Solution of the symmetric eigenproblem AX=lambda BX by delayed division

Delayed division is an iterative method for solving the linear eigenvalue problem AX = lambda BX for a limited number of small eigenvalues and their corresponding eigenvectors. The distinctive feature of the method is the reduction of the problem to an approximate triangular form by systematically dropping quadratic terms in the eigenvalue lambda. The report describes the pivoting strategy in the reduction and the method for preserving symmetry in submatrices at each reduction step. Along with the approximate triangular reduction, the report extends some techniques used in the method of inverse subspace iteration. Examples are included for problems of varying complexity

Asymmetric thymocyte death underlies the CD4:CD8 T-cell ratio in the adaptive immune system

It has long been recognized that the T-cell compartment has more CD4 helper than CD8 cytotoxic T cells, and this is most evident looking at T-cell development in the thymus. However, it remains unknown how thymocyte development so favors CD4 lineage development. To identify the basis of this asymmetry, we analyzed development of synchronized cohorts of thymocytes in vivo and estimated rates of thymocyte death and differentiation throughout development, inferring lineage-specific efficiencies of selection. Our analysis suggested that roughly equal numbers of cells of each lineage enter selection and found that, overall, a remarkable ∼75% of cells that start selection fail to complete the process. Importantly it revealed that class I-restricted thymocytes are specifically susceptible to apoptosis at the earliest stage of selection. The importance of differential apoptosis was confirmed by placing thymocytes under apoptotic stress, resulting in preferential death of class I-restricted thymocytes. Thus, asymmetric death during selection is the key determinant of the CD4:CD8 ratio in which T cells are generated by thymopoiesis

Models of self-peptide sampling by developing T cells identify candidate mechanisms of thymic selection

Conventional and regulatory T cells develop in the thymus where they are exposed to samples of self-peptide MHC (pMHC) ligands. This probabilistic process selects for cells within a range of responsiveness that allows the detection of foreign antigen without excessive responses to self. Regulatory T cells are thought to lie at the higher end of the spectrum of acceptable self-reactivity and play a crucial role in the control of autoimmunity and tolerance to innocuous antigens. While many studies have elucidated key elements influencing lineage commitment, we still lack a full understanding of how thymocytes integrate signals obtained by sampling self-peptides to make fate decisions. To address this problem, we apply stochastic models of signal integration by T cells to data from a study quantifying the development of the two lineages using controllable levels of agonist peptide in the thymus. We find two models are able to explain the observations; one in which T cells continually re-assess fate decisions on the basis of multiple summed proximal signals from TCR-pMHC interactions; and another in which TCR sensitivity is modulated over time, such that contact with the same pMHC ligand may lead to divergent outcomes at different stages of development. Neither model requires that T and T are differentially susceptible to deletion or that the two lineages need qualitatively different signals for development, as have been proposed. We find additional support for the variable-sensitivity model, which is able to explain apparently paradoxical observations regarding the effect of partial and strong agonists on T and T development

Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars

We present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O masers are found in thick expanding shells with inner radii ~ 6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly-filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4 au and filling factors of only ~ 0.01 with respect to the whole H2O maser shells. The CSE velocity structure gives additional evidence the maser clouds are density bounded. Masing clouds can be identified over a similar timescale to their sound crossing time (~2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.Comment: 21 pages, including 14 figures and 8 tables. Accepted for publication in MNRA

User's manual for the Shuttle Electric Power System analysis computer program (SEPS), volume 2 of program documentation

The Shuttle Electric Power System Analysis SEPS computer program which performs detailed load analysis including predicting energy demands and consumables requirements of the shuttle electric power system along with parameteric and special case studies on the shuttle electric power system is described. The functional flow diagram of the SEPS program is presented along with data base requirements and formats, procedure and activity definitions, and mission timeline input formats. Distribution circuit input and fixed data requirements are included. Run procedures and deck setups are described

Venus' Atmospheric Chemistry and Cloud Characteristics Are Compatible with Venusian Life

Venus is Earth's sister planet, with similar mass and density but an uninhabitably hot surface, an atmosphere with a water activity 50-100 times lower than anywhere on Earths' surface, and clouds believed to be made of concentrated sulfuric acid. These features have been taken to imply that the chances of finding life on Venus are vanishingly small, with several authors describing Venus' clouds as "uninhabitable", and that apparent signs of life there must therefore be abiotic, or artefactual. In this article, we argue that although many features of Venus can rule out the possibility that Earth life could live there, none rule out the possibility of all life based on what we know of the physical principle of life on Earth. Specifically, there is abundant energy, the energy requirements for retaining water and capturing hydrogen atoms to build biomass are not excessive, defenses against sulfuric acid are conceivable and have terrestrial precedent, and the speculative possibility that life uses concentrated sulfuric acid as a solvent instead of water remains. Metals are likely to be available in limited supply, and the radiation environment is benign. The clouds can support a biomass that could readily be detectable by future astrobiology-focused space missions from its impact on the atmosphere. Although we consider the prospects for finding life on Venus to be speculative, they are not absent. The scientific reward from finding life in such an un-Earthlike environment justifies considering how observations and missions should be designed to be capable of detecting life if it is there.Comment: Published in Astrobiology, June 12, 2023: https://www.liebertpub.com/doi/full/10.1089/ast.2022.011

The magnetic field of the proto-planetary nebula candidate IRAS 19296+2227

Context: Magnetic fields are thought to be one of the possible mechanisms responsible for shaping the generally spherical outflow of evolved stars into often aspherical planetary nebulae. However, direct measurements of magnetic fields during the transition to the planetary nebula phase are rare. Aims: The aim of this project is to expand the number of magnetic field measurements of stars in the (proto-)planetary nebula phase and find if the magnetic field strength is sufficient to affect the stellar outflow. Methods: We used Very Long Baseline Array observations to measure the circular polarization due to the Zeeman splitting of 22 GHz water masers in the envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the planetary nebula K3-35. Results: A strong magnetic field of B||=-135+-28 is detected in the water maser region of the proto-planetary nebula candidate IRAS 19296+2227. The water masers of K3-35 are too weak to detect circular polarization although we do present the measurements of weak linear polarization in those masers. Conclusions: The field measured in the masers of IRAS 19296+2227 is dynamically important and, if it is representative of the large scale field, is an important factor in driving the stellar mass loss and shaping the stellar outflow.Comment: 5 pages, 3 figures; A&A accepte

Seasonal changes in carbohydrate and mineral composition of overvigorous and devitalized Anab-e-Shahi grapevines in relation to uniruitiulness

Total carbohydrates in shoots, canes and old wood were higher in fruitful vines than in overvigorous or devitalized ones. The highest percentage of total carbohydrates was observed from December to February in all three types of vines. The minimum values for total carbohydrates in shoots of bearing vines were recorded in May. Total sugars were highest in bearing vines, followed by overvigorous and devitalized ones, respectively. Non-bearing (overvigorous and devitalized) vines had maximum sugar content in February, while bearing vines reached the maximum in January. Non-reducing sugars were practically absent in all vines from March to September. The first evidence of their presence was obtained in October.Nitrogen concentration was higher in overvigorous vines, followed by bearing and devitalized ones, respectively. Its level in devitalized vines was very low throughout the year. Also the nttrogen status of devitalized vineyard was very low, as observed in soil analysis. In bearing and overvigorous vines the percentage nitrogen was higher in shoots than in old wood. There was no significant difference in the phosphorus levels of bearing and overvigorous vines. Potassium level of overvigorous vines was slightly higher than that of bearing and devitalized vines.Overvigorous vines had low carbohydrates and very high ni,trogen, bearing vines had moderate carbohydrates and moderate nitrogen, while the devitalized vines had low carbohydrates and very low nitrogen level.Die jahreszeitlichen Veränderungen der Kohlenhydrat- und Mineralstoffzusammensetzung bei luxurierenden und kümmernden Anab-e-Shahi-Reben in Beziehung zu ihrer UnfruchtbarkeitDer Gesamtkohlenhydratgehalt in grünen Trieben, Tragruten und altem Holz war bei fruchttragenden Anab-e-Shahi-Reben größer als bei luxurierenden oder kümmernden Reben. Die höchste Konzentration der Gesamtkohlenhydrate wurde bei allen drei Rebtypen von Dezember bis Februar beobachtet; in den Trieben der traubentragenden Reben wurden im Mai die niedrigsten Werte festgestellt. Die Gesamtzuckerkonzentration nahm in der Reihenfolge fruchttragende, luxurierende, kümmernde Reben ab. Bei nicht-traubentragenden (luxurierenden und kümmernden) Reben war der Zuckergehalt im Februar am größten; traubentragende Reben erreichten ihr Zuckermaximum schon im Januar. Nichtreduzierende Zucker fehlten bei allen Reben von März bis September praktisch vollständig und waren erst von Oktober an nachzuweisen.Luxurierende Reben zeigten einen höheren Stickstoffgehalt als traubentragende und kümmernde Reben. Die letzteren wiesen das ganze Jahr über sehr niedrige Stickstoffkonzentrationen auf. Bodenanalysen ergaben, daß auch ihre Parzelle sehr schwach mit Stickstoff versorgt war. Bei fruchttragenden und luxurierenden Reben enthielten die grünen Triebe prozentual mehr Stickstoff als das alte Holz. Zwischen eiern Phosphorgehalt normaler und luxurierender Reben bestand kein signifikanter Unterschied. Bei den luxurierenden Reben war der Kaliumgehalt etwas höher als bei den traubentragenden und kümmernden Reben.Luxurierende Reben besaßen wenig Kohlenhydrate und sehr viel Stickstoff, traubentragende Reben wiesen mittlere Kohlenhydrat- und Stickstoffkonzentrationen auf, während die kümmernden Reben einen niedrigen Kohlenhydrat- und einen sehr geringen Stickstoffgehalt zeigten

Iridium-Catalyzed Enantioselective Hydrogenation of Vinylsilanes

We have screened a diverse array of iridium complexes derived from chiral N,P ligands as catalysts for the asymmetric hydrogenation of vinylsilanes, a transformation for which generally applicable catalysts were lacking. Several catalysts emerged from this study that enabled the highly enantioselective hydrogenation of a wide range of vinylsilanes with trisubstituted or disubstituted terminal C=C bonds bearing aryl, alkyl, ethoxycarbonyl, or hydroxymethyl substituents. In addition to trimethylsilyl and dimethyl(phenyl) silyl derivatives, trialkoxysilyl- and silacyclobutyl-substituted alkenes were used as substrates

Production of ammonia makes Venusian clouds habitable and explains observed cloud-level chemical anomalies.

The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O2 in the cloud layers, an unknown composition of large particles in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO2 and H2O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH3), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH3 dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO2 as ammonium sulfite salts. This trapping of SO2 in the clouds, together with the release of SO2 below the clouds as the droplets settle out to higher temperatures, explains the vertical SO2 abundance anomaly. A consequence of the presence of NH3 is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH3 is unknown but could involve biological production; if so, then the most energy-efficient NH3-producing reaction also creates O2, explaining the detection of O2 in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements