2,921 research outputs found
The Overlooked Potential of Generalized Linear Models in Astronomy-III: Bayesian Negative Binomial Regression and Globular Cluster Populations
In this paper, the third in a series illustrating the power of generalized
linear models (GLMs) for the astronomical community, we elucidate the potential
of the class of GLMs which handles count data. The size of a galaxy's globular
cluster population is a prolonged puzzle in the astronomical
literature. It falls in the category of count data analysis, yet it is usually
modelled as if it were a continuous response variable. We have developed a
Bayesian negative binomial regression model to study the connection between
and the following galaxy properties: central black hole mass,
dynamical bulge mass, bulge velocity dispersion, and absolute visual magnitude.
The methodology introduced herein naturally accounts for heteroscedasticity,
intrinsic scatter, errors in measurements in both axes (either discrete or
continuous), and allows modelling the population of globular clusters on their
natural scale as a non-negative integer variable. Prediction intervals of 99%
around the trend for expected comfortably envelope the data,
notably including the Milky Way, which has hitherto been considered a
problematic outlier. Finally, we demonstrate how random intercept models can
incorporate information of each particular galaxy morphological type. Bayesian
variable selection methodology allows for automatically identifying galaxy
types with different productions of GCs, suggesting that on average S0 galaxies
have a GC population 35% smaller than other types with similar brightness.Comment: 14 pages, 12 figures. Accepted for publication in MNRA
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Hybrid propagation physics for the design and modeling of astronomical observatories: a coronagraphic example
For diffraction-limited optical systems an accurate physical optics model is
necessary to properly evaluate instrument performance. Astronomical
observatories outfitted with coronagraphs for direct exoplanet imaging require
physical optics models to simulate the effects of misalignment and diffraction.
Accurate knowledge of the observatory's PSF is integral for the design of
high-contrast imaging instruments and simulation of astrophysical observations.
The state of the art is to model the misalignment, ray aberration, and
diffraction across multiple software packages, which complicates the design
process. Gaussian Beamlet Decomposition (GBD) is a ray-based method of
diffraction calculation that has been widely implemented in commercial optical
design software. By performing the coherent calculation with data from the ray
model of the observatory, the ray aberration errors can be fed directly into
the physical optics model of the coronagraph, enabling a more integrated model
of the observatory. We develop a formal algorithm for the transfer-matrix
method of GBD, and evaluate it against analytical results and a traditional
physical optics model to assess the suitability of GBD for high-contrast
imaging simulations. Our GBD simulations of the observatory PSF, when compared
to the analytical Airy function, have a sum-normalized RMS difference of
~10^-6. These fields are then propagated through a Fraunhofer model of a
exoplanet imaging coronagraph where the mean residual numerical contrast is
4x10^-11, with a maximum near the inner working angle at 5x10^-9. These results
show considerable promise for the future development of GBD as a viable
propagation technique in high-contrast imaging. We developed this algorithm in
an open-source software package and outlined a path for its continued
development to increase the fidelity and flexibility of diffraction simulations
using GBD.Comment: 58 pages, 15 figures, preprint version for article in press. Accepted
to SPIE's Journal of Astronomical Telescopes, Instruments, and Systems on
October 23 202
The String Calculation of QCD Wilson Loops on Arbitrary Surfaces
Compact string expressions are found for non-intersecting Wilson loops in
SU(N) Yang-Mills theory on any surface (orientable or nonorientable) as a
weighted sum over covers of the surface. All terms from the coupled chiral
sectors of the 1/N expansion of the Wilson loop expectation values are
included.Comment: 10 pages, LaTeX, no figure
Intraoperative electrocochleographic characteristics of auditory neuropathy spectrum disorder in cochlear implant subjects
Auditory neuropathy spectrum disorder (ANSD) is characterized by an apparent discrepancy between measures of cochlear and neural function based on auditory brainstem response (ABR) testing. Clinical indicators of ANSD are a present cochlear microphonic (CM) with small or absent wave V. Many identified ANSD patients have speech impairment severe enough that cochlear implantation (CI) is indicated. To better understand the cochleae identified with ANSD that lead to a CI, we performed intraoperative round window electrocochleography (ECochG) to tone bursts in children (n = 167) and adults (n = 163). Magnitudes of the responses to tones of different frequencies were summed to measure the “total response” (ECochG-TR), a metric often dominated by hair cell activity, and auditory nerve activity was estimated visually from the compound action potential (CAP) and auditory nerve neurophonic (ANN) as a ranked “Nerve Score”. Subjects identified as ANSD (45 ears in children, 3 in adults) had higher values of ECochG-TR than adult and pediatric subjects also receiving CIs not identified as ANSD. However, nerve scores of the ANSD group were similar to the other cohorts, although dominated by the ANN to low frequencies more than in the non-ANSD groups. To high frequencies, the common morphology of ANSD cases was a large CM and summating potential, and small or absent CAP. Common morphologies in other groups were either only a CM, or a combination of CM and CAP. These results indicate that responses to high frequencies, derived primarily from hair cells, are the main source of the CM used to evaluate ANSD in the clinical setting. However, the clinical tests do not capture the wide range of neural activity seen to low frequency sounds
A Climate-Data Record of the "Clear-Sky" Surface Temperature of the Greenland Ice Sheet
We are developing a climate-data record (CDR of daily "clear-sky" ice-surface temperature (IST) of the Greenland Ice Sheet, from 1982 to the present using Advanced Very High Resolution Radiometer (AVHRR) (1982 - present) and Moderate-Resolution Imaging Spectroradiometer (MODIS) data (2000 - present) at a resolution of approximately 5 km. The CDR will be continued in the National Polar-orbiting Operational Environmental Satellite System Visible/Infrared Imager Radiometer Suite era. Two algorithms remain under consideration. One algorithm under consideration is based on the split-window technique used in the Polar Pathfinder dataset (Fowler et al., 2000 & 21007). Another algorithm under consideration, developed by Comiso (2006), uses a single channel of AVHRR data (channel 4) in conjunction with meteorological-station data to account for atmospheric effects and drift between AVHRR instruments. Known issues being addressed in the production of the CDR are: tune-series bias caused by cloud cover (surface temperatures can be different under clouds vs. clear areas) and cross-calibration in the overlap period between AVHRR instruments, and between AVHRR and MODIS instruments. Because of uncertainties, mainly due to clouds (Stroeve & Steffen, 1998; Wang and Key, 2005; Hall et al., 2008 and Koenig and Hall, submitted), time-series of satellite 1S'1" do not necessarily correspond to actual surface temperatures. The CDR will be validated by comparing results with automatic-,",eather station (AWS) data and with satellite-derived surface-temperature products. Regional "clear-sky" surface temperature increases in the Arctic, measured from AVHRR infrared data, range from 0.57+/-0.02 deg C (Wang and Key, 2005) to 0.72+/-0.10 deg C (Comiso, 2006) per decade since the early 1980s. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near 0 deg C during the melt season, and is thus vulnerable to rapid melting if temperatures continue to increase. Reference
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
PROBER: oligonucleotide FISH probe design software
PROBER is an oligonucleotide primer design software application that designs multiple primer pairs for generating PCR probes useful for fluorescence in situ hybridization (FISH). PROBER generates Tiling Oligonucleotide Probes (TOPs) by masking repetitive genomic sequences and delineating essentially unique regions that can be amplified to yield small (100-2000 bp) DNA probes that in aggregate will generate a single, strong fluorescent signal for regions as small as a single gene. TOPs are an alternative to bacterial artificial chromosomes (BACs) that are commonly used for FISH but may be unstable, unavailable, chimeric, or non-specific to small (10-100 kb) genomic regions. PROBER can be applied to any genomic locus, with the limitation that the locus must contain at least 10 kb of essentially unique blocks. To test the software, we designed a number of probes for genomic amplifications and hemizygous deletions that were initially detected by Representational Oligonucleotide Microarray Analysis of breast cancer tumors. AVAILABILITY: http://prober.cshl.ed
Possible scale invariant linear magnetoresistance in pyrochlore iridates Bi2Ir2O7
We report the observation of a linear magnetoresistance in single crystals and epitaxial thin films of the pyrochlore iridate Bi2Ir2O7. The linear magnetoresistance is positive and isotropic at low temperatures, without any sign of saturation up to 35 T. As temperature increases, the linear field dependence gradually evolves to a quadratic field dependence. The temperature and field dependence of magnetoresistance of Bi2Ir2O7 bears strikingly resemblance to the scale invariant magnetoresistance observed in the strange metal phase in high Tc cuprates. However, the residual resistivity of Bi2Ir2O7 is more than two orders of magnitude higher than the curpates. Our results suggest that the correlation between linear magnetoresistance and quantum fluctuations may exist beyond high temperature superconductors
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