Elemental ratios in stars vs planets

Context. The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same. Aims. Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O. Methods. A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances. Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.Comment: 8 pages, 5 figures, 3 tables. Accepted for publication in A&

From stellar nebula to planets: the refractory components

We computed the abundance of refractory elements in planetary bodies formed in stellar systems with solar chemical composition by combining models for chemical composition and planet formation. We also consider the formation of refractory organic compounds, which have been ignored in previous studies on this topic. We used the commercial software package HSC Chemistry in order to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e. organic compounds are formed in the gas phase and are nonreactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets, with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides; pyroxene and olivine in similar quantities are formed. The model predicts planets that are similar in composition to those of the Solar system. It also shows that, starting from a common initial nebula composition, a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.Comment: 18 pages, 29 figures. Accepted for publication in A&

The Galactic Cosmic Ray Intensity over the Past 106-109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive nuclide 40K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5-1 billion years within a factor of ∼1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30-50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climat

A Bio-Inspired Tensegrity Manipulator with Multi-DOF, Structurally Compliant Joints

Most traditional robotic mechanisms feature inelastic joints that are unable to robustly handle large deformations and off-axis moments. As a result, the applied loads are transferred rigidly throughout the entire structure. The disadvantage of this approach is that the exerted leverage is magnified at each subsequent joint possibly damaging the mechanism. In this paper, we present two lightweight, elastic, bio-inspired tensegrity robotics arms which mitigate this danger while improving their mechanism's functionality. Our solutions feature modular tensegrity structures that function similarly to the human elbow and the human shoulder when connected. Like their biological counterparts, the proposed robotic joints are flexible and comply with unanticipated forces. Both proposed structures have multiple passive degrees of freedom and four active degrees of freedom (two from the shoulder and two from the elbow). The structural advantages demonstrated by the joints in these manipulators illustrate a solution to the fundamental issue of elegantly handling off-axis compliance.Comment: IROS 201

Production rates for cosmogenic krypton and argon isotopes in H-chondrites with known ^<36>Cl-^<36>Ar ages

We present physical model calculations for the production of cosmogenic Kr isotopes in stony meteorites and compare the model results with measured data for bulk samples of 12 H-chondrites which recently had been investigated for their ^Cl-^Ar cosmic-ray exposure ages and light noble gas production rates. The correlation between P(^Kr)P(^Kr) and P(^Kr)P(^Kr) modelled here is significantly different from the classical relation commonly used to derive ^Kr-Kr exposure ages. For both relations, the ^Kr ages scatter considerably around the respective ^Cl-^Ar ages, but the new relation on average yields a somewhat better agreement between ^Kr-Kr and ^Cl-^Ar ages. The calculations combined with concentration measurements of the main target elements for the production of cosmogenic Kr (Rb, Sr, Y, Zr, and Nb) show that target element chemistry does hardly influence the isotopic composition of cosmogenic Kr in bulk chondrites. These calculations also confirm earlier conclusions that the isotopic systematics of cosmogenic Kr in lunar samples are applicable for chondrites too. We derived an average ^Ar production rate at average shielding (^Ne^Ne=1.11) of (0.0431±0.0035)×10^ cm^3 STP(g×Myr)

A miniature mass analyser for in-situ elemental analysis of planetary material-performance studies

The performance of a laser ablation mass analyser designed for in-situ exploration of the chemical composition of planetary surfaces has been investigated. The instrument measures the elemental and isotopic composition of raw solid materials with high spatial resolution. The initial studies were performed on NIST standard materials using IR laser irradiance (< 1 GW cm−2) at which a high temporal stability of ion formation and sufficiently low sample consumption was achieved. Measurements of highly averaged spectra could be performed with typical mass resolution of m/Δm ≈ 600 in an effective dynamic range spanning seven decades. Sensitive detection of several trace elements can be achieved at the ~ ppm level and lower. The isotopic composition is usually reproduced with 1% accuracy, implying good performance of the instrument for quantitative analysis of the isotopic fractionation effects caused by natural processes. Using the IR laser, significant elemental fractionation effects were observed for light elements and elements with a high ionization potential. Several diatomic clusters of major and minor elements could also be measured, and sometimes these interfere with the detection of trace elements at the same nominal mass. The potential of the mass analyser for application to sensitive detection of elements and their isotopes in realistic samples is exemplified by measurements of minerals. The high resolution and large dynamic range of the spectra makes detection limits of ~100ppb possible. Figure The mass spectrum of Allende meteorite measured by a miniature laser ablation mass spectrometer. Similar mass spectra of planetary materials in-situ could be measured with spatial resolution of 10-100 μm (white circles) providing means for chemical analysis of planetary surface

Evolution of the Solar Nebula. IX. Gradients in the Spatial Heterogeneity of the Short-Lived Radioisotopes 60^{60}Fe and 26^{26}Al and the Stable Oxygen Isotopes

Short-lived radioisotopes (SLRI) such as $^{60}$Fe and $^{26}$Al were likely injected into the solar nebula in a spatially and temporally heterogeneous manner. Marginally gravitationally unstable (MGU) disks, of the type required to form gas giant planets, are capable of rapid homogenization of isotopic heterogeneity as well as of rapid radial transport of dust grains and gases throughout a protoplanetary disk. Two different types of new models of a MGU disk in orbit around a solar-mass protostar are presented. The first set has variations in the number of terms in the spherical harmonic solution for the gravitational potential, effectively studying the effect of varying the spatial resolution of the gravitational torques responsible for MGU disk evolution. The second set explores the effects of varying the initial minimum value of the Toomre $Q$ stability parameter, from values of 1.4 to 2.5, i.e., toward increasingly less unstable disks. The new models show that the basic results are largely independent of both sets of variations. MGU disk models robustly result in rapid mixing of initially highly heterogeneous distributions of SLRIs to levels of $\sim$ 10% in both the inner ($$ 10 AU) disk regions, and to even lower levels ($\sim$ 2%) in intermediate regions, where gravitational torques are most effective at mixing. These gradients should have cosmochemical implications for the distribution of SLRIs and stable oxygen isotopes contained in planetesimals (e.g., comets) formed in the giant planet region ($\sim$ 5 to $\sim$ 10 AU) compared to those formed elsewhere.Comment: 37 pages, 1 table, 19 figures, ApJ accepte

Aspiring india: The Politics of Mothering, Education Reforms, and English

This dissertation is an ethnography of aspirational mobilities emergent under contexts of profound material and social change. To explore the unprecedented expansion of educational aspirations in post market reform India, specifically surging parental desires for English-medium schooling, I conducted fieldwork at a low-fee private English-medium school and a neighboring state-funded Malayalam-medium school in the southern Indian state of Kerala. Further, to record state responses to non-elite educational aspirations, my fieldwork was distributed along diverse agencies that supported and regulated English learning in Kerala and across the country. This dissertation makes two key arguments. Firstly, transitions from a previously austere socialist economy to a consumption saturated society has radically altered gendered everyday lives and unsettled entrenched social hierarchies. Negotiating these changes, non-elite mothers are reimagining possible futures for their children. Since social recognition and economic security was and continues to be entangled with higher education and English proficiencies, this has intensified desires for English-medium schooling from the earliest grades. Secondly, intensifying non-elite desires for English learning reveals how educational systems in India are geared towards meeting the aspirations of privileged citizens. Analyzing the provision of English language learning in state-funded and private school systems, I argue that emergent emphases on conversational skills defines “knowing” English as predicated on the ability to socialize in English. While this shift benefits internationally mobile elite Indians, it marginalizes non-elite learning communities whose pedagogic resources are skewed towards literacy rather than orality skills. To conclude, aspirational mobilities in contemporary India are diverse and even oppositional, and dependent on aspirational locations as well as the resources that groups are able to mobilize

Improved modelling of helium and tritium production for spallation targets

Reliable predictions of light charged particle production in spallation reactions are important to correctly assess gas production in spallation targets. In particular, the helium production yield is important for assessing damage in the window separating the accelerator vacuum from a spallation target, and tritium is a major contributor to the target radioactivity. Up to now, the models available in the MCNPX transport code, including the widely used default option Bertini-Dresner and the INCL4.2-ABLA combination of models, were not able to correctly predict light charged particle yields. The work done recently on both the intranuclear cascade model INCL4, in which cluster emission through a coalescence process has been introduced, and on the de-excitation model ABLA allows correcting these deficiencies. This paper shows that the coalescence emission plays an important role in the tritium and $^3He$ production and that the combination of the newly developed versions of the codes, INCL4.5-ABLA07, now lead to good predictions of both helium and tritium cross sections over a wide incident energy range. Comparisons with other available models are also presented.Comment: 6 pages, 9 figure

Dynamic Motivation to Lead: Construct Validity of Motivation to Lead

Although motivation to lead (MTL) was characterized as stable, recent research suggested otherwise. This study explored the malleability of MTL and its predictors. Individuals with high affective-identity MTL are motivated to lead because they enjoy leading. Individuals with high social normative MTL are motivated by an obligation to lead. Individuals with high noncalculative MTL are drawn to leadership because they avoid weighing the costs and benefits of leading. Applicants to a California college were sent a questionnaire on MTL and leadership self-efficacy (LSE) (Time 1 assessment, N = 2704). Four years later (Time 2), participants who responded at Time 1 were sent a survey on motivation to lead, leadership self-efficacy, college leadership experience, and leader identity (LID) (N = 96). Results showed that participants’ affective-identity and noncalculative MTL have decreased over time. Leadership self-efficacy at Time 2 and leader identity at Time 2 were related to the changes in all 3 categories of MTL. Only specific college leadership experiences related to changes in affective-identity MTL. Lastly, leader identity at Time 2 mediated the relationship between affective-identity MTL at Time 1 and Time 2. Most high school students applied to college aspiring to be leaders, but only students who cultivate their leader identity should continue to be motivated to lead. Implications are discussed in the context of the construct validity of MTL, specifically for student leadership development in higher education