Empirical Constraints Research Articles

Spectroscopic Measurements of the Ion Velocity Distribution at the Base of the Fast Solar Wind

Abstract In situ measurements of the fast solar wind reveal non-thermal distributions of electrons, protons, and minor ions extending from 0.3 au to the heliopause. The physical mechanisms responsible for these non-thermal properties and the location where these properties originate remain open questions. Here, we present spectroscopic evidence, from extreme ultraviolet spectroscopy, that the velocity distribution functions (VDFs) of minor ions are already non-Gaussian at the base of the fast solar wind in a coronal hole, at altitudes of <1.1 R ⊙. Analysis of Fe, Si, and Mg spectral lines reveals a peaked line-shape core and broad wings that can be characterized by a kappa VDF. A kappa distribution fit gives very small kappa indices off-limb of κ ≈ 1.9–2.5, indicating either (a) ion populations far from thermal equilibrium, (b) fluid motions such as non-Gaussian turbulent fluctuations or non-uniform wave motions, or (c) some combination of both. These observations provide important empirical constraints for the source region of the fast solar wind and for the theoretical models of the different acceleration, heating, and energy deposition processes therein. To the best of our knowledge, this is the first time that the ion VDF in the fast solar wind has been probed so close to its source region. The findings are also a timely precursor to the upcoming 2018 launch of the Parker Solar Probe, which will provide the closest in situ measurements of the solar wind at approximately 0.04 au (8.5 solar radii).

Young star clusters in nearby molecular clouds

The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study of the young stellar populations in 22 star forming regions with distances <=1 kpc designed to extend our earlier MYStIX survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using parametric mixture models applied homogeneously to the catalog of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association to molecular clouds, absorption, age (AgeJX), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and AgeJX are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high ellipticities aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ~0.08 to ~0.9 pc over the age range 1--3.5 Myr. Inferred gas removal timescales are longer than 1 Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An Appendix compares the performance of the mixture models and nonparametric Minimum Spanning Tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disk longevity, age gradients, and dynamical modeling.

Late Neoproterozoic adakitic magmatism of the eastern Arabian Nubian Shield

Late Neoproterozoic adakitic magmatism within the Eastern Arabian Nubian Shield has been dated at 633.2 ± 9.0 Ma (2σ). These magmas intrude the forearc Ad Dawadimi Basin, which is composed of metapelitic schists and greywacke along with ophiolitic mélanges of boninitic affinity which underwent inversion and deformation by ∼620 Ma. This adakitic magmatism, while intruding parts of the Ad Dawadimi Basin, predates this deformation, but is possibly coincident with basin closure. As adakitic magmatism requires melting of an amphibolite or eclogitic source, empirical and experimental constraints require anomalously hot supra-subduction zone mantle. Considering that this magmatism immediately predates basin inversion, these magmas possibly pinpoint the timing of the slab breaking, marking the terminal stages of arc magmatism, terrane accretion and the influx of hot oceanic asthenospheric mantle. This influx of hot asthenospheric mantle may also be responsible for post-collisional A-type magmatism.

Rates of Deep Continental Burial From Lu‐Hf Garnet Chronology and Zr‐in‐Rutile Thermometry on (Ultra)high‐Pressure Rocks

AbstractThe burial of continental lithosphere in collision zones is a first‐order process in global tectonics. Decades of interdisciplinary research have provided models for continental subduction; however, few empirical constraints exist on the processes and rates of burial, and assessments of the impact of continental collision on the geodynamics of convergent margins are still purely qualitative. A spatially resolved analysis of continental burial in collisional orogens is needed to progress in this field. To this end, we subjected samples collected along a vector parallel to the paleo‐subduction direction of the Western Gneiss Complex—one of the world's largest and best preserved continental ultrahigh‐pressure terranes—to Lu‐Hf garnet chronology and Zr‐in‐rutile thermometry. The Lu‐Hf ages range from 420 to 400 Ma but do not mimic pressure and temperature trends, which decrease up‐slab. Zirconium‐in‐rutile data demonstrate that Lu‐Hf ages of 405–400 Ma represent the cessation of recrystallization at peak conditions during deep burial, whereas ages of 420–410 Ma represent prograde garnet growth, preserved as relics. The differences in P‐T‐t of garnet growth are used to calculate a burial rate of ~5 mm yr−1, which is much slower than the burial of subducting mature oceanic crust. Comparing our observations with those from other collisional settings, including the India‐Asia Collision Zone, demonstrates that burial rates for continental crust are globally uniform and independent of the precollisional convergence rate and slab angle. These results provide a new quantitative framework for evaluating and predicting changes in the geodynamics of active margins during the collision of (super)continents.

Vegetation demographics in Earth System Models: A review of progress and priorities.

Numerous current efforts seek to improve the representation of ecosystem ecology and vegetation demographic processes within Earth System Models (ESMs). These developments are widely viewed as an important step in developing greater realism in predictions of future ecosystem states and fluxes. Increased realism, however, leads to increased model complexity, with new features raising a suite of ecological questions that require empirical constraints. Here, we review the developments that permit the representation of plant demographics in ESMs, and identify issues raised by these developments that highlight important gaps in ecological understanding. These issues inevitably translate into uncertainty in model projections but also allow models to be applied to new processes and questions concerning the dynamics of real-world ecosystems. We argue that stronger and more innovative connections to data, across the range of scales considered, are required to address these gaps in understanding. The development of first-generation land surface models as a unifying framework for ecophysiological understanding stimulated much research into plant physiological traits and gas exchange. Constraining predictions at ecologically relevant spatial and temporal scales will require a similar investment of effort and intensified inter-disciplinary communication.

Soil respiration across a permafrost transition zone: spatial structure and environmental correlates

Abstract. Soil respiration is a key ecosystem function whereby shifts in respiration rates can shift systems from carbon sinks to sources. Soil respiration in permafrost-associated systems is particularly important given climate change driven permafrost thaw that leads to significant uncertainty in resulting ecosystem carbon dynamics. Here we characterize the spatial structure and environmental drivers of soil respiration across a permafrost transition zone. We find that soil respiration is characterized by a non-linear threshold that occurs at active-layer depths greater than 140 cm. We also find that within each season, tree basal area is a dominant driver of soil respiration regardless of spatial scale, but only in spatial domains with significant spatial variability in basal area. Our analyses further show that spatial variation (the coefficient of variation) and mean-variance power-law scaling of soil respiration in our boreal system are consistent with previous work in other ecosystems (e.g., tropical forests) and in population ecology, respectively. Comparing our results to those in other ecosystems suggests that temporally stable features such as tree-stand structure are often primary drivers of spatial variation in soil respiration. If so, this provides an opportunity to better estimate the magnitude and spatial variation in soil respiration through remote sensing. Combining such an approach with broader knowledge of thresholding behavior – here related to active layer depth – would provide empirical constraints on models aimed at predicting ecosystem responses to ongoing permafrost thaw.

Empirical constraints on the effects of radiation damage on helium diffusion in zircon

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523Ma and 88.0 to 138.9Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15×1019α/g, spanning the complete damage spectrum.We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample’s thermal history.

The relationship between galaxy and dark matter halo size from z ∼ 3 to the present

We explore empirical constraints on the statistical relationship between the radial size of galaxies and the radius of their host dark matter halos from $z\sim 0.1$--3 using the GAMA and CANDELS surveys. We map dark matter halo mass to galaxy stellar mass using relationships from abundance matching, applied to the Bolshoi-Planck dissipationless N-body simulation. We define SRHR$\equiv r_e/R_h$ as the ratio of galaxy radius to halo virial radius, and SRHR$\lambda \equiv r_e/(\lambda R_h)$ as the ratio of galaxy radius to halo spin parameter times halo radius. At $z\sim 0.1$, we find an average value of SRHR $\simeq 0.018$ and SRHR$\lambda \simeq 0.5$ with very little dependence on stellar mass. SRHR and SRHR$\lambda$ have a weak dependence on cosmic time since $z\sim 3$. SRHR shows a mild decrease over cosmic time for low mass galaxies, but increases slightly or does not evolve for more massive galaxies. We find hints that at high redshift ($z\sim 2$--3), SRHR$\lambda$ is lower for more massive galaxies, while it shows no significant dependence on stellar mass at $z\lesssim 0.5$. We find that for both the GAMA and CANDELS samples, at all redshifts from $z\sim 0.1$--3, the observed conditional size distribution in stellar mass bins is remarkably similar to the conditional distribution of $\lambda R_h$. We discuss the physical interpretation and implications of these results.

Inflation as an amplifier: The case of Lorentz violation

Modified gravity theories are supposed to incorporate low-energy quantum-gravity effects and, at the same time, they could shed light into the dark matter and dark energy problems. Here we study a particular modification of general relativity where local Lorentz invariance is spontaneously broken and whose physical effects, despite a decade-long effort, were unknown. We show that, during inflation, this modification produces anisotropies that would generate measurable effects on the cosmic microwave background. Then, by using empirical constraints on the B-mode polarization spectrum, we can estimate that the ``coefficient'' components absolute value have to be smaller than ${10}^{\ensuremath{-}43}$. This is a remarkably strong limit; in fact, it is 29 orders of magnitude better than the best constraints on similar coefficients. Thus, we propose that inflation could stringently test other modified gravity theories.

On the Age of the TRAPPIST-1 System

Abstract The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928−0502285) hosts a compact system of at least seven exoplanets with sizes similar to Earth. Given its importance for testing planet formation and evolution theories, and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy, we present a comprehensive assessment of the age of this system. We collate empirical age constraints based on the color-absolute magnitude diagram, average density, lithium absorption, surface gravity features, metallicity, kinematics, rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6 ± 2.2 Gyr. The star’s color–magnitude position indicates that it is slightly metal-rich, which is consistent with the previously reported near-infrared spectroscopic metallicity; and it has a radius ) that is larger by 8%–14% than the predictions of solar-metallicity evolutionary models. We discuss some implications of the old age of this system with regard to the stability and habitability of its planets.

Empirical Constraints on the Origin of Fast Radio Bursts: Volumetric Rates and Host Galaxy Demographics as a Test of Millisecond Magnetar Connection

Abstract The localization of the repeating fast radio burst (FRB) 121102 to a low-metallicity dwarf galaxy at z = 0.193, and its association with a luminous quiescent radio source, suggests the possibility that FRBs originate from magnetars, formed by the unusual supernovae that occur in such galaxies. We investigate this possibility via a comparison of magnetar birth rates, the FRB volumetric rate, and host galaxy demographics. We calculate average volumetric rates of possible millisecond magnetar production channels, such as superluminous supernovae (SLSNe), long and short gamma-ray bursts (GRBs), and general magnetar production via core-collapse supernovae (CCSNe). For each channel, we also explore the expected host galaxy demographics using their known properties. We determine for the first time the number density of FRB emitters (the product of their volumetric birth rate and lifetime), Gpc−3, assuming that FRBs are predominantly emitted from repetitive sources similar to FRB 121102 and adopting a beaming factor of 0.1. By comparing rates, we find that production via rare channels (SLSNe, GRBs) implies a typical FRB lifetime of ∼30–300 years, in good agreement with other lines of argument. The total energy emitted over this time is consistent with the available energy stored in the magnetic field. On the other hand, any relation to magnetars produced via normal CCSNe leads to a very short lifetime of ∼0.5 years, in conflict with both theory and observation. We demonstrate that due to the diverse host galaxy distributions of the different progenitor channels, many possible sources of FRB birth can be ruled out with host galaxy identifications. Conversely, targeted searches of galaxies that have previously hosted decades-old SLSNe and GRBs may be a fruitful strategy for discovering new FRBs and related quiescent radio sources, and determining the nature of their progenitors.

Wolf-Rayet spin at low metallicity and its implication for black hole formation channels

The spin of Wolf-Rayet (WR) stars at low metallicity (Z) is most relevant for our understanding of gravitational wave sources such as GW 150914, as well as the incidence of long-duration gamma-ray bursts (GRBs). Two scenarios have been suggested for both phenomena: one of them involves rapid rotation and quasi-chemical homogeneous evolution (CHE), the other invokes classical evolution through mass loss in single and binary systems. WR spin rates might enable us to test these two scenarios. In order to obtain empirical constraints on black hole progenitor spin, we infer wind asymmetries in all 12 known WR stars in the Small Magellanic Cloud (SMC) at Z = 1/5 Zsun, as well as within a significantly enlarged sample of single and binary WR stars in the Large Magellanic Cloud (LMC at Z = 1/2 Zsun), tripling the sample of Vink (2007). This brings the total LMC sample to 39, making it appropriate for comparison to the Galactic sample. We measure WR wind asymmetries with VLT-FORS linear spectropolarimetry. We report the detection of new line effects in the LMC WN star BAT99-43 and the WC star BAT99-70, as well as the famous WR/LBV HD 5980 in the SMC, which might be evolving chemically homogeneously. With the previous reported line effects in the late-type WNL (Ofpe/WN9) objects BAT99-22 and BAT99-33, this brings the total LMC WR sample to 4, i.e. a frequency of ~10%. Perhaps surprisingly, the incidence of line effects amongst low-Z WR stars is not found to be any higher than amongst the Galactic WR sample, challenging the rotationally-induced CHE model. As WR mass loss is likely Z-dependent, our Magellanic Cloud line-effect WR stars may maintain their surface rotation and fulfill the basic conditions for producing long GRBs, both via the classical post-red supergiant (RSG) or luminous blue variable (LBV) channel, as well as resulting from CHE due to physics specific to very massive stars (VMS).

Empirical constraints on partitioning of platinum group elements between Cr-spinel and primitive terrestrial magmas

Recent experimental studies and in situ LA-ICP-MS analysis on natural Cr-spinel have shown that Rh and IPGEs (Ir-group platinum group elements: Ru, Ir, Os) are enriched in the lattice of Cr-spinel. However, the factors controlling the partitioning behaviour of these elements are not well constrained. In this study, we report the Rh, IPGE, and trace element contents in primitive Cr-spinel, measured by LA-ICP-MS, from nine volcanic suites covering various tectonic settings including island arc picrites, boninites, large igneous province picrites and mid-ocean ridge basalts. The aim is to understand the factors controlling the enrichment of Rh and IPGEs in Cr-spinels, to estimate empirical partition coefficients between Cr-spinel and silicate melts, and to investigate the role of Cr-spinel fractional crystallization on the PGE geochemistry of primitive magmas during the early stages of fractional crystallization.There are systematic differences in trace elements, Rh and IPGEs in Cr-spinels from arc-related magmas (Arc Group Cr-spinel), intraplate magmas (Intraplate Group Cr-spinel), and mid-ocean ridge magmas (MORB Group Cr-spinel). Arc Group Cr-spinels are systematically enriched in Sc, Co and Mn and depleted in Ni compared to the MORB Group Cr-spinels. Intraplate Group Cr-spinels are distinguished from the Arc Group Cr-spinels by their high Ni contents. Both the Arc and Intraplate Group Cr-spinels have total Rh and IPGE contents of 22–689ppb whereas the MORB Group Cr-spinels are depleted in Rh and IPGE (total<20ppb). Palladium and Pt contents are below detection limit for all of the studied Cr-spinels (<1–5ppb). The time-resolved spectra of LA-ICP-MS data for Cr-spinels mostly show constant count rates for trace element and Rh and IPGEs, suggesting homogeneous distribution of these elements in Cr-spinels. The PGE spikes observed in several Cr-spinels were interpreted to be PGE-bearing mineral inclusions and excluded from calculating the PGE contents of the Cr-spinels.On primitive mantle normalized diagrams the Arc Group Cr-spinels are characterized by a fractionated pattern with high Rh and low Os. The Intraplate Group Cr-spinels show flat patterns with positive Ru anomalies. Our results, together with the experimental and empirical data from previous studies, show that PGE patterns of Cr-spinel largely mimic that of the rock in which they are found, and that Rh, Ir and Os contents increase with increasing Fe3+ contents (i.e. magnetite component) in Cr-spinel, although Ru does not. These observations suggest that the enrichment of Rh and IPGEs in Cr-spinel is controlled by a combination of the Rh and IPGE contents in parental melts and the magnetite component of the spinel. Empirical partition coefficients (D) for Rh and IPGEs between Cr-spinels and silicate melts were calculated using the Rh and IPGE contents of the Cr-spinel and their host volcanic rocks after subtracting the accumulation effect of Cr-spinel. The D values for the Intraplate and MORB Group Cr-spinels increase with increasing magnetite component in Cr-spinel and range from 6 to 512, which is consistent with previously reported experimental and empirical values. In contrast, the Arc Group Cr-spinels have significantly higher D values (e.g. up to ∼3700 for Ru) than those of the Intraplate and MORB Group at the same magnetite concentration in the Cr-spinel, suggesting Rh and IPGEs dissolved in silicate melt have stronger affinity for Cr spinel under arc magma conditions than in intraplate magmas. This may be partly attributed to the low temperature of arc magmas relative to intraplate magmas, which leads to the Arc Group Cr-spinels having more octahedral sites at the same magnetite components than the Intraplate Group Cr-spinels. Because of significantly higher D values for the Arc Group Cr-spinels, compared with the Intraplate Group and MORB Group spinels, fractional crystallization of Cr-spinel will more efficiently fractionate Rh and IPGE from Pd and Pt in arc systems than in intraplate and MORB systems, which accounts for the highly fractionated PGE patterns in arc basalts.

Star formation rates and efficiencies in the Galactic Centre

The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from $0.1 - 5$ Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on $\sim$ parsec scales and find $1 - 4$ per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered "volumetric" star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.

Adaptive optimization on ultrasonic transmission tomography-based temperature image for biomedical treatment**Project supported by the DoD/BCRP Idea Award BC095397P1, the National Natural Science Foundation of China (Grant No. 61201425), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, China.

Hyperthermia has proven to be beneficial to treating superficial malignancies, particularly chest wall recurrences of breast cancer. During hyperthermia, monitoring the time–temperature profiles in the target and surrounding areas is of great significance for the effect of therapy. An ultrasound-based temperature imaging method has advantages over other approaches. When the temperature around the tumor is calculated by using the propagation speed of ultrasound, there always exist overshoot artifacts along the boundary between different tissues. In this paper, we present a new method combined with empirical mode decomposition (EDM), similarity constraint, and continuity constraint to optimize the temperature images. Simulation and phantom experiment results compared with those from our previously proposed method prove that the EMD-based method can build a better temperature field image, which can adaptively yield better temperature images with less computation for assistant medical treatment control.

Fast shading correction for cone beam CT in radiation therapy via sparse sampling on planning CT.

The image quality of cone beam computed tomography (CBCT) is limited by severe shading artifacts, hindering its quantitative applications in radiation therapy. In this work, we propose an image-domain shading correction method using planning CT (pCT) as prior information which is highly adaptive to clinical environment. We propose to perform shading correction via sparse sampling on pCT. The method starts with a coarse mapping between the first-pass CBCT images obtained from the Varian TrueBeam system and the pCT. The scatter correction method embedded in the Varian commercial software removes some image errors but the CBCT images still contain severe shading artifacts. The difference images between the mapped pCT and the CBCT are considered as shading errors, but only sparse shading samples are selected for correction using empirical constraints to avoid carrying over false information from pCT. A Fourier-Transform-based technique, referred to as local filtration, is proposed to efficiently process the sparse data for effective shading correction. The performance of the proposed method is evaluated on one anthropomorphic pelvis phantom and 17 patients, who were scheduled for radiation therapy. (The codes of the proposed method and sample data can be downloaded from https://sites.google.com/view/linxicbct) RESULTS: The proposed shading correction substantially improves the CBCT image quality on both the phantom and the patients to a level close to that of the pCT images. On the phantom, the spatial nonuniformity (SNU) difference between CBCT and pCT is reduced from 74 to 1HU. The root of mean square difference of SNU between CBCT and pCT is reduced from 83 to 10HU on the pelvis patients, and from 101 to 12HU on the thorax patients. The robustness of the proposed shading correction is fully investigated with simulated registration errors between CBCT and pCT on the phantom and mis-registration on patients. The sparse sampling scheme of our method successfully avoids false structures in the corrected CBCT even when the maximum registration error is as high as 8mm. We develop an effective shading correction algorithm for CBCT readily implementable on clinical data as a software plug-in without modifications of current imaging hardware and protocol. The algorithm is directly applied on the output images from a commercial CBCT scanner with high computational efficiency and negligible memory burden.

Defining shear zone boundaries using fabric intensity gradients: An example from the east-central Nepal Himalaya

Uranium-lead (U-Pb) zircon geochronology, whole-rock geochemistry, and petrographic observations indicate that specimens from a suite of variably deformed granite and orthogneiss from the Okhaldungha region of east-central Nepal share a common origin. Microtextural characterization and quartz crystallographic fabric preferred orientation analyses of these same specimens outline a strain gradient that marks the location of a shear zone boundary. The location of this boundary, at the base of the orthogneiss, coincides with one of the interpreted locations of the Main Central thrust, though it cannot be uniquely identified as such. This study provides the first steps toward empirical constraints on the location and geometry of thrust structures in the region, helping to clarify the complex local kinematic framework. These methods not only help in assessing orogenic models of the Himalaya, but may also be applied to investigating other orogenic systems where the potential location of shear structures is contested.

The dispersal of planet-forming discs: theory confronts observations.

Discs of gas and dust around million-year-old stars are a by-product of the star formation process and provide the raw material to form planets. Hence, their evolution and dispersal directly impact what type of planets can form and affect the final architecture of planetary systems. Here, we review empirical constraints on disc evolution and dispersal with special emphasis on transition discs, a subset of discs that appear to be caught in the act of clearing out planet-forming material. Along with observations, we summarize theoretical models that build our physical understanding of how discs evolve and disperse and discuss their significance in the context of the formation and evolution of planetary systems. By confronting theoretical predictions with observations, we also identify the most promising areas for future progress.

Rage against the machine: Evaluation metrics in the 21st century

ABSTRACTI review the classic literature in generative grammar and Marr’s three-level program for cognitive science to defend the Evaluation Metric as a psychological theory of language learning. Focusing on well-established facts of language variation, change, and use, I argue that optimal statistical principles embodied in Bayesian inference models are ill-suited for language acquisition. Specific attention will be given to the Subset Problem: Indirect negative evidence, which can be attractively formulated in the Bayesian framework, is ineffective when the statistical properties of language are examined in detail. As an alternative, I suggest that the Tolerance Principle (Yang 2016) provides a unified solution for the problem of induction and generalization: It bridges the computational and algorithm levels in Marr’s formulation, while retaining the commitment to the formal and empirical constraints in child language development.

The past and future of growth rate estimation in demographic temporal frequency analysis: Biodemographic interpretability and the ascendance of dynamic growth models

Population growth rate estimators have recently emerged in demographic temporal frequency analysis (dTFA) as further tools to monitor pre-census population dynamics. The information that such estimators supply affords considerable heuristic potential for population-ecological research both because they implicate the environmental, behavioral, and physiological mechanisms that condition population growth, and because they impose much-needed empirical constraints on our efforts to build theory addressing long-run human population dynamics. However, the earnestness with which these estimators have been applied warrants caution. First, several nonidentical measures of population growth are current in both formal demography and dTFA, creating an opportunity for their equivocation. Second, our ability to insightfully interrogate growth estimates for population-ecological information has been checked by a tendency to interpret them in the framework of off-the-shelf parametric growth models ill-suited to long-run population dynamics. This paper evaluates the biodemographic merit of three estimators recently applied in dTFA. It also advocates a transition away from parametric and toward dynamic growth models and introduces an inductive, regression-based approach to the latter. A Monte Carlo simulation study indicates that this inductive approach can successfully recapture information about environmental influences on population growth from archaeological summed probability distributions, less so from kernel density estimates.