We report a precision measurement of the parity-violating asymmetry APV in the elastic scattering of longitudinally polarized electrons from 208Pb. We measure APV= 550 ± 16 (stat) ±8 (syst) parts per ...billion, leading to an extraction of the neutral weak form factor FW(Q2= 0.00616 GeV2) = 0.368 ± 0.013. Combined with our previous measurement, the extracted neutron skin thickness is Rn-Rp= 0.283 ± 0.071 fm. The result also yields the first significant direct measurement of the interior weak density of 208Pb: ρ$^0_W$ = -0.0796 ± 0.0036(exp) ± 0.0013(theo) fm-3 leading to the interior baryon density ρ$^0_b$ = 0.1480 ± 0.0036(exp) ± 0.0013(theo) fm-3. Finally, the measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.
We present the first radiation-hydrodynamic model of a protoplanetary disc irradiated with an X-ray extreme ultraviolet (X-EUV) spectrum. In a model where the total ionizing luminosity is divided ...equally between X-ray and EUV luminosity, we find a photoevaporation rate of 1.4 × 10−8 M⊙ yr−1, which is two orders of magnitude greater than the case of EUV photoevaporation alone. Thus, it is clear that the X-rays are the dominant driving mechanism for photoevaporation. This can be understood inasmuch as X-rays are capable of penetrating much larger columns (∼1022 cm−2) and can thus effect heating in denser regions and at larger radius than the EUV. The radial extent of the launching region of the X-ray-heated wind is 1–70 au compared with the pure EUV case where the launch region is concentrated around a few au. When we couple our wind mass-loss rates with models for the disc's viscous evolution, we find that, as in the pure EUV case, there is a photoevaporative switch, such that an inner hole develops at ∼1 au at the point when the accretion rate in the disc drops below the wind mass-loss rate. At this point, the remaining disc material is quickly removed in the final 15–20 per cent of the disc's lifetime. This is consistent with the 105 yr transitional time-scale estimated from observations of T Tauri stars. We however note several key differences to previous EUV-driven photoevaporation models. The two orders of magnitude higher photoevaporation rate is now consistent with the average accretion rate observed in young stars and will cut the disc off in its prime. Moreover, the extended mass-loss profile subjects the disc to a significant period (∼20 per cent of the disc's lifetime) of ‘photoevaporation-starved accretion’. We also caution that although our mass-loss rates are high compared to some accretion rates observed in young stars, our model has a rather large X-ray luminosity of 2 × 1030 erg s−1; further modelling is required in order to investigate the evolutionary implications of the large observed spread of X-ray luminosities in T Tauri stars.
The coexistence of different species of large herbivores (ungulates) in grasslands and savannas has fascinated ecologists for decades. However, changes in climate, land‐use and trophic structure of ...ecosystems increasingly jeopardise the persistence of such diverse assemblages. Body size has been used successfully to explain ungulate niche differentiation with regard to food requirements and predation sensitivity. But this single trait axis insufficiently captures interspecific differences in water requirements and thermoregulatory capacity and thus sensitivity to climate change. Here, we develop a two‐dimensional trait space of body size and minimum dung moisture content that characterises the combined food and water requirements of large herbivores. From this, we predict that increased spatial homogeneity in water availability in drylands reduces the number of ungulate species that will coexist. But we also predict that extreme droughts will cause the larger, water‐dependent grazers as wildebeest, zebra and buffalo–dominant species in savanna ecosystems – to be replaced by smaller, less water‐dependent species. Subsequently, we explore how other constraints such as predation risk and thermoregulation are connected to this two‐dimensional framework. Our novel framework integrates multiple simultaneous stressors for herbivores and yields an extensive set of testable hypotheses about the expected changes in large herbivore community composition following climate change.
The Solar Orbiter mission Müller, D.; St. Cyr, O. C.; Zouganelis, I. ...
Astronomy and astrophysics (Berlin),
10/2020, Letnik:
642
Journal Article
Recenzirano
Odprti dostop
Aims.
Solar Orbiter, the first mission of ESA’s Cosmic Vision 2015–2025 programme and a mission of international collaboration between ESA and NASA, will explore the Sun and heliosphere from close up ...and out of the ecliptic plane. It was launched on 10 February 2020 04:03 UTC from Cape Canaveral and aims to address key questions of solar and heliospheric physics pertaining to how the Sun creates and controls the Heliosphere, and why solar activity changes with time. To answer these, the mission carries six remote-sensing instruments to observe the Sun and the solar corona, and four in-situ instruments to measure the solar wind, energetic particles, and electromagnetic fields. In this paper, we describe the science objectives of the mission, and how these will be addressed by the joint observations of the instruments onboard.
Methods.
The paper first summarises the mission-level science objectives, followed by an overview of the spacecraft and payload. We report the observables and performance figures of each instrument, as well as the trajectory design. This is followed by a summary of the science operations concept. The paper concludes with a more detailed description of the science objectives.
Results.
Solar Orbiter will combine in-situ measurements in the heliosphere with high-resolution remote-sensing observations of the Sun to address fundamental questions of solar and heliospheric physics. The performance of the Solar Orbiter payload meets the requirements derived from the mission’s science objectives. Its science return will be augmented further by coordinated observations with other space missions and ground-based observatories.
The Solar Orbiter magnetometer Horbury, T. S.; O’Brien, H.; Carrasco Blazquez, I. ...
Astronomy and astrophysics (Berlin),
10/2020, Letnik:
642
Journal Article
Recenzirano
Odprti dostop
The magnetometer instrument on the Solar Orbiter mission is designed to measure the magnetic field local to the spacecraft continuously for the entire mission duration. The need to characterise not ...only the background magnetic field but also its variations on scales from far above to well below the proton gyroscale result in challenging requirements on stability, precision, and noise, as well as magnetic and operational limitations on both the spacecraft and other instruments. The challenging vibration and thermal environment has led to significant development of the mechanical sensor design. The overall instrument design, performance, data products, and operational strategy are described.
We present the first multievent study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near‐Earth plasma instability which causes the substorm onset arc. ...Using data from ground‐based auroral imagers, we study repeatable signatures of along‐arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near‐Earth plasma sheet. We show that the growth and spatial scales of these wave‐like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wave number with the most unstable spatial scales mapping to an azimuthal wavelength λ≈ 1700–2500 km in the equatorial magnetosphere at around 9–12 RE. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the Cross‐Field Current Instability and the Shear Flow Ballooning Instability. We conclude that, although the Cross‐Field Current instability can generate similar magnitude of growth rates, the range of unstable wave numbers indicates that the Shear Flow Ballooning Instability is the most likely explanation for our observations.
Key Points
Auroral beads are a repeatable signature along the substorm onset arc
Bead brightness grows exponentially across a wide range of spatial scales
Growth rates peak at those spatial scales predicted by shear flow ballooning
In order to enhance the experience of learning, many educators applied learning analytics in a classroom, the major principle of learning analytics is targeting at-risk student and given timely ...intervention according to the results of student behavior analysis. However, when researchers applied machine learning to train a risk identifying model, the reason which affected the performance of the model was overlooked. This study collected seven datasets within three universities located in Taiwan and Japan and listed performance metrics of risk identification model after fed data into eight classification methods. U1, U2, and U3 were used to denote the three universities, which have three, two, and two cases of datasets (learning logs), respectively. According to the results of this study, the factors influencing the predictive performance of classification methods are the number of significant features, the number of categories of significant features, and Spearman correlation coefficient values. In U1 dataset case 1.3 and U2 dataset case 2.2, the numbers of significant features, numbers of categories of significant features, and Spearman correlation coefficient values for significant features were all relatively high, which is the main reason why these datasets were able to perform classification with high predictive ability.
Closing the Gap Dynarski, Susan; Libassi, CJ; Michelmore, Katherine ...
The American economic review,
06/2021, Letnik:
111, Številka:
6
Journal Article
Recenzirano
Odprti dostop
High-achieving, low-income students attend selective colleges at far lower rates than upper-income students with similar achievement. Behavioral biases, intensified by complexity and uncertainty in ...the admissions and aid process, may explain this gap. In a large-scale experiment we test an early commitment of free tuition at a flagship university. The intervention did not increase aid: rather, students were guaranteed before application the same grant aid that they would qualify for in expectation if admitted. The offer substantially increased application (68 percent versus 26 percent) and enrollment rates (27 percent versus 12 percent). The results suggest that uncertainty, present bias, and loss aversion loom large in students’ college decisions.
Whether supernovae are major sources of dust in galaxies is a long-standing debate. We present infrared and submillimeter photometry and spectroscopy from the Herschel Space Observatory of the Crab ...Nebula between 51 and 670 mu m as part of the Mass Loss from Evolved StarS program. We compare the emission detected with Herschel with multiwavelength data including millimeter, radio, mid-infrared, and archive optical images. We carefully remove the synchrotron component using the Herschel and Planck fluxes measured in the same epoch. The contribution from line emission is removed using Herschel spectroscopy combined with Infrared Space Observatory archive data. Several forbidden lines of carbon, oxygen, and nitrogen are detected where multiple velocity components are resolved, deduced to be from the nitrogen-depleted, carbon-rich ejecta. No spectral lines are detected in the SPIRE wavebands; in the PACS bands, the line contribution is 5% and 10% at 70 and 100 mu m and negligible at 160 mu m. After subtracting the synchrotron and line emission, the remaining far-infrared continuum can be fit with two dust components. Assuming standard interstellar silicates, the mass of the cooler component is 0.24 super(+0.32) sub(-0.08) M sub(middot in circle) for T = 28.1 super(+5.5) sub(-3.2) K. Amorphous carbon grains require 0.11 + or - 0.01 M sub(middot in circle) of dust with T = 33.8 super(+2.3) sub(-1.8) K. A single temperature modified blackbody with 0.14 M sub(middot in circle) and 0.08 M sub(middot in circle) for silicate and carbon dust, respectively, provides an adequate fit to the far-infrared region of the spectral energy distribution but is a poor fit at 24-500 mu m. The Crab Nebula has condensed most of the relevant refractory elements into dust, suggesting the formation of dust in core-collapse supernova ejecta is efficient.
We report precision determinations of the beam-normal single spin asymmetries (A_{n}) in the elastic scattering of 0.95 and 2.18 GeV electrons off ^{12}C, ^{40}Ca, ^{48}Ca, and ^{208}Pb at very ...forward angles where the most detailed theoretical calculations have been performed. The first measurements of A_{n} for ^{40}Ca and ^{48}Ca are found to be similar to that of ^{12}C, consistent with expectations and thus demonstrating the validity of theoretical calculations for nuclei with Z≤20. We also report A_{n} for ^{208}Pb at two new momentum transfers (Q^{2}) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the Z≤20 nuclei. These data confirm our basic understanding of the underlying dynamics that govern A_{n} for nuclei containing ≲50 nucleons, but point to the need for further investigation to understand the unusual A_{n} behavior discovered for scattering off ^{208}Pb.
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