Mass Constraints Research Articles

Neutron star mass estimates from gamma-ray eclipses in spider millisecond pulsar binaries

Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling.

Shape optimization of light structures and the vanishing mass conjecture

This work proves rigorous results about the vanishing mass limit of the classical problem to find a shape with minimal elastic compliance. Contrary to all previous results in the mathematical literature, which utilize a soft mass constraint by introducing a Lagrange multiplier, we here consider the hard mass constraint. Our results are the first to establish the convergence of approximately optimal shapes of (exact) size ε↓0 to a limit generalized shape represented by a (possibly diffuse) probability measure. This limit generalized shape is a minimizer of the limit compliance, which involves a new integrand, namely the one conjectured by Bouchitté in 2001 and predicted heuristically before in works of Allaire, Kohn, and Strang from the 1980s and 1990s. This integrand gives the energy of the limit generalized shape understood as a fine oscillation of (optimal) lower-dimensional structures. Its appearance is surprising since the integrand in the original compliance is just a quadratic form and the nonconvexity of the problem is not immediately obvious. In fact, it is the interaction of the mass constraint with the requirement of attaining the loading (in the form of a divergence constraint) that gives rise to this new integrand. We also present connections to the theory of Michell trusses, first formulated in 1904, and show how our results can be interpreted as a rigorous justification of that theory on the level of functionals in both two and three dimensions, settling this open problem. Our proofs rest on compensated compactness arguments applied to an explicit family of (symmetric) div-quasiconvex quadratic forms, computations involving the Hashin–Shtrikman bounds for the Kohn–Strang integrand, and the characterization of limit minimizers due to Bouchitté and Buttazzo.

Constraining the Densities of the Three Kepler-289 Planets with Transit Timing Variations

Kepler-289 is a three-planet system containing two sub-Neptunes and one cool giant planet orbiting a young, Sun-like star. All three planets exhibit transit timing variations (TTVs), with both adjacent planet pairs having orbital periods close to the 2:1 orbital resonance. We observe two transits of Kepler-289c with the Wide-field InfraRed Camera on the 200″ Hale Telescope at Palomar Observatory, using diffuser-assisted photometry to achieve space-like photometric precision from the ground. These new transit observations extend the original four-year Kepler TTV baseline by an additional 7.5 yr. We rereduce the archival Kepler data with an improved stellar activity correction and carry out a joint fit with the Palomar data to constrain the transit shapes and derive updated transit times. We then model the TTVs to determine the masses of the three planets and constrain their densities and bulk compositions. Our new analysis improves on previous mass and density constraints by a factor of two or more for all three planets, with the innermost planet showing the largest improvement. Our updated atmospheric mass fractions for the inner two planets indicate that they have hydrogen-rich envelopes, consistent with their location on the upper side of the radius valley. We also constrain the heavy element composition of the outer Saturn-mass planet, Kepler-289c, for the first time, finding that it contains 30.5 ± 6.9 M ⊕ of metals. We use dust evolution models to show that Kepler-289c must have formed beyond 1 au, and likely beyond 3 au, and then migrated inward.

Harnessing cavity dissipation for enhanced sound absorption in Helmholtz resonance metamaterials.

Helmholtz resonance, based on resonance through a pore-and-cavity structure, constitutes the primary sound absorption mechanism in majority of sound-absorbing metamaterials. Typically, enhancing sound absorption in such absorbers necessitates substantial geometrical redesign or the addition of dissipative materials, which is non-ideal considering the volume and mass constraints. Herein, we introduce a new approach - that is to simply reshape the cavity, without alterations to its overall mass and volume - to drastically enhance sound absorption. This is achieved by bringing the cavity walls close to the pores where additional thermoviscous dissipation along these boundaries can occur. Experimentally validated, with three sides of the cuboid cavity close to the pore and at a particular pore-cavity geometry, a 44% gain in maximum absorption is achieved compared to the original structure. Through numerical simulations, we fully elucidate structure-property relationships and their mechanisms, and propose analytical models for design and optimization. Ultimately, utilizing this concept, we demonstrate a heterogeneously porous broadband (1500 to 6000 Hz) absorber that exhibits an excellent average absorption coefficient of 0.74 at a very low thickness of 18 mm. Overall, we introduce a new and universal concept that could revolutionize the design principles of Helmholtz resonators, and demonstrate its potential for designing advanced sound-absorbing metamaterials.

Signature of Massive Neutrinos from the Clustering of Critical Points. I. Density-threshold-based Analysis in Configuration Space

Critical points represent a subset of special points tracing cosmological structures, carrying remarkable topological properties. They thus offer a richer high-level description of the multiscale cosmic web, being more robust to systematic effects. For the first time, we characterize here their clustering statistics in massive neutrino cosmologies, including cross-correlations, and quantify their simultaneous imprints on the corresponding web constituents—i.e., halos, filaments, walls, and voids—for a series of rarity levels. Our first analysis is centered on a density-threshold-based approach in configuration space. In particular, we show that the presence of massive neutrinos does affect the baryon acoustic oscillation peak amplitudes of all of the critical point correlation functions above/below the rarity threshold, as well as the positions of their correspondent inflection points at large scales: departures from analogous measurements carried out in the baseline massless neutrino scenario can reach up to ∼7% in autocorrelations and ∼9% in cross-correlations at z = 0 when M ν = 0.1 eV and are more pronounced for higher neutrino mass values. In turn, these combined multiscale effects can be used as a novel technique to set upper limits on the summed neutrino mass and infer the type of hierarchy. Our study is particularly relevant for ongoing and future large-volume redshift surveys such as the Dark Energy Spectroscopic Instrument and the Rubin Observatory Legacy Survey of Space and Time, which will provide unique data sets suitable for establishing competitive neutrino mass constraints.

Enhancement of the vibration attenuation characteristics in local resonance metamaterial beams: Theory and experiment

Enhancing vibration isolation with local resonance metamaterials has attracted widespread attention due to the low-frequency band gap. However, the narrow band gaps limit its application. To improve vibration suppression properties over a wide frequency range under the constraints of space and mass in engineering, this paper presents the modeling techniques and design strategies for a local resonance elastic metamaterial beam with multiple resonators. The local resonance sub-system contains multi-degree-of-freedom resonators and is periodically mounted on the elastic beam. Two special cases, the elastic metamaterial beam with one-degree-of-freedom local resonator sub-system (1-DOF) and two-degree-of-freedom local resonator sub-system (2-DOF) are used to investigate the vibration attenuation characteristics in detail. Results are presented in the form of attenuation constant, which are calculated by the extended plane wave expansion (EPWE) method. The effects of frequency spacing, damping and mass ratio on the vibration attenuation characteristics of elastic metamaterial beams are comprehensively investigated. The results show that a wider and more stable attenuation range can be obtained by reasonably adjusting these three parameters. Finally, the comparison of experimental and simulation results demonstrates that the design strategies proposed in this paper can broaden the vibration attenuation regions. The theoretical approaches and design schemes can provide effective guidance for passive vibration control.

Universal relations for neutron star f -mode and g -mode oscillations

Among the various oscillation modes of neutron stars, $f$- and $g$- modes are the most likely to be ultimately observed in binary neutron star mergers due to their relatively large coupling and shared frequencies with tidal excitations. The $f$-mode frequency and damping time are known to correlate in normal neutron stars with their compactness, and previous fits to hadronic stars are extended and shown to be valid for an extremely broad sampling of equations of state using a piecewise polytropic parametrization scheme for hadrons and a constant sound-speed parametrization for quark matter. Separate fits applicable to quark (self-bound) stars are improved. Much more significant correlations exist with tidal deformability, and therefore with moment of inertia and quadrupole moment. It is conclusively demonstrated that these correlations are the same for all types of stars, whether hadronic, hybrid, or pure quark, and its accuracy is quantified. A novel 1-node branch of the $f$-mode that occurs in low-mass hybrid stars in a narrow mass range just beyond the critical mass necessary for a phase transition to appear is identified. This 1-node branch shows the largest, but still small, deviations from the universal correlation for any configuration. It is characterized by a nonmonotonic relation between neutron star mass and $f$-mode frequency, in contrast to the behavior otherwise observed in normal, quark and hybrid stars. The $g$-mode only exists in matter with a nonbarotropic equation of state involving temperature, chemical potential or composition (such as being out of beta equilibrium), or a phase transition in barotropic matter. The $g$-mode therefore could serve as a probe for studying phase transitions in hybrid stars. In contrast with the $f$-mode, $g$-mode frequencies do not correlate well with tidal deformability, but depend strongly on properties of the transition (the density and the magnitude of the discontinuity) at the transition. Imposing causality and maximum mass constraints, a fit involving neutron star and phase transition properties is found and the $g$-mode frequency is determined to have an upper bound of about 1.25 kHz. However, if the sound speed ${c}_{s}$ in the inner core at densities above the phase transition density is restricted to ${c}_{s}^{2}\ensuremath{\le}1/3$, $g$-mode frequencies can only reach about 0.8 kHz, which are significantly lower than $f$-mode frequencies (1.3--2.8 kHz). $g$-mode gravitational wave damping times are found to be extremely long, $>{10}^{4}\text{ }\text{ }\mathrm{s}$ (${10}^{2}\text{ }\text{ }\mathrm{s}$) in the inner core with ${c}_{s}^{2}\ensuremath{\le}1/3$ (1), in comparison with $f$-mode damping times (0.1--1 s).

Revisiting the assignment of $$1^{3}D_{1}$$ meson nonet

Based on the mass relations from the Regge phenomenology, we revisit the mass spectrum of \(1^{3}D_{1}\) meson nonet. The masses of kaon and \(s{\bar{s}}\) member of \(1^{3}D_{1}\) (\(1^{3}D_{1}(s{\bar{s}})\)) meson nonet are obtained and the results are compared with the values from different theoretical models. Moreover, the strong decay properties of the kaon and \(1^{3}D_{1}(s{\bar{s}})\) are presented in the \(^{3}P_{0}\) model. On the basis of results, we suggest the assignment of \(1^{3}D_{1}\) meson nonet need further tested in the new experiment in the future. Our results also provide mass constraints for the study of these states.

Extending empirical constraints on the SZ–mass scaling relation to higher redshifts via HST weak lensing measurements of nine clusters from the SPT-SZ survey at z ≳ 1

We present a Hubble Space Telescope (HST) weak gravitational lensing study of nine distant and massive galaxy clusters with redshifts 1.0 ≲ z ≲ 1.7 (zmedian = 1.4) and Sunyaev Zel’dovich (SZ) detection significance ξ > 6.0 from the South Pole Telescope Sunyaev Zel’dovich (SPT-SZ) survey. We measured weak lensing galaxy shapes in HST/ACS F606W and F814W images and used additional observations from HST/WFC3 in F110W and VLT/FORS2 in UHIGH to preferentially select background galaxies at z ≳ 1.8, achieving a high purity. We combined recent redshift estimates from the CANDELS/3D-HST and HUDF fields to infer an improved estimate of the source redshift distribution. We measured weak lensing masses by fitting the tangential reduced shear profiles with spherical Navarro-Frenk-White (NFW) models. We obtained the largest lensing mass in our sample for the cluster SPT-CL J2040−4451, thereby confirming earlier results that suggest a high lensing mass of this cluster compared to X-ray and SZ mass measurements. Combining our weak lensing mass constraints with results obtained by previous studies for lower redshift clusters, we extended the calibration of the scaling relation between the unbiased SZ detection significance ζ and the cluster mass for the SPT-SZ survey out to higher redshifts. We found that the mass scale inferred from our highest redshift bin (1.2 < z < 1.7) is consistent with an extrapolation of constraints derived from lower redshifts, albeit with large statistical uncertainties. Thus, our results show a similar tendency as found in previous studies, where the cluster mass scale derived from the weak lensing data is lower than the mass scale expected in a PlanckνΛCDM (i.e. νΛ cold dark matter) cosmology given the SPT-SZ cluster number counts.

Once in a summer: Fall history of the JaH 073 strewn field, Sultanate of Oman

AbstractModeling of a prehistoric fall can be successful if a strewn field is very well documented and coordinates, masses, and shapes of all individual stones are recorded. In combination with meteoroid mass and wind model constraints, a detailed scenario of the atmospheric passage is obtained for the ~20 × 6 km‐sized JaH 073 L6 strewn field in Oman. The wide mass ranges from 52.2 kg to <1 g together with the large number of ~3500 stones offer the statistical basis to reconstruct the trajectory and the fragmentation sequence. The size of the meteoroid, constrained by noble gas analyses, corresponds to an initial mass of about 12 t at atmospheric entry using an L‐chondrite bulk density of 3400–3500 kg m−3. Assuming typical ablation behavior, these data are compatible with an entry velocity of 20 ± 3 km s−1. The best model fit is achieved for a serial fragmentation scenario starting at an altitude of ~34 km and showing a main fragmentation at 26 km. A resolved event seems to have occurred at 22 km, followed by a more diffuse fragmentation at 19 km. The vertical trajectory angle is calculated at 43 ± 2° and the azimuth at 329 ± 1°. The position of numerous outlying meteorites in the strewn field can only be reproduced by repeated fragmentation with cumulated transverse velocities from explosive events. The wind model adopted from modern data fits surprisingly well and indicates summer monsoon with strong easterly winds during the fall event, consistent with paleoclimatic data.

Fermion masses and mixings, dark matter, leptogenesis and $$g-2$$ muon anomaly in an extended 2HDM with inverse seesaw

We propose a predictive \(Q_4\) flavored 2HDM model, where the scalar sector is enlarged by the inclusion of several gauge singlet scalars and the fermion sector by the inclusion of right-handed Majorana neutrinos. In our model, the \(Q_4\) family symmetry is supplemented by several auxiliary cyclic symmetries, whose spontaneous breaking produces the observed pattern of SM charged fermion masses and quark mixing angles. The light active neutrino masses are generated from an inverse seesaw mechanism at one loop level thanks to a remnant preserved \(Z_2\) symmetry. Our model successfully reproduces the measured dark matter relic abundance and is consistent with direct detection constraints for masses of the DM candidate around \(\sim\) 6.3 TeV. Furthermore, our model is also consistent with the lepton and baryon asymmetries of the Universe as well as with the muon anomalous magnetic moment.

Cloud Detection Autonomous System Based on Machine Learning and COTS Components On-Board Small Satellites

One of the main applications of small satellites is Earth observation. CubeSats and different kinds of nanosatellites usually form constellations that obtain images mainly using an optical payload. There is a massive amount of data generated by these satellites and a limited capacity of download due to volume and mass constraints that make it difficult to use high-speed communication systems and high-power systems. For this reason, it is important to develop satellites with the autonomy to process data on board. In this way, the limited communication channel can be used efficiently to download relevant images containing the required information. In this paper, a system for the satellite on-board processing of RGB images is proposed, which automatically detects the cloud coverage level to prioritize the images and effectively uses the download time and the mission operation center. The system implements a Convolutional Neural Network (CNN) on a Commercial off-the-Shelf (COTS) microcontroller that receives the image and returns the cloud level (priority). After training, the system was tested on a dataset of 100 images with an accuracy of 0.9 and it was also evaluated with CubeSat images to evaluate the performance of a different image sensor. This implementation contributes to the development of autonomous satellites with processing on board.

Neutron star mass formula with nuclear saturation parameters for asymmetric nuclear matter

Low-mass neutron stars are directly associated with the nuclear saturation parameters because their central density is definitely low. We have already found a suitable combination of nuclear saturation parameters for expressing the neutron star mass and gravitational redshift, i.e., $\eta\equiv (K_0L^2)^{1/3}$ with the incompressibility for symmetric nuclear matter, $K_0$, and the density-dependent nuclear symmetry energy, $L$. In this study, we newly find another suitable combination given by $\eta_\tau\equiv (-K_\tau L^5)^{1/6}$ with the isospin dependence of incompressibility for asymmetric nuclear matter, $K_\tau$, and derive the empirical relations for the neutron star mass and gravitational redshift as a function of $\eta_\tau$ and the normalized central number density. With these empirical relations, one can evaluate the mass and gravitational redshift of the neutron star, whose central number density is less than threefold the saturation density, within $\sim 10\%$ accuracy, and the radius within a few \% accuracies. In addition, we discuss the neutron star mass and radius constraints from the terrestrial experiments, using the empirical relations, together with those from the astronomical observations. Furthermore, we find a tight correlation between $\eta_\tau$ and $\eta$. With this correlation, we derive the constraint on $K_\tau$ as $-348\le K_\tau\le -237$ MeV, assuming that $L=60\pm 20$ and $K_0=240\pm 20$ MeV.

Lightweight Optimization Design of Structures with Multiple Cellular Materials

Cellular materials have been widely applied to a lightweight design of structures. The mechanical properties of those materials depend on their microstructures at the microlevel/mesolevel, and the optimizaiton design of lightweight structures using multiple cellular materials is still challenging. This paper develops a topology optimization algorithm for a lightweight design of structures constructed by multiple cellular materials with specified microstructures. The mechanical properties of cellular materials are homogenized according to their microstructures and then integrated into topology optimization. The topology optimization problem is defined by minimizing structural compliance subject to a specified mass constraint. In order to identify the distribution of multiple cellular materials within the design domain, the multiple design variables are introduced based on the volume fractions of multiple cellular materials within each element. Meanwhile, the homogenized mechanical properties are linearly interpolated, and multiple floating projection constraints are imposed on the relaxed design variables to push them toward 0 or 1. Numerical examples demonstrate the successful implementation of the proposed algorithm by the optimal distribution and selection of multiple cellular materials.

Effects of Impurities and Operating Mode on The Dynamics of Direct Butane Proton-Exchange Membrane Fuel Cells

Maximizing the energy density of a portable power system is an important design factor since those systems have stringent maximum mass constraints. While Lithium-ion batteries are currently the standard choice for power systems in the scale of 10-100 watts, direct hydrocarbon Proton-exchange membrane fuel cells (PEMFC) have the potential to be more energy-dense than batteries at those scales even with efficiencies as low as %10. PEMFC are simple and operate at low temperatures which make them a suitable choice for portable power generation. Because of their low operating temperature, they are usually fueled by hydrogen. However, hydrogen is hard to store and handle, which makes it an unsuitable fuel for portable power applications. Alternatively, Hydrocarbon fuels, such as butane, are easier to store and safer to handle. Additionally, hydrocarbon fuels are ~50 times more energy dense than state of the art Lithium-ion batteries. For those reasons, it is desirable to develop a direct butane PEMFC for portable power applications. Limited work has been done in the past to demonstrate the viability of direct butane fuel cells, mainly because of the low reactivity of hydrocarbon in low temperature PEMFC. EH Kong et. al. demonstrated a working direct butane PEMFC by adding trace amounts of unsaturated hydrocarbons to the butane and operating the cell in “load-interrupt” mode. The authors proposed that butane might be converting active catalyst sites to inactive sites, causing the fuel cell to ‘extinguish’. Thus, the addition of unsaturated hydrocarbons and the load-interrupt operating mode can circumcise this saturation process. In this work, we demonstrate the repeatability of the direct butane PEMFC results produced by EH Kong et. al. on a custom-built fuel cell testing apparatus and explore methods to explain the current dynamics of the cell.For the experimental setup, we used a 25cm2 membrane electrode assembly (MEA). It consisted of treated Nafion N-177 layer directly painted with a catalyst layer on the anode and cathode sides. The catalyst layer is obtained from an ink that is prepared by mixing platinumblack, water, and 5% Nafionsolution. The Nafion N-177 layer is prepared using the manufacturer standard pretreatment procedure. Once assembled, the MEA layer is then pressed at 120 °C and 500 lbs. for 25 min. We then load the MEA layer into the fuel cell hardware, which includes the fuel cell, humidifiers, and heaters, and test the cell electrical performance at temperatures below (100C).Using the fuel cell testing apparatus, we confirm that for high purity butane (>%99.99) the PEMFC does not start and no open circuit voltage is detected. However, the PEMFC is started when unsaturated hydrocarbons are added to the butane fuel stream as shown in previous work. Specifically, the addition of ethylene at concentrations of 2000 ppm allows the PEMFC to operate continuously at low temperatures (<100C). Previously observed current dynamics were also reproduced. The current density of the fuel cell peaks in the beginning of the cell operation and decreases over time until it reaches steady state. It was observed that operating the cell in a load-interrupt mode, where the current is set to zero for 5 seconds every 25 second of operation, increases the average power density of the cell, when compared to the galvanostatic operating mode. Further we investigate the effect of different additives to butane. Specifically, we explore the effect of adding different concentration of hydrogen to the butane stream to analyze whether the interaction between the two fuels is independent or synergetic. Furthermore, we produce Electrochemical Impedance spectrums of the direct butane PEMFC to gain insights about cell resistances including mass transport limitations.For future work, we are interested in in-situ measurement techniques that allow us to observe the intermediates on the catalyst surface. Such data might allow us to verify and inform the empirical dynamical model proposed in previous work to explain the direct butane PEMFC current dynamics.

Normalized solutions to p-Laplacian equations with combined nonlinearities* *Supported by National Natural Science Foundation of China (12031015, 11771428, 12026217, 11871302).

In this paper, we study the p-Laplacian equation with a L p -norm constraint: where N ⩾ 2, a > 0, , , and is a Lagrange multiplier, which appears due to the mass constraint ‖u‖ p = a. We assume that g is odd and L p -supercritical. When and μ > 0, we use Schwarz rearrangement and Ekeland variational principle to prove the existence of positive radial ground states for suitable μ. When and μ > 0 or and μ ⩽ 0, with an additional condition of g, we obtain a positive radial ground state if μ lies in a suitable range, by the Schwarz rearrangement and minimax theorems. Via a fountain theorem type argument, with suitable , we obtain infinitely many radial solutions for any N ⩾ 2 and establish the existence of infinitely many nonradial sign-changing solutions for N = 4 or N ⩾ 6. In any case mentioned above, the range of μ depends on the value of a: |μ| can be large if a > 0 is small.

Bayesian inference of signatures of hyperons inside neutron stars

The possible signatures of the presence of hyperons inside neutron stars are discussed within a Bayesian inference framework applied to a set of models based on a density-dependent relativistic mean-field description of hadronic matter. Nuclear matter properties, hypernuclei properties, and observational information are used to constrain the models. General properties of neutron stars such as the maximum mass, radius, tidal deformability, proton fraction, hyperon fraction, and speed of sound are discussed. It is shown that the two solar mass constraint imposes that neutron stars described by equations of state that include hyperons have on average a larger radius, $\gtrsim 0.5$km, and a larger tidal deformability, $\gtrsim 150$, than the stars determined from a nucleonic equation of state, while the speed of sound at the center of the star is more than 25\% smaller. If a 1.4 M$_\odot$ star with a radius $\lesssim 12.5$ km is measured it is quite improbable that a massive star described by the same model contains hyperons. A similar conclusion is drawn if a two solar mass star with a radius $\lesssim 11.5$ km or a neutron star with a mass above 2.2 M$_\odot$ is observed: the possible hyperon content of these stars is ruled out or very reduced. The hyperon presence inside neutron stars is compatible with the present NICER mass-radius observations of the pulsars PSR J0030+0451 and PSR J0740+6620 and the gravitational wave detection GW170817. {It is shown that if the polytropic index $\gamma=\partial\ln p/\partial\ln\epsilon$ takes values of the order of 1.75 at not too large densities, it may indicate the onset of some kind of exotic matter, but not necessarily of deconfined quark matter.

Estimation of a Fission Product Concentration Distribution in a Spherical Body from Surface Release Measurements

The diffusion of fission products (FPs) in reactor materials affects the nuclear source term. The diffusion coefficient itself is measured through various techniques. In the release method, it is of interest to know the initial distribution of the FPs in nuclear graphite or other materials from an exterior measurement like mass surface flux or cumulative mass release. In this paper, a Fredholm integral of the first kind is considered, relating the initial distribution to the cumulative release fraction of a diffusant from a spherically symmetric body. The Tikhonov regularization, conjugate gradient least-squares (CGLS) method, and algebraic reconstruction technique (ART) with nonnegativity and conserved mass constraints were compared to fractional release data from a simulated linear profile using data for Cs diffusion in a 0.32-cm sphere NBG-18 at 1090 K. The Tikhonov regularization was shown to provide a better estimation of the initial linear distribution than the CGLS and ART methods. The performance of the Tikhonov regularization was further examined with simulated constant, quartic, and exponential initial distributions. The Tikhonov regularization provided a reasonable recovery of the exponential profile, while the estimation of the linear, constant, and quartic profiles suffers from several issues.

Scalable Microfabrication of Multi-Emitter Arrays in Silicon for a Compact Microfluidic Electrospray Propulsion System.

The recent proliferation of SmallSats and their use in increasingly demanding applications require the development of onboard electric propulsion compatible with the power, mass, and volume constraints of these spacecraft. Electrospray propulsion is a promising technology for SmallSats due to its unique high efficiency and scalability across the wide power range of these platforms, for example, from a few watts available in a CubeSat to a few hundred watts in a MiniSat. The implementation of electrospray propulsion requires the use of microfabrication techniques to create compact arrays of thousands of electrospray emitters. This article demonstrates the microfabrication of multi-emitter electrospray sources of a scalable size for electrospray propulsion. In particular, a microfabrication and assembly process is developed and demonstrated by fabricating sources with arrays of 1, 64, and 256 emitters. The electrospray sources are tested in a relevant environment for space propulsion (inside a vacuum chamber), exhibiting excellent propulsive performance (e.g., absence of beam impingement in the extractor electrode, absence of hysteresis in the beam current versus propellant flow rate characteristic, proper operation in the cone-jet electrospraying mode, etc.) and nearly coincident output per emitter. Several design elements contribute to this performance: the even distribution of the propellant among all emitters made possible by the implementation of a network of microfluidic channels in the backside of the emitter array; the small dead volume of the network of microfluidic channels; the accurate alignment between the emitters and extractor orifices; and the use of a pipe-flow configuration to drive the propellant through closed conduits, which protects the propellant.

Mass Upper Bounds for Over 50 Kepler Planets Using Low-S/N Transit Timing Variations

Prospects for expanding the available mass measurements of the Kepler sample are limited. Planet masses have typically been inferred via radial velocity (RV) measurements of the host star or time-series modeling of transit timing variations (TTVs) in multiplanet systems; however, the majority of Kepler hosts are too dim for RV follow-up, and only a select number of systems have strong enough TTVs for time-series modeling. Here, we develop a method of constraining planet mass in multiplanet systems using low signal-to-noise ratio (S/N) TTVs. For a sample of 175 planets in 79 multiplanet systems from the California-Kepler Survey, we infer posteriors on planet mass using publicly available TTV time series from Kepler. For 53 planets (>30% of our sample), low-S/N TTVs yield informative upper bounds on planet mass, i.e., the mass constraint strongly deviates from the prior on mass and yields a physically reasonable bulk composition. For 25 small planets, low-S/N TTVs favor volatile-rich compositions. Where available, low-S/N TTV-based mass constraints are consistent with RV-derived masses. TTV time series are publicly available for each Kepler planet, and the compactness of Kepler systems makes TTV-based constraints informative for a substantial fraction of multiplanet systems. Leveraging low-S/N TTVs offers a valuable path toward increasing the available mass constraints of the Kepler sample.