738 research outputs found
Transit Timing Observations of the Extrasolar Hot-Neptune Planet GL 436b
Gliese 436 is an M dwarf with a mass of 0.45 Msun and hosts the extrasolar
planet GL 436b [3, 6, 7, 2], which is currently the least massive transiting
planet with a mass of ~23.17 Mearth [10], and the only planet known to transit
an M dwarf. GL 436b represents the first transiting detection of the class of
extrasolar planets known as "Hot Neptunes" that have masses within a few times
that of Neptune's mass (~17 Mearth) and orbital semimajor axis <0.1 AU about
the host star. Unlike most other known transiting extrasolar planets, GL 436b
has a high eccentricity (e~0.16). This brings to light a new parameter space
for habitability zones of extrasolar planets with host star masses much smaller
than typical stars of roughly a solar mass. This unique system is an ideal
candidate for orbital perturbation and transit-time variation (TTV) studies to
detect smaller, possibly Earth-mass planets in the system. In April 2008 we
began a long-term intensive campaign to obtain complete high-precision light
curves using the Apache Point Observatory's 3.5-meter telescope, NMSU's 1-meter
telescope (located at APO), and Sommers Bausch Observatory's 24" telescope.
These light curves are being analyzed together, along with amateur and other
professional astronomer observations. Results of our analysis are discussed.
Continued measurements over the next few years are needed to determine if
additional planets reside in the system, and to study the impact of other
manifestations on the light curves, such as star spots and active regions.Comment: 4 pages, 3 figures. To appear in "Proceedings of the 15th Cambridge
Workshop on Cool Stars, Stellar Systems and the Sun", 2009, AIP Conference
Proceedings vol. 1094, ed. Eric Stempel
Water Demand Management in England and Wales: constructions of the domestic water-user
YesMeasures to manage demand include implicit and explicit messages about domestic water-users which have important potential impacts on their perceptions and practices. Drawing on recent literature, this paper identifies three different ĀædimensionsĀæ along which demand management measuresĀæ constructions of the water-user may vary: these relate to whether the water user is passive or active, whether they are motivated by individual or common needs, and whether they perceive water as a right or a commodity. Demand management measures currently used in England and Wales are then discussed and analysed. The paper concludes by highlighting the importance of communications associated with demand management, and in particular, notes the need to consider the cumulative impact of messages and their interactions with peopleĀæs existing understandings
Thermodynamic Model for Energy-Constrained Open-System Evolution of Crustal Magma Bodies Undergoing Simultaneous Recharge, Assimilation and Crystallization: the Magma Chamber Simulator
The Magma Chamber Simulator quantifies the impact of simultaneous recharge, assimilation and crystallization through mass and enthalpy balance in a multicomponentāmultiphase (melt + solids Ā± fluid) composite system. As a rigorous thermodynamic model, the Magma Chamber Simulator computes phase equilibria and geochemical evolution self-consistently in resident magma, recharge magma and wallrock, all of which are connected by specified thermodynamic boundaries, to model an evolving open-system magma body. In a simulation, magma cools from its liquidus temperature, and crystals Ā± fluid are incrementally fractionated to a separate cumulate reservoir. Enthalpy from cooling, crystallization, and possible magma recharge heats wallrock from its initial subsolidus temperature. Assimilation begins when a critical wallrock melt volume fraction (0Ā·04ā0Ā·12) in a range consistent with the rheology of partially molten rock systems is achieved. The mass of melt above this limit is removed from the wallrock and homogenized with the magma body melt. New equilibrium states for magma and wallrock are calculated that reflect conservation of total mass, mass of each element and enthalpy. Magma cooling and crystallization, addition of recharge magma and anatectic melt to the magma body (where appropriate), and heating and partial melting of wallrock continue until magma and wallrock reach thermal equilibrium. For each simulation step, mass and energy balance and thermodynamic assessment of phase relations provide major and trace element concentrations, isotopic characteristics, masses, and thermal constraints for all phases (melt + solids Ā± fluid) in the composite system. Model input includes initial compositional, thermal and mass information relevant to each subsystem, as well as solidāmelt and solidāfluid partition coefficients for all phases. Magma Chamber Simulator results of an assimilationāfractional crystallization (AFC) scenario in which dioritic wallrock at 0Ā·1 GPa contaminates high-alumina basalt are compared with results in which no assimilation occurs [fractional crystallization only (FC-only)]. Key comparisons underscore the need for multicomponentāmultiphase energy-constrained thermodynamic modeling of open systems, as follows. (1) Partial melting of dioritic wallrock yields cooler silicic melt that contaminates hotter magma. Magma responds by cooling, but a pulse of crystallization, possibly expected based on thermal arguments, does not occur because assimilation suppresses crystallization by modifying the topology of multicomponent phase saturation surfaces. As a consequence, contaminated magma composition and crystallizing solids are distinct compared with the FC-only case. (2) At similar stages of evolution, contaminated melt is more voluminous (ā¼3Ā·5Ć) than melt formed by FC-only. (3) In AFC, some trace element concentrations are lower than their FC-only counterparts at the same stage of evolution. Elements that typically behave incompatibly in mafic and intermediate magmas (e.g. La, Nd, Ba) may not be āenrichedā by crustal contamination, and the most ācrustalā isotope signatures may not correlate with the highest concentrations of such elements. (4) The proportion of an element contributed by anatectic melt to resident magma is typically different for each element, and thus the extent of mass exchange between crust and magma should be quantified using total mass rather than the mass of a single element. Based on these sometimes unexpected results, it can be argued that progress in quantifying the origin and evolution of open magmatic systems and documenting how mantle-derived magmas and the crust interact rely not only on improvements in instrumentation and generation of larger datasets, but also on continued development of computational tools that couple thermodynamic assessment of phase equilibria in multicomponent systems with energy and mass conservation
Asymmetric Synthesis of anti -Ī²-Amino-Ī±-Hydroxy Esters via Dynamic Kinetic Resolution of Ī²-Amino-Ī±-Keto Esters
A method for the asymmetric synthesis of enantioenriched anti-Ī±-hydroxy-Ī²-amino acid derivatives by enantioconvergent reduction of the corresponding racemic Ī±-keto esters is presented. The requisite Ī±-keto esters are prepared via Mannich addition of ethyl diazoacetate to imines followed by oxidation of the diazo group with Oxone. Implementation of a recently developed dynamic kinetic resolution of Ī²-substituted-Ī±-keto esters via Ru(II)-catalyzed asymmetric transfer hydrogenation provides the title motif in routinely high diastereo- and enantioselectivity
Dynamic nuclear polarization at 9 T using a novel 250 GHz gyrotron microwave source
In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9 T (250 GHz for g = 2 electrons, 380 MHz for [superscript 1]H). In these experiments, [superscript 1]H enhancements of up to 170 Ā± 50 have been observed in 1-[superscript 13]C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20 K; in addition, we have observed significant enhancements in [superscript 15]N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ~17 have been obtained in two-dimensional [superscript 13]Cā[superscript 13]C chemical shift correlation spectra of the amino acid Uā[superscript 13]C, [superscript 15]N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250 GHz gyrotron oscillator used in these experiments.Natural Sciences and Engineering Research Council of Canada (Postgraduate Scholarship Fellowship)National Institutes of Health (U.S.) (Grant GM-35382)National Institutes of Health (U.S.) (Grant GM-55327)National Institutes of Health (U.S.) (Grant RR-00995
Probably Approximately Knowing
Whereas deterministic protocols are typically guaranteed to obtain particular
goals of interest, probabilistic protocols typically provide only probabilistic
guarantees. This paper initiates an investigation of the interdependence
between actions and subjective beliefs of agents in a probabilistic setting. In
particular, we study what probabilistic beliefs an agent should have when
performing actions, in a protocol that satisfies a probabilistic constraint of
the form: 'Condition C should hold with probability at least p when action a is
performed'. Our main result is that the expected degree of an agent's belief in
C when it performs a equals the probability that C holds when a is performed.
Indeed, if the threshold of the probabilistic constraint should hold with
probaility p=1-x^2 for some small value of x then, with probability 1-x, when
the agent acts it will assign a probabilistic belief no smaller than 1-x to the
possibility that C holds. In other words, viewing strong belief as,
intuitively, approximate knowledge, the agent must probably approximately know
(PAK-know) that C is true when it acts.Comment: 23 pages, 2 figures, a full version of a paper whose extended
abstract appears in the proceeding of PODC 202
Asymmetric Synthesis of Diverse Glycolic Acid Scaffolds via Dynamic Kinetic Resolution of Ī±-Keto Esters
The dynamic kinetic resolution of Ī±-keto esters via asymmetric transfer hydrogenation has been developed as a technique for the highly stereoselective construction of structurally diverse Ī²-substituted-Ī±-hydroxy carboxylic acid derivatives. Through the development of a privileged m-terphenylsulfonamide for (arene)RuCl(monosulfonamide) complexes with a high affinity for selective Ī±-keto ester reduction, excellent levels of chemo-, diastereo-, and enantiocontrol can be realized in the reduction of Ī²-aryl- and Ī²-chloro-Ī±-keto esters
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