2,698 research outputs found
Testing asteroseismology with Gaia DR2: Hierarchical models of the Red Clump
Asteroseismology provides fundamental stellar parameters independent of
distance, but subject to systematics under calibration. Gaia DR2 has provided
parallaxes for a billion stars, which are offset by a parallax zero-point. Red
Clump (RC) stars have a narrow spread in luminosity, thus functioning as
standard candles to calibrate these systematics. This work measures how the
magnitude and spread of the RC in the Kepler field are affected by changes to
temperature and scaling relations for seismology, and changes to the parallax
zero-point for Gaia. We use a sample of 5576 RC stars classified through
asteroseismology. We apply hierarchical Bayesian latent variable models,
finding the population level properties of the RC with seismology, and use
those as priors on Gaia parallaxes to find the parallax zero-point offset. We
then find the position of the RC using published values for the zero-point. We
find a seismic temperature insensitive spread of the RC of ~0.03 mag in the
2MASS K band and a larger and slightly temperature-dependent spread of ~0.13
mag in the Gaia G band. This intrinsic dispersion in the K band provides a
distance precision of ~1% for RC stars. Using Gaia data alone, we find a mean
zero-point of -41 10 as. This offset yields RC absolute magnitudes
of -1.634 0.018 in K and 0.546 0.016 in G. Obtaining these same
values through seismology would require a global temperature shift of ~-70 K,
which is compatible with known systematics in spectroscopy.Comment: Accepted for publication in MNRA
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Precise Throughput Determination of the PanSTARRS Telescope and the Gigapixel Imager using a Calibrated Silicon Photodiode and a Tunable Laser: Initial Results
We have used a precision calibrated photodiode as the fundamental metrology reference in order to determine the relative throughput of the PanSTARRS telescope and the Gigapixel imager, from 400 nm to 1050 nm. Our technique uses a tunable laser as a source of illumination on a transmissive flat-field screen. We determine the full-aperture system throughput as a function of wavelength, including (in a single integral measurement) the mirror reflectivity, the transmission functions of the filters and the corrector optics, and the detector quantum efficiency, by comparing the light seen by each pixel in the CCD array to that measured by a precision-calibrated silicon photodiode. This method allows us to determine the relative throughput of the entire system as a function of wavelength, for each pixel in the instrument, without observations of celestial standards. We present promising initial results from this characterization of the PanSTARRS system, and we use synthetic photometry to assess the photometric perturbations due to throughput variation across the field of view.Physic
Evidence for Two Populations of Classical Transneptunian Objects: The Strong Inclination Dependence of Classical Binaries
We have searched 101 Classical transneptunian objects for companions with the
Hubble Space Telescope. Of these, at least 21 are binary. The heliocentric
inclinations of the objects we observed range from 0.6-34 degrees. We find a
very strong anticorrelation of binaries with inclination. Of the 58 targets
that have inclinations of less than 5.5 degrees, 17 are binary, a binary
fraction of 29 +7/-6%. All 17 are similar-brightness systems. On the contrary,
only 4 of the 42 objects with inclinations greater than 5.5 degrees have
satellites and only 1 of these is a similar-brightness binary. This striking
dichotomy appears to agree with other indications that the low eccentricity,
non-resonant Classical transneptunian objects include two overlapping
populations with significantly different physical properties and dynamical
histories.Comment: 32 pages, 4 figures, 2 tables in press for Icarus volume 194 Note
added in proof and reference updates in this corrected versio
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Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite
Layered hybrid perovskites have attracted much attention in recent years due
to their emergent physical properties and exceptional functional performances,
but the coexistence of lattice order and structural disorder severely hinders
our understanding of these materials. One unsolved problem regards how the
lattice dynamics are affected by the dimensional engineering of the inorganic
frameworks and the interaction with the molecular moieties. Here, we address
this question by using a combination of high-resolution spontaneous Raman
scattering, high-field terahertz spectroscopy, and molecular dynamics
simulations. This approach enables us to reveal the structural vibrations and
disorder in and out of equilibrium and provides surprising observables that
differentiate single- and double-layered perovskites. While no distinct
vibrational coherence is observed in double-layer perovskites, we discover that
an off-resonant terahertz pulse can selectively drive a long-lived coherent
phonon mode through a two-photon process in the single-layered system. This
difference highlights the dramatic change in the lattice environment as the
dimension is reduced. The present findings pave the way for the ultrafast
structural engineering of hybrid lattices as well as for developing high-speed
optical modulators based on layered perovskites
Pluralism about Knowledge
In this paper I consider the prospects for pluralism about knowledge, that is, the view that there is a plurality of knowledge relations. After a brief overview of some views that entail a sort of pluralism about knowledge, I focus on a particular kind of knowledge pluralism I call standards pluralism. Put roughly, standards pluralism is the view that one never knows anything simpliciter. Rather, one knows by this-or-that epistemic standard. Because there is a plurality of epistemic standards, there is a plurality of knowledge relations. In §1 I argue that one can construct an impressive case for standards pluralism. In §2 I clarify the relationship between standards pluralism, epistemic contextualism and epistemic relativism. In §3 I argue that standards pluralism faces a serious objection. The gist of the objection is that standards pluralism is incompatible with plausible claims about the normative role of knowledge. In §4 I finish by sketching the form that a standards pluralist response to this objection might take
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Heterogenization of Homogeneous Catalysts: the Effect of the Support
We have studied the influence of placing a soluble, homogeneous catalyst onto a solid support. We determined that such a 'heterogenized' homogeneous catalyst can have improved activity and selectivity for the asymmetric hydrogenation of enamides to amino acid derivatives. The route of heterogenization of RhDuPhos(COD){sup +} cations occurs via electrostatic interactions with anions that are capable of strong hydrogen bonding to silica surfaces. This is a novel approach to supported catalysis. Supported RhDuPhos(COD){sup +} is a recyclable, non-leaching catalyst in non-polar media. This is one of the few heterogenized catalysts that exhibits improved catalytic performance as compared to its homogeneous analog
Quantitative approaches in climate change ecology
Contemporary impacts of anthropogenic climate change on ecosystems are increasingly being recognized. Documenting the extent of these impacts requires quantitative tools for analyses of ecological observations to distinguish climate impacts in noisy data and to understand interactions between climate variability and other drivers of change. To assist the development of reliable statistical approaches, we review the marine climate change literature and provide suggestions for quantitative approaches in climate change ecology. We compiled 267 peer-reviewed articles that examined relationships between climate change and marine ecological variables. Of the articles with time series data (n = 186), 75% used statistics to test for a dependency of ecological variables on climate variables. We identified several common weaknesses in statistical approaches, including marginalizing other important non-climate drivers of change, ignoring temporal and spatial autocorrelation, averaging across spatial patterns and not reporting key metrics. We provide a list of issues that need to be addressed to make inferences more defensible, including the consideration of (i) data limitations and the comparability of data sets; (ii) alternative mechanisms for change; (iii) appropriate response variables; (iv) a suitable model for the process under study; (v) temporal autocorrelation; (vi) spatial autocorrelation and patterns; and (vii) the reporting of rates of change. While the focus of our review was marine studies, these suggestions are equally applicable to terrestrial studies. Consideration of these suggestions will help advance global knowledge of climate impacts and understanding of the processes driving ecological change
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