492 research outputs found
MIZ1 regulates ECA1 to generate a slow, long-distance phloem-transmitted Ca2+ signal essential for root water tracking in Arabidopsis
No full textEver since Darwin postulated that the tip of the root is sensitive to moisture differences and that it "transmits an influence to the upper adjoining part, which bends towards the source of moisture" [Darwin C, Darwin F (1880) The Power of Movement in Plants, pp 572-574], the signal underlying this tropic response has remained elusive. Using the FRET-based Cameleon Ca2+ sensor in planta, we show that a water potential gradient applied across the root tip generates a slow, long-distance asymmetric cytosolic Ca2+ signal in the phloem, which peaks at the elongation zone, where it is dispersed laterally and asymmetrically to peripheral cells, where cell elongation occurs. In addition, the MIZ1 protein, whose biochemical function is unknown but is required for root curvature toward water, is indispensable for generating the slow, long-distance Ca2+ signal. Furthermore, biochemical and genetic manipulations that elevate cytosolic Ca2+ levels, including mutants of the endoplasmic reticulum (ER) Ca2+-ATPase isoform ECA1, enhance root curvature toward water. Finally, coimmunoprecipitation of plant proteins and functional complementation assays in yeast cells revealed that MIZ1 directly binds to ECA1 and inhibits its activity. We suggest that the inhibition of ECA1 by MIZ1 changes the balance between cytosolic Ca2+ influx and efflux and generates the cytosolic Ca2+ signal required for water tracking
Path planning in 1000+ dimensions using a task-space Voronoi bias
No full textThe reduction of the kinematics and/or dynamics of a high-DOF robotic manipulator to a low-dimension ldquotask spacerdquo has proven to be an invaluable tool for designing feedback controllers. When obstacles or other kinodynamic constraints complicate the feedback design process, motion planning techniques can often still find feasible paths, but these techniques are typically implemented in the high-dimensional configuration (or state) space. Here we argue that providing a Voronoi bias in the task space can dramatically improve the performance of randomized motion planners, while still avoiding non-trivial constraints in the configuration (or state) space. We demonstrate the potential of task-space search by planning collision-free trajectories for a 1500 link arm through obstacles to reach a desired end-effector position.United States. Defense Advanced Research Projects Agency (Learning Locomotion program (AFRL contract # FA8650-05-C-7262)
Reachability-guided sampling for planning under differential constraints
No full textRapidly-exploring random trees (RRTs) are widely used to solve large planning problems where the scope prohibits the feasibility of deterministic solvers, but the efficiency of these algorithms can be severely compromised in the presence of certain kinodynamics constraints. Obstacle fields with tunnels, or tubes are notoriously difficult, as are systems with differential constraints, because the tree grows inefficiently at the boundaries. Here we present a new sampling strategy for the RRT algorithm, based on an estimated feasibility set, which affords a dramatic improvement in performance in these severely constrained systems. We demonstrate the algorithm with a detailed look at the expansion of an RRT in a swing up task, and on path planning for a nonholonomic car.United States. Defense Advanced Research Projects Agency (Learning Locomotion program (AFRL contract # FA8650-05-C-7262)
A search for star-planet interactions in the υ Andromedae system at X-ray and optical wavelengths
No full textContext. Close-in, giant planets are expected to influence their host stars via tidal or magnetic interaction. But are these effects in X-rays strong enough in suitable targets known so far to be observed with today's instrumentation? Aims: The υ And system, an F8V star with a Hot Jupiter, was observed to undergo cyclic changes in chromospheric activity indicators with its innermost planet's period. We aim to investigate the stellar chromospheric and coronal activity over several months. Methods: We therefore monitored the star in X-rays as well as at optical wavelengths to test coronal and chromospheric activity indicators for planet-induced variability, making use of the Chandra X-ray Observatory as well as the echelle spectrographs FOCES and HRS at Calar Alto (Spain) and the Hobby-Eberly Telescope (Texas, US). Results: The stellar activity level is low, as seen both in X-rays as in Ca ii line fluxes; the chromospheric data show variability with the stellar rotation period. We do not find activity variations in X-rays or in the optical that can be traced back to the planet. Conclusions: Gaining observational evidence of star-planet interactions in X-rays remains challenging
A DETAILED SPECTROPOLARIMETRIC ANALYSIS OF THE PLANET-HOSTING STAR WASP-12
No full textThe knowledge of accurate stellar parameters is paramount in several fields of stellar astrophysics, particularly in the study of extrasolar planets, where often the star is the only visible component and therefore used to infer the planet's fundamental parameters. Another important aspect of the analysis of planetary systems is the stellar activity and the possible star planet interaction. Here, we present a self-consistent abundance analysis of the planet-hosting star WASP-12 and a high-precision search for a structured stellar magnetic field on the basis of spectropolarimetric observations obtained with the ESPaDOnS spectropolarimeter. Our results show that the star does not have a structured magnetic field, and that the obtained fundamental parameters are in good agreement with what was previously published. In addition, we derive improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M(circle dot)), and distance (295-465 pc). WASP-12 is an ideal object in which to look for pollution signatures in the stellar atmosphere. We analyze the WASP-12 abundances as a function of the condensation temperature and compare them with those published by several other authors on planet-hosting and non-planet-hosting stars. We find hints of atmospheric pollution in WASP-12's photosphere but are unable to reach firm conclusions with our present data. We conclude that a differential analysis based on WASP-12 twins will probably clarify whether an atmospheric pollution is present as well as the nature of this pollution and its implications in planet formation and evolution. We also attempt the direct detection of the circumstellar disk through infrared excess, but without success
HAZMAT. IX. An Analysis of the UV and X-Ray Evolution of Low-Mass Stars in the Era of Gaia
No full textLow mass stars ( M) are some of the best candidates for
hosting planets with detectable life because of these stars' long lifetimes and
relative planet to star mass and radius ratios. An important aspect of these
stars to consider is the amount of ultraviolet (UV) and X-ray radiation
incident on planets in the habitable zones due to the ability of UV and X-ray
radiation to alter the chemistry and evolution of planetary atmospheres. In
this work, we build on the results of the HAZMAT I (Shkolnik & Barman 2014) and
HAZMAT III (Schneider & Shkolnik 2018) M star studies to determine the
intrinsic UV and X-ray flux evolution with age for M stars using Gaia
parallactic distances. We then compare these results to the intrinsic fluxes of
K stars adapted from HAZMAT V (Richey-Yowell et al. 2019). We find that
although the intrinsic M star UV flux is 10 to 100 times lower than that of K
stars, the UV fluxes in their respective habitable zone are similar. However,
the habitable zone X-ray flux evolutions are slightly more distinguishable with
a factor of 3 -- 15 times larger X-ray flux for late-M stars than for K stars.
These results suggest that there may not be a K dwarf advantage compared to M
stars in the UV, but one may still exist in the X-ray.Comment: 16 pages, 14 figure
A DETAILED SPECTROPOLARIMETRIC ANALYSIS OF THE PLANET-HOSTING STAR WASP-12
No full textThe knowledge of accurate stellar parameters is paramount in several fields of stellar astrophysics, particularly in the study of extrasolar planets, where often the star is the only visible component and therefore used to infer the planet's fundamental parameters. Another important aspect of the analysis of planetary systems is the stellar activity and the possible star planet interaction. Here, we present a self-consistent abundance analysis of the planet-hosting star WASP-12 and a high-precision search for a structured stellar magnetic field on the basis of spectropolarimetric observations obtained with the ESPaDOnS spectropolarimeter. Our results show that the star does not have a structured magnetic field, and that the obtained fundamental parameters are in good agreement with what was previously published. In addition, we derive improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M(circle dot)), and distance (295-465 pc). WASP-12 is an ideal object in which to look for pollution signatures in the stellar atmosphere. We analyze the WASP-12 abundances as a function of the condensation temperature and compare them with those published by several other authors on planet-hosting and non-planet-hosting stars. We find hints of atmospheric pollution in WASP-12's photosphere but are unable to reach firm conclusions with our present data. We conclude that a differential analysis based on WASP-12 twins will probably clarify whether an atmospheric pollution is present as well as the nature of this pollution and its implications in planet formation and evolution. We also attempt the direct detection of the circumstellar disk through infrared excess, but without success
A PULSEIRA AZUL DO FILÓLOGO: UMA LEITURA DE NOTA DE RODAPÉ, DE JOSEPH CEDAR
No full textO presente trabalho traz a leitura do filme Nota de Rodapé, de Joseph Cedar a partir dos conceitos de memória cultural, fama e lixo apresentados por Aleida Assmann em seu livro Espaços de Recordação. O filme é a narrativa da tensão entre dois pesquisadores do Talmude, Eliezer Shkolnik e Uriel Shkolnik que são, também, pai e filho. O ápice do enredo é quando o Shkolnik pai recebe uma ligação informando-o que foi escolhido para receber o prêmio Israel. Porém, a ligação foi um engano, visto que o verdadeiro ganhador foi seu filho. Uriel se esforça para manter o prêmio destinado ao pai. O objetivo deste trabalho é apontar, nas cenas, que o move Uriel a manter a mentira é o desejo der qu fazer com que o pai seja valorizado pela coletividade. Com isso, ele acredita que alcançará seu próprio reconhecimento no espaço de recordação familiar. A conclusão foi que Eliezer Shkolnik não consegue, de fato, uma entrada para a memória cultural, mas alcança a fama, visto que a premiação não é pautada em uma verdade e, para além disso, a cena final do filme é uma encenação do recebimento do prêmio, em uma ilustração do conceito de fama apresentado por Aleida Assmann
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Sparse and Low-rank Matrix Decomposition – Application in Finance
No full textThe field of machine learning is witnessing a rapid expansion in the literature that explores techniques and applications of sparse and low-rank matrix decompositions. Typically formulated as an optimization problem involving nuclear norm minimization, this paradigm offers computational efficiency and robust statistical recovery guarantees, contrasting with the NP-hard nature of rank-based objectives. This thesis dedicates attention to the development of new methodology (Chapter 2) and also its application to finance (Chapter 3), as described below. Chapter 1 furnishes the necessary background and conducts a comprehensive survey of the related literature.Chapter 2 concerns dimensionality reduction methods such as principal component analysis (PCA) and factor analysis, which are central to many problems in data science. There are, however, serious and well-understood challenges to finding robust low dimensional approximations for data with significant heteroscedastic noise. This Chapter introduces a relaxed version of Minimum Trace Factor Analysis (MTFA), a convex optimization method with roots dating back to the work of Ledermann in 1940. This relaxation is particularly effective at not overfitting to heteroskedastic perturbations and addresses the commonly cited Heywood cases in factor analysis and the recently identified ``curse of ill-conditioning" for existing spectral methods. We provide theoretical guarantees on the accuracy of the resulting low rank subspace and the convergence rate of the proposed algorithm to compute that matrix. We develop a number of interesting connections to existing methods, including Hetero PCA, Lasso, and Soft-Impute, to fill an important gap in the already large literature on low rank matrix estimation. Numerical experiments benchmark our results against several recent proposals for dealing with heteroskedastic noise.In Chapter 3, we shift focus to factor analysis applied to security returns. Traditionally, commercially successful factor analysis relies on fundamental models, despite a rich academic literature exploring statistical models. Traditional statistical approaches like PCA and maximum likelihood exhibit success but suffer from drawbacks, such as a lack of robustness and insensitivity to narrow factors. To address these limitations, we propose convex optimization methods inspired by the techniques from Chapter 2. These methods aim to decompose a security return covariance matrix into its low-rank and sparse components. The low-rank component captures broad factors affecting most securities, while the sparse component accounts for narrow factors and security-specific effects. We illustrate the efficacy of this approach by measuring the variance forecasting accuracy of a low-rank plus sparse covariance matrix estimator through simulations and an empirical analysis of global equity data, showcasing improvements over PCA-based methods
A note on increasing lamb production of fat-tailed Awassi and German Mutton Merino sheep grazing in a semi-arid area
No full textThe native fat-tailed Awassi and the imported German Mutton Merino (GMM) sheep are well adapted to semi-arid conditions (Degen, 1977; Degen and Shkolnik, 1978). This enables these sheep to graze large areas of the Negev Desert in Israel that is marginal for farming. However, under extensive conditions both these sheep breeds yield low lamb crops producing fewer than 1·0 lamb per ewe per year.</jats:p
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