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Limits On Planets Around Pulsating White Dwarf Stars
We present limits on planetary companions to pulsating white dwarf stars. A subset of these stars exhibit extreme stability in the period and phase of some of their pulsation modes; a planet can be detected around such a star by searching for periodic variations in the arrival time of these pulsations. We present limits on companions greater than a few Jupiter masses around a sample of 15 white dwarf stars as part of an ongoing survey. One star shows a variation in arrival time consistent with a 2M(J) planet in a 4.5 yr orbit. We discuss other possible explanations for the observed signal and conclude that a planet is the most plausible explanation based on the data available.NASA Origins NAG5-13094Astronom
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A high-wavenumber boundary-element method for an acoustic scattering problem
In this paper we show stability and convergence for a novel Galerkin boundary element method approach to the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data. This problem models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve a good approximation with a relatively low number of degrees of freedom we employ a graded mesh with smaller elements adjacent to discontinuities in impedance, and a special set of basis functions for the Galerkin method so that, on each element, the approximation space consists of polynomials (of degree ) multiplied by traces of plane waves on the boundary. In the case where the impedance is constant outside an interval , which only requires the discretization of , we show theoretically and experimentally that the error in computing the acoustic field on is , where is the number of degrees of freedom and is the wavenumber. This indicates that the proposed method is especially commendable for large intervals or a high wavenumber. In a final section we sketch how the same methodology extends to more general scattering problems
Modelling the functional genomics of Parkinson’s disease in Caenorhabditis elegans: LRRK2 and beyond
For decades, Parkinson’s disease (PD) cases have been genetically categorised into familial, when caused by mutations in single genes with a clear inheritance pattern in affected families, or idiopathic, in the absence of an evident monogenic determinant. Recently, genome-wide association studies (GWAS) have revealed how common genetic variability can explain up to 36% of PD heritability and that PD manifestation is often determined by multiple variants at different genetic loci. Thus, one of the current challenges in PD research stands in modelling the complex genetic architecture of this condition and translating this into functional studies. Caenorhabditis elegans provide a profound advantage as a reductionist, economical model for PD research, with a short lifecycle, straightforward genome engineering and high conservation of PD relevant neural, cellular and molecular pathways. Functional models of PD genes utilising C. elegans show many phenotypes recapitulating pathologies observed in PD. When contrasted with mammalian in vivo and in vitro models, these are frequently validated, suggesting relevance of C. elegans in the development of novel PD functional models. This review will discuss how the nematode C. elegans PD models have contributed to the uncovering of molecular and cellular mechanisms of disease, with a focus on the genes most commonly found as causative in familial PD and risk factors in idiopathic PD. Specifically, we will examine the current knowledge on a central player in both familial and idiopathic PD, Leucine-rich repeat kinase 2 (LRRK2) and how it connects to multiple PD associated GWAS candidates and Mendelian disease-causing genes
Out-Of-Focus Holography at the Green Bank Telescope
We describe phase-retrieval holography measurements of the 100-m diameter
Green Bank Telescope using astronomical sources and an astronomical receiver
operating at a wavelength of 7 mm. We use the technique with parameterization
of the aperture in terms of Zernike polynomials and employing a large defocus,
as described by Nikolic, Hills & Richer (2006). Individual measurements take
around 25 minutes and from the resulting beam maps (which have peak signal to
noise ratios of 200:1) we show that it is possible to produce low-resolution
maps of the wavefront errors with accuracy around a hundredth of a wavelength.
Using such measurements over a wide range of elevations, we have calculated a
model for the wavefront-errors due to the uncompensated gravitational
deformation of the telescope. This model produces a significant improvement at
low elevations, where these errors are expected to be the largest; after
applying the model, the aperture efficiency is largely independent of
elevation. We have also demonstrated that the technique can be used to measure
and largely correct for thermal deformations of the antenna, which often exceed
the uncompensated gravitational deformations during daytime observing.
We conclude that the aberrations induced by gravity and thermal effects are
large-scale and the technique used here is particularly suitable for measuring
such deformations in large millimetre wave radio telescopes.Comment: 10 pages, 7 figures (accepted by Astronomy & Astrophysics
Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
We use time-resolved dynamic light scattering to investigate the slow
dynamics of a colloidal gel. The final decay of the average intensity
autocorrelation function is well described by , with and
decreasing from 1.5 to 1 with increasing . We show that the dynamics is not
due to a continuous ballistic process, as proposed in previous works, but
rather to rare, intermittent rearrangements. We quantify the dynamical
fluctuations resulting from intermittency by means of the variance
of the instantaneous autocorrelation function, the analogous of
the dynamical susceptibility studied in glass formers. The amplitude
of is found to grow linearly with . We propose a simple --yet
general-- model of intermittent dynamics that accounts for the dependence
of both the average correlation functions and .Comment: Revised and improved, to appear in Europhys. Let
Completion of a SCUBA survey of Lynds dark clouds and implications for low-mass star formation
We have carried out a survey of optically-selected dark clouds using the
bolometer array SCUBA on the James Clerk Maxwell Telescope, at 850 microns
wavelength. The survey covers a total of 0.5 square degrees and is unbiased
with reference to cloud size, star formation activity, or the presence of
infrared emission. Several new protostars and starless cores have been
discovered; the protostars are confirmed through the detection of their
accompanying outflows in CO(2-1) emission. The survey is believed to be
complete for Class 0 and Class I protostars, and yields two important results
regarding the lifetimes of these phases. First, the ratio of Class 0 to Class
protostars in the sample is roughly unity, very different from the 1:10 ratio
that has previously been observed for the rho Ophiuchi star-forming region.
Assuming star formation to be a homogeneous process in the dark clouds, this
implies that the Class 0 lifetime is similar to the Class I phase, which from
infrared surveys has been established to be approximately 200,000 yr. It also
suggests there is no rapid initial accretion phase in Class 0 objects. A burst
of triggered star formation some 100,000 yr ago can explain the earlier results
for rho Ophiuchus. Second, the number of starless cores is approximately twice
that of the total number of protostars, indicating a starless core lifetime of
approximately 800,000 yr. These starless cores are therefore very short-lived,
surviving only two or three free-fall times. This result suggests that, on size
scales of 10,000 AU at least, the dynamical evolution of starless cores is
probably not controlled by magnetic processes.Comment: 67 pages including 32 figures (highly compressed). Accepted for
publication in the Astronomical Journal. Available with full resolution
(legible) figures at http://www.mrao.cam.ac.uk/%7ejsr/papers/lynds2.ps.g
Anthropocene, Capitalocene and Liberal Cosmopolitan IR: A Response to Burke et al.’s Planet Politics
This article is a collective response to Anthony Burke et al’s ‘Planet Politics’, published in this journal in 2016, and billed as a ‘Manifesto from the end of IR’. We dispute this claim on the basis that rather than breaking from the discipline, the Manifesto provides a problematic global governance agenda which is dangerously authoritarian and deeply depoliticising. We substantiate this analysis in the claim that Burke et al reproduce an already failed and discredited liberal cosmopolitan framework through the advocacy of managerialism rather than transformation; the top-down coercive approach of international law; and use of abstract modernist political categories. In the closing sections of the article, we discuss the possibility of different approaches, which, taking the Anthropocene as both an epistemological and ontological break with modernist assumptions, could take us beyond IR’s disciplinary confines
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