3,010 research outputs found
Comment: Struggles with Survey Weighting and Regression Modeling
Comment: Struggles with Survey Weighting and Regression Modeling
[arXiv:0710.5005]Comment: Published in at http://dx.doi.org/10.1214/088342307000000159 the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Forecasting Organic Food Prices: Testing and Evaluating Conditional Predictive Ability
Organic farmers, wholesalers, and retailers need reliable price forecasts to improve their decision- making practices. This paper presents a methodology and protocol to select the best-performing method from several time and frequency domain candidates. Weekly farmgate prices for organic fresh produce are used. Forecasting methods are evaluated on the basis of an aggregate accuracy measure and several out-of-sample predictive ability tests. Combining forecasts to improve on individual forecasts is investigated.Demand and Price Analysis,
Quantum chemical studies of carbon-13 equilibrium fractionation in ionâmolecule reactions
Ab initio computational quantum chemical methods are used to calculate reduced partition function ratios for all isotopomers of CO, HCO+,HCO+, and HOC+HOC+ involving the nuclides 1H,1H, 2H2H (D), 12C,12C, 13C,13C, 16O,16O, and 18O.18O. The ratios are used to calculate equilibrium constants for the reaction pairs HCO+/CO,HCO+/CO, HOC+/CO,HOC+/CO, and C+/CO.C+/CO. Both simple proton transfers and more complex isotopic variants involving the breaking and reforming of CO bonds are considered. The probable pathways for the HCO+/COHCO+/CO and C+/COC+/CO exchange reactions are explored in detail using high-accuracy quantum chemical calculations. It appears most likely that the HCO+/COHCO+/CO reaction proceeds through exothermic formation of the linear adduct OCHCO+OCHCO+ with DâhDâh symmetry. Similarly, the C+/COC+/CO reaction proceeds along a spin-allowed pathway with exothermic formation of the linear adduct COC+COC+ with DâhDâh symmetry. An alternate but higher energy spin-allowed pathway for the C+/COC+/CO reaction passes through a transition state with only CsCs symmetry and a locally stable intermediate with C2vC2v symmetry. In the ISM these reactions may proceed by these direct pathways or indirectly through coupled exothermic reaction pairs involving other species to achieve 13C/12C13C/12C isotope exchange. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70679/2/JCPSA6-108-19-8012-1.pd
An updated stellar census of the Quintuplet cluster
Context. Found within the central molecular zone, the Quintuplet is one of the most massive young clusters in the Galaxy. As a consequence it offers the prospect of constraining stellar formation and evolution in extreme environments. However, current observations suggest that it comprises a remarkably diverse stellar population that is difficult to reconcile with an instantaneous formation event.
Aims. To better understand the nature of the cluster our aim is to improve observational constraints on the constituent stars.
Methods. In order to accomplish this goal we present Hubble Space Telescope/NICMOS+WFC3 photometry and Very Large Telescope/SINFONI+KMOS spectroscopy for âŒ100 and 71 cluster members, respectively.
Results. Spectroscopy of the cluster members reveals the Quintuplet to be far more homogeneous than previously expected. All supergiants are classified as either O7â8 Ia or O9âB0 Ia, with only one object of earlier (O5 IâIII) spectral type. These stars form a smooth morphological sequence with a cohort of seven early-B hypergiants and six luminous blue variables and WN9-11h stars, which comprise the richest population of such stars of any stellar aggregate known. In parallel, we identify a smaller population of late-O hypergiants and spectroscopically similar WN8â9ha stars. No further H-free WolfâRayet (WR) stars are identified, leaving an unexpectedly extreme ratio of 13:1 for WC/WN stars. A subset of the O9âB0 supergiants are unexpectedly faint, suggesting they are both less massive and older than the greater cluster population. Finally, no main sequence objects were identifiable.
Conclusions. Due to uncertainties over which extinction law to apply, it was not possible to quantitatively determine a cluster age via isochrone fitting. Nevertheless, we find an impressive coincidence between the properties of cluster members preceding the H-free WR phase and the evolutionary predictions for a single, non-rotating 60 Mâ star; in turn this implies an age of âŒ3.0â3.6 Myr for the Quintuplet. Neither the late O-hypergiants nor the low luminosity supergiants are predicted by such a path; we suggest that the former either result from rapid rotators or are the products of binary driven mass-stripping, while the latter may be interlopers. The H-free WRs must evolve from stars with an initial mass in excess of 60 Mâ but it appears difficult to reconcile their observational properties with theoretical expectations. This is important since one would expect the most massive stars within the Quintuplet to be undergoing core-collapse/SNe at this time; since the WRs represent an evolutionary phase directly preceding this event,their physical properties are crucial to understanding both this process and the nature of the resultant relativistic remnant. As such, the Quintuplet provides unique observational constraints on the evolution and death of the most massive stars forming in the local, high metallicity Universe
Proton pump inhibitors are associated with increased risk of development of chronic kidney disease
Background Acute interstitial nephritis secondary to proton pump inhibitors (PPIs) frequently goes undiagnosed due to its subacute clinical presentation, which may later present as chronic kidney disease (CKD). We investigated the association of PPI use with the development of CKD and death. Methods Two separate retrospective caseâcontrol study designs were employed with a prospective logistic regression analysis of data to evaluate the association of development of CKD and death with PPI use. The population included 99,269 patients who were seen in primary care VISN2 clinics from 4/2001 until 4/2008. For evaluation of the CKD outcome, 22,807 with preexisting CKD at the first observation in Veterans Affairs Health Care Upstate New York (VISN2) network data system were excluded. Data obtained included use of PPI (Yes/No), demographics, laboratory data, pre-PPI comorbidity variables. Results A total of 19,311/76,462 patients developed CKD. Of those who developed CKD 24.4 % were on PPI. Patients receiving PPI were less likely to have vascular disease, COPD, cancer and diabetes. Of the total of 99,269 patients analyzed for mortality outcome, 11,758 died. A prospective logistic analysis of caseâcontrol data showed higher odds for development of CKD (OR 1.10 95 % CI 1.05â1.16) and mortality (OR 1.76, 95 % CI 1.67â1.84) among patients taking PPIs versus those not on PPIs. Conclusions Use of proton pump inhibitors is associated with increased risk of development of CKD and death. With the large number of patients being treated with proton pump inhibitors, healthcare providers need to be better educated about the potential side effects of these medications
The Arches cluster revisited: II. A massive eclipsing spectroscopic binary in the Arches cluster
We have carried out a spectroscopic variability survey of some of the most massive stars in the Arches cluster, using K-band observations obtained with SINFONI on the VLT. One target, F2, exhibits substantial changes in radial velocity; in combination with new KMOS and archival SINFONI spectra, its primary component is found to undergo radial velocity variation with a period of 10.483+/-0.002 d and an amplitude of ~350 km/s-1. A secondary radial velocity curve is also marginally detectable. We reanalyse archival NAOS-CONICA photometric survey data in combination with our radial velocity results to confirm this object as an eclipsing SB2 system, and the first binary identified in the Arches. We model it as consisting of an 82+/-12 Mâ WN8-9h primary and a 60+/-8 Mâ O5-6 Ia+ secondary, and as having a slightly eccentric orbit, implying an evolutionary stage prior to strong binary interaction. As one of four X-ray bright Arches sources previously proposed as colliding-wind massive binaries, it may be only the first of several binaries to be discovered in this cluster, presenting potential challenges to recent models for the Arches' age and composition. It also appears to be one of the most massive binaries detected to date; the primary's calculated initial mass of >~120 Mâ would arguably make this the most massive binary known in the Galaxy
Simple Model of the Dynamic JahnâTeller Effect in SixâCoordinated Copper(II) Complexes
A model potential is assumed for describing the vibrational degrees of freedom associated with the JahnâTeller effect in sixâcoordinated copper (II) complexes. The pseudorotational limit is characterized by a potential that is constant in the region between two concentric cylinders, but becomes infinite elsewhere. The energy spectrum is obtained for both angular and radial excitations. A squareâwell periodic angular potential is applied as a perturbation, yielding a localization of states. The results are used to describe the temperature dependence of the electron spin resonance spectra of copper (II) complexes with emphasis on the system NaCl:Cu(II) containing the hexachlorocuprate (II) complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70781/2/JCPSA6-57-2-702-1.pd
Rotational energy dispersions for van der Waals molecular clusters: Analytic descriptions for Rg3, Rg4, and Rg6
We have obtained analytic expressions, parametric in centrifugal displacement coordinates, which provide exact classical descriptions of the rotational energy dispersions, that is, the dependence of the combined rotational and ââelectronicââ (vibrational potential) energies on the rotational angular momenta, for small molecular clusters bound by van der Waals interactions modeled by pairwise additive LennardâJones 6â12 potential energies. The clusters considered consist of three (equilateral triangle), four (tetrahedron), and six (octahedron) units and serve as models for small clusters of rareâgas atoms such as argon. This work represents an extension of our recently published study of analytic rotational energy dispersions for diatomic molecules bound by harmonic oscillator, Morse, or LennardâJones potentials [J. Mol. Spectrosc. 155, 205 (1992)]. A parallel set of studies were made using an angular momentumâconserving simulation program. The physical properties of the clusters that are addressed using our results include calculation of quartic and higherâorder spectroscopic constants, location of rotational instabilities, and characterization of the ââcubicââ anisotropies for the spherical top clusters A4 and A6. Of particular interest is the result that for each of these cluster types the preferred direction of the rotational angular momentum is parallel to a molecular fourfold axis, leading to reduced symmetries of D2d for tetrahedral A4 and D4h for octahedral A6.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70411/2/JCPSA6-99-9-6369-1.pd
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