81,169 research outputs found
Towards precision distances and 3D dust maps using broadband Period--Magnitude relations of RR Lyrae stars
We determine the period-magnitude relations of RR Lyrae stars in 13
photometric bandpasses from 0.4 to 12 {\mu}m using timeseries observations of
134 stars. The Bayesian formalism, extended from our previous work to include
the effects of line-of-sight dust extinction, allows for the simultaneous
inference of the posterior distribution of the mean absolute magnitude, slope
of the period-magnitude power-law, and intrinsic scatter about a perfect
power-law for each bandpass. In addition, the distance modulus and
line-of-sight dust extinction to each RR Lyrae star in the calibration sample
is determined, yielding a sample median fractional distance error of 0.66%. The
intrinsic scatter in all bands appears to be larger than the photometric
errors, except in WISE W1 (3.4 {\mu}m) and W2 (4.6 {\mu}m) where the
photometric error ( mag) is to be comparable or larger
than the intrinsic scatter. Additional observations at these wavelengths could
improve the inferred distances to these sources further. As an application of
the methodology, we infer the distance to the RRc-type star RZCep at low
Galactic latitude () to be mag
( pc) with colour excess mag. This
distance, equivalent to a parallax of microarcsec, is consistent
with the published HST parallax measurement but with an uncertainty that is 13
times smaller than the HST measurement. If our measurements (and methodology)
hold up to scrutiny, the distances to these stars have been determined to an
accuracy comparable to those expected with Gaia. As RR Lyrae are one of the
primary components of the cosmic distance ladder, the achievement of sub-1%
distance errors within a formalism that accounts for dust extinction may be
considered a strong buttressing of the path to eventual 1% uncertainties in
Hubble's constant.Comment: 21 pages, 29 figures, 2 tables, abstract abridged for arXiv. Comments
solicited on referee report (received June 9, 2014) linked:
https://gist.github.com/profjsb/c6c4e2f3a20ea02f1762 . Public archive of code
used to generate results and figures:
https://github.com/ckleinastro/period_luminosity_relation_fittin
Asteroid Models from Multiple Data Sources
In the past decade, hundreds of asteroid shape models have been derived using
the lightcurve inversion method. At the same time, a new framework of 3-D shape
modeling based on the combined analysis of widely different data sources such
as optical lightcurves, disk-resolved images, stellar occultation timings,
mid-infrared thermal radiometry, optical interferometry, and radar
delay-Doppler data, has been developed. This multi-data approach allows the
determination of most of the physical and surface properties of asteroids in a
single, coherent inversion, with spectacular results. We review the main
results of asteroid lightcurve inversion and also recent advances in multi-data
modeling. We show that models based on remote sensing data were confirmed by
spacecraft encounters with asteroids, and we discuss how the multiplication of
highly detailed 3-D models will help to refine our general knowledge of the
asteroid population. The physical and surface properties of asteroids, i.e.,
their spin, 3-D shape, density, thermal inertia, surface roughness, are among
the least known of all asteroid properties. Apart for the albedo and diameter,
we have access to the whole picture for only a few hundreds of asteroids. These
quantities are nevertheless very important to understand as they affect the
non-gravitational Yarkovsky effect responsible for meteorite delivery to Earth,
or the bulk composition and internal structure of asteroids.Comment: chapter that will appear in a Space Science Series book Asteroids I
Correlation of the rate of Type Ia supernovae with the parent galaxy properties: Light and shadows
The identification of the progenitors of Type Ia Supernovae (SNIa) is
extremely important in several astrophysical contexts, ranging from stellar
evolution in close binary systems to evaluating cosmological parameters.
Determining the distribution of the delay times (DTD) of SNIa progenitors can
shed light on their nature. In this paper we investigate on the diagnostic
capabilities on the DTD of the correlation between the SNIa rate and the parent
galaxy properties by examining its systematics with the various parameters at
play: simple stellar population models, the adopted description for the star
formation history in galaxies, and the way in which the masses of the galaxies
are evaluated. We compute models for the correlations of the SNIa rate with the
parent galaxy color and specific star formation rate for a variety of input
ingredients, and for a few astrophysically motivated DTD laws. The models are
compared to the results of three independent observational surveys. We find
that the scaling of the SNIa rate with the properties of the parent galaxy is
sensitive to all input ingredients mentioned above. This is a severe limitation
on the possibility to discriminate alternative DTDs. In addition, current
surveys show some discrepancies for the rate measured in the reddest and bluest
galaxies, likely due to limited statistics and inhomogeneity of the
observations. For galaxies with intermediate colors the rates are in agreement,
leading to a robust determination of the productivity of SNIa from stellar
populations of 0.8 events per 1000 \msun. Large stastistics of SNIa
events along with accurate measurements of the star formation history in the
galaxies are required to derive firm constraints on the DTD. LSST will achieve
these results by providing the homogeneous, unbiased and vast database on both
SNIa and galaxies.Comment: Astronomy and Astrophysics in press. Includes one more figure in the
appendix. Notice the slight change of titl
Approximate Bayesian Image Interpretation using Generative Probabilistic Graphics Programs
The idea of computer vision as the Bayesian inverse problem to computer
graphics has a long history and an appealing elegance, but it has proved
difficult to directly implement. Instead, most vision tasks are approached via
complex bottom-up processing pipelines. Here we show that it is possible to
write short, simple probabilistic graphics programs that define flexible
generative models and to automatically invert them to interpret real-world
images. Generative probabilistic graphics programs consist of a stochastic
scene generator, a renderer based on graphics software, a stochastic likelihood
model linking the renderer's output and the data, and latent variables that
adjust the fidelity of the renderer and the tolerance of the likelihood model.
Representations and algorithms from computer graphics, originally designed to
produce high-quality images, are instead used as the deterministic backbone for
highly approximate and stochastic generative models. This formulation combines
probabilistic programming, computer graphics, and approximate Bayesian
computation, and depends only on general-purpose, automatic inference
techniques. We describe two applications: reading sequences of degraded and
adversarially obscured alphanumeric characters, and inferring 3D road models
from vehicle-mounted camera images. Each of the probabilistic graphics programs
we present relies on under 20 lines of probabilistic code, and supports
accurate, approximately Bayesian inferences about ambiguous real-world images.Comment: The first two authors contributed equally to this wor
BayesNAS: A Bayesian Approach for Neural Architecture Search
One-Shot Neural Architecture Search (NAS) is a promising method to
significantly reduce search time without any separate training. It can be
treated as a Network Compression problem on the architecture parameters from an
over-parameterized network. However, there are two issues associated with most
one-shot NAS methods. First, dependencies between a node and its predecessors
and successors are often disregarded which result in improper treatment over
zero operations. Second, architecture parameters pruning based on their
magnitude is questionable. In this paper, we employ the classic Bayesian
learning approach to alleviate these two issues by modeling architecture
parameters using hierarchical automatic relevance determination (HARD) priors.
Unlike other NAS methods, we train the over-parameterized network for only one
epoch then update the architecture. Impressively, this enabled us to find the
architecture on CIFAR-10 within only 0.2 GPU days using a single GPU.
Competitive performance can be also achieved by transferring to ImageNet. As a
byproduct, our approach can be applied directly to compress convolutional
neural networks by enforcing structural sparsity which achieves extremely
sparse networks without accuracy deterioration.Comment: International Conference on Machine Learning 201
The Science Case for an Extended Spitzer Mission
Although the final observations of the Spitzer Warm Mission are currently
scheduled for March 2019, it can continue operations through the end of the
decade with no loss of photometric precision. As we will show, there is a
strong science case for extending the current Warm Mission to December 2020.
Spitzer has already made major impacts in the fields of exoplanets (including
microlensing events), characterizing near Earth objects, enhancing our
knowledge of nearby stars and brown dwarfs, understanding the properties and
structure of our Milky Way galaxy, and deep wide-field extragalactic surveys to
study galaxy birth and evolution. By extending Spitzer through 2020, it can
continue to make ground-breaking discoveries in those fields, and provide
crucial support to the NASA flagship missions JWST and WFIRST, as well as the
upcoming TESS mission, and it will complement ground-based observations by LSST
and the new large telescopes of the next decade. This scientific program
addresses NASA's Science Mission Directive's objectives in astrophysics, which
include discovering how the universe works, exploring how it began and evolved,
and searching for life on planets around other stars.Comment: 75 pages. See page 3 for Table of Contents and page 4 for Executive
Summar
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
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