1450 ~ −23. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U − I color interval, with I ~ 21–23, and 3.6 &lt; z &lt; 4.2. These AGNs are three magnitudes fainter than the typical SDSS QSOs (M1450 ≲−26) which are known to ionize their surrounding IGM at z ≳ 4. Methods. We acquired deep spectra of these faint AGNs with spectrographs available at the VLT, LBT, and Magellan telescopes, that is, FORS2, MODS1-2, and LDSS3, respectively. The emission in the Lyman Continuum region, close to 900 Å rest frame, has been detected with a signal to noise ratio of ~10–120 for all 16 AGNs. The flux ratio between the 900 Å rest-frame region and 930 Å provides a robust estimate of the escape fraction of HI ionizing photons. Results. We have found that the Lyman Continuum escape fraction is between 44 and 100% for all the observed faint AGNs, with a mean value of 74% at 3.6 &lt; z &lt; 4.2 and − 25.1 ≲ M1450 ≲−23.3, in agreement with the value found in the literature for much brighter QSOs (M1450 ≲−26) at the same redshifts. The Lyman Continuum escape fraction of our faint AGNs does not show any dependence on the absolute luminosities or on the observed U − I colors of the objects. Assuming that the Lyman Continuum escape fraction remains close to ~75% down to M1450 ~ − 18, we find that the AGN population can provide between 16 and 73% (depending on the adopted luminosity function) of the whole ionizing UV background at z ~ 4, measured through the Lyman forest. This contribution increases to 25–100% if other determinations of the ionizing UV background are adopted from the recent literature. Conclusions. Extrapolating these results to z ~ 5–7, there are possible indications that bright QSOs and faint AGNs can provide a significant contribution to the reionization of the Universe, if their space density is high at M1450 ~ −23.

Fully-differential top-pair production at a lepton collider: from threshold to continuum

We present an approach to predict exclusive {W}^{+}b{W}^{-}overline{b} production at lepton colliders that correctly describes the top-anti-top threshold as well as the continuum region. We incorporate toverline{t} form factors for the NLL threshold resummation derived in NRQCD into a factorized relativistic cross section using an extended double-pole approximation, which accounts for fixed-order QCD corrections to the top decays at NLO. This is combined with the full fixed-order QCD result at NLO for {W}^{+}b{W}^{-}overline{b} production to obtain predictions that are not only valid at threshold but smoothly transition to the continuum region. Our implementation is based on the Monte Carlo event generator Whizard and the code Toppik and allows to compute fully-differential threshold-resummed cross sections including the interference with non-resonant background processes. For the first time it is now possible to systematically study general differential observables at future lepton colliders involving the decay products of the top quarks at energies close to the pair production threshold and beyond.

O/H-N/O: the curious case of NGC 4670

We use integral field spectroscopic (IFS) observations from Gemini North Multi-Object Spectrograph (GMOS-N) of a group of four H II regions and the surrounding gas in the central region of the blue compact dwarf (BCD) galaxy NGC 4670. At spatial scales of $\sim$ 9 pc, we map the spatial distribution of a variety of physical properties of the ionised gas: internal dust attenuation, kinematics, stellar age, star-formation rate, emission line ratios and chemical abundances. The region of study is found to be photoionised. Using the robust direct T$_e$-method, we estimate metallicity, nitrogen-to-oxygen ratio and helium abundance of the four H II regions. The same parameters are also mapped for the entire region using the HII-CHI-mistry code. We find that log(N/O) is increased in the region where the Wolf-Rayet bump is detected.The region coincides with the continuum region, around which we detect a slight increase in He abundance. We estimate the number of WC4, WN2-4 and WN7-9 stars from the integrated spectrum of WR bump region. We study the relation between log(N/O) and 12 + log(O/H) using the spatially-resolved data of the FOV as well as the integrated data of the H II regions from ten BCDs. We find an unexpected negative trend between N/O and metallicity. Several scenarios are explored to explain this trend, including nitrogen enrichment, and variations in star formation efficiency via chemical evolution models.

Modeling of heat and mass transfer for dropwise condensation of moist air and the experimental validation

A single droplet model is developed to describe the droplet growth during dropwise condensation of moist air on a cold substrate. The condensation process is divided by the droplet surface into two parts. The first step, i.e. the processes of mass transfer from the surroundings to the droplet surface, is modeled by the Kinetic theory and the laws of continuum fluid dynamics formulated using the two-region concept (Knudsen layer and continuum region) at any droplet size and at any concentration of non-condensable gas (NCG). The second step, i.e. the heat transfer across the droplet, is governed by Fourier’s law of heat conduction. These three regions (the continuum region, the Knudsen layer and the region inside the droplet) are incorporated by the matching both the mass flow rates and the energy flow rates. From these, the droplet growth rate, the nucleation size of droplet, the temperature at the droplet surface and Knudsen layer interface can be evaluated depending on different conditions. For this, a numerical algorithm is developed to reflect the droplet dynamics sufficiently detailed, including nucleation, growth/coalescence, slide-off/fall-off, re-nucleation. This is applied to the simulation of the entire condensation process by putting the growth rate and minimum radius from single droplet model into the growth algorithm. Additionally, dropwise condensation experiments of moist air in different relative humidity (RH) are carried out for validation of the simulation results. Good agreement is obtained which demonstrates that the present droplet growth model for dropwise condensation of moist air is credible. The current model and experiments also indicate that the diffusion resistance of water vapor in air from the free stream toward the droplet surface has a significant influence for the heat transfer performance of dropwise condensation of moist air.

Analysis of the Residual-Type and the Gradient Recovery-Type a Posteriori Error Estimators for a Consistent Atomistic-to-Continuum Coupling Method in One-Dimension

We consider the a posteriori error estimation for an atomistic-to-continuum coupling scheme for a generic one-dimensional many-body next-nearest-neighbor interaction model in one dimension. We derive and rigorously prove the efficiency of the residual type estimator. We prove the equivalence between the residual type and the gradient recovery-type estimator in the continuum region and propose a (novel) hybrid a posteriori error estimator by combining the two types of estimators. Our numerical experiments illustrate the optimal convergence rate of the adaptive algorithms using these estimators, whose effectivity indices are also presented.

Saturation technique for the measurement of photoionization cross-section of atomic excited states—A review

A review on the saturation technique is presented which has made utilized extensively for the absolute measurements of the photoionization cross section of the atomic excited states. This technique employed two step photoionization, in first step the state whose photoionization cross section to be measured is excited and in second step the excited state is photoionized. The energy of the exciting laser kept fixed and that of the ionizing laser is varied until the photo-ion signal gets saturated. Least square fitting of the photo-ion signals verses the ionizing laser energy density yields the photoionization cross section. Different experimental methodologies in combination with saturation technique have been employed for the measurement of photoionization cross section. These include heat cell with capacitive electrodes, atomic beam apparatus, thermionic diode ion detector, magneto optical traps (MOT), atomic beam-time of flight (TOF) mass spectrometer and DC/RF glow discharge cell. Thermionic diode ion detector and DC discharge cell offer an additional advantage of the measurement of photo absorption cross section (oscillator strength) of the atomic excited states. In addition, the availability of intense tunable dye laser made it possible to probe different region of continuum i.e. to search the regions of copper minima.

Passing waves from atomistic to continuum

Progress in the development of coupled atomistic–continuum methods for simulations of critical dynamic material behavior has been hampered by a spurious wave reflection problem at the atomistic–continuum interface. This problem is mainly caused by the difference in material descriptions between the atomistic and continuum models, which results in a mismatch in phonon dispersion relations. In this work, we introduce a new method based on atomistic dynamics of lattice coupled with a concurrent atomistic–continuum method to enable a full phonon representation in the continuum description. This permits the passage of short-wavelength, high-frequency phonon waves from the atomistic to continuum regions. The benchmark examples presented in this work demonstrate that the new scheme enables the passage of all allowable phonons through the atomistic–continuum interface; it also preserves the wave coherency and energy conservation after phonons transport across multiple atomistic–continuum interfaces. This work is the first step towards developing a concurrent atomistic–continuum simulation tool for non-equilibrium phonon-mediated thermal transport in materials with microstructural complexity.

Coupling atomistic and continuum modelling of magnetism

In this article, a new energy-based atomistic–continuum partitioned-domain multiscale method for magnetisation dynamics is proposed. The main feature of the method is the minimised mismatch between the local continuum description and the non-local atomistic description (in which non-nearest neighbour interatomic interactions are present). The error at the atomistic–continuum interface is minimised by constructing an intermediate region consisting of transition atoms, which interact differently with the neighbouring atomistic and continuum regions. When the mesh of the continuum region is refined to the atomistic scale at the atomistic–continuum interface and an energy-conserving time-stepping scheme is used for the entire computational domain, the method conserves the total energy of the system. The second feature of the method is the introduction of a damping band at the atomistic–continuum interface, which absorbs high-frequency waves that are otherwise reflected from the interface and thereby contribute to the error inside the atomistic region. Several examples of domain wall motion and spin wave propagation in one- and two-dimensional structures are used to illustrate the applicability of the method and to investigate its limitations.