4,186 research outputs found
A Preliminary Discussion of the Kinematics of BHB and RR Lyrae Stars near the North Galactic Pole
The radial velocity dispersion of 67 RR Lyrae variable and blue horizontal
branch (BHB) stars that are more than 4 kpc above the galactic plane at the
North Galactic Pole is 110 km/sec and shows no trend with Z (the height above
the galactic plane). Nine stars with Z < 4 kpc show a smaller velocity
dispersion (40 +/-9 km/sec) as is to be expected if they mostly belong to a
population with a flatter distribution. Both RR Lyrae stars and BHB stars show
evidence of stream motion; the most significant is in fields RR2 and RR3 where
24 stars in the range 4.0 < Z < 11.0 kpc have a mean radial velocity of -59 +/-
16 km/sec. Three halo stars in field RR 2 appear to be part of a moving group
with a common radial velocity of -90 km/sec. The streaming phenomenon therefore
occurs over a range of spatial scales. The BHB and RR Lyrae stars in our sample
both have a similar range of metallicity (-1.2 < [Fe/H] < -2.2). Proper motions
of BHB stars in fields SA 57 (NGP) and the Anticenter field (RR 7) (both of
which lie close to the meridional plane of the Galaxy) show that the stars that
have Z 4 kpc have a Galactic V motion that is
< -200 km/sec and which is characteristic of the halo. Thus the stars that have
a flatter distribution are really halo stars and not members of the metal-weak
thick-disk.Comment: Accepted for publication in the March 1996 AJ. 15 pages, AASTeX V4.0
latex format (including figures), 2 eps figures, 2 separate AASTeX V4.0 latex
table
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Exploratory analysis using machine learning to predict for chest wall pain in patients with stage I non-small-cell lung cancer treated with stereotactic body radiation therapy.
Background and purposeChest wall toxicity is observed after stereotactic body radiation therapy (SBRT) for peripherally located lung tumors. We utilize machine learning algorithms to identify toxicity predictors to develop dose-volume constraints.Materials and methodsTwenty-five patient, tumor, and dosimetric features were recorded for 197 consecutive patients with Stage I NSCLC treated with SBRT, 11 of whom (5.6%) developed CTCAEv4 grade â„2 chest wall pain. Decision tree modeling was used to determine chest wall syndrome (CWS) thresholds for individual features. Significant features were determined using independent multivariate methods. These methods incorporate out-of-bag estimation using Random forests (RF) and bootstrapping (100 iterations) using decision trees.ResultsUnivariate analysis identified rib dose to 1 cc < 4000 cGy (P = 0.01), chest wall dose to 30 cc < 1900 cGy (P = 0.035), rib Dmax < 5100 cGy (P = 0.05) and lung dose to 1000 cc < 70 cGy (P = 0.039) to be statistically significant thresholds for avoiding CWS. Subsequent multivariate analysis confirmed the importance of rib dose to 1 cc, chest wall dose to 30 cc, and rib Dmax. Using learning-curve experiments, the dataset proved to be self-consistent and provides a realistic model for CWS analysis.ConclusionsUsing machine learning algorithms in this first of its kind study, we identify robust features and cutoffs predictive for the rare clinical event of CWS. Additional data in planned subsequent multicenter studies will help increase the accuracy of multivariate analysis
The Relationship Between Galaxies and Low Redshift Weak Lyman alpha Absorbers in the Directions of H1821+643 and PG1116+215
To study the nature of low z Lya absorbers in the spectra of QSOs, we have
obtained high signal-to-noise UV spectra of H 1821+643 (z = 0.297) and PG
1116+215 (z = 0.177) with the GHRS on the HST. The spectra have minimum S/N of
70-100 and 3 sigma limiting equivalent widths of 50-75 mA. We detect 26 Lya
lines with Wr > 50 mA toward H1821+643 and 13 toward PG1116+215, which implies
a density of 102+/-16 lines per unit redshift. The two-point correlation
function shows marginal evidence of clustering on ~500 km/s scales, but only if
the weakest lines are excluded. We have also used the WIYN Observatory to
measure galaxy redshifts in the ~1 degree fields centered on each QSO. We find
17 galaxy-absorber pairs within projected distances of 1 Mpc with velocity
separations of 350 km/s or less. Monte Carlo simulations show that if the Lya
lines are randomly distributed, the probability of observing this many close
pairs is 3.6e-5. We find that all galaxies with projected distances of 600 kpc
or less have associated Lya absorbers within 1000 km/s, and the majority of
these galaxies have absorbers within 350 km/s. We also find that the Lya
equivalent width is anticorrelated with the projected distance of the nearest
galaxy out to at least 600 kpc, but this should be interpreted cautiously
because there are potential selection biases. Statistical tests using the
entire sample also indicate that the absorbers are not randomly distributed. We
discuss the nature of the Lya absorbers in light of the new data.Comment: Accepted for publication in ApJ. 17 pages plus 11 tables and 17
figure
The BTC40 Survey for Quasars at 4.8 < z < 6
The BTC40 Survey for high-redshift quasars is a multicolor search using
images obtained with the Big Throughput Camera (BTC) on the CTIO 4-m telescope
in V, I, and z filters to search for quasars at redshifts of 4.8 < z < 6. The
survey covers 40 sq. deg. in B, V, & I and 36 sq. deg. in z. Limiting
magnitudes (3 sigma) reach to V = 24.6, I = 22.9 and z = 22.9. We used the
(V-I) vs. (I-z) two-color diagram to select high-redshift quasar candidates
from the objects classified as point sources in the imaging data. Follow-up
spectroscopy with the AAT and CTIO 4-m telescopes of candidates having I < 21.5
has yielded two quasars with redshifts of z = 4.6 and z = 4.8 as well as four
emission line galaxies with z = 0.6. Fainter candidates have been identified
down to I = 22 for future spectroscopy on 8-m class telescopes.Comment: 27 pages, 8 figures; Accepted for publication in the Astronomical
Journa
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Building more accurate decision trees with the additive tree.
The expansion of machine learning to high-stakes application domains such as medicine, finance, and criminal justice, where making informed decisions requires clear understanding of the model, has increased the interest in interpretable machine learning. The widely used Classification and Regression Trees (CART) have played a major role in health sciences, due to their simple and intuitive explanation of predictions. Ensemble methods like gradient boosting can improve the accuracy of decision trees, but at the expense of the interpretability of the generated model. Additive models, such as those produced by gradient boosting, and full interaction models, such as CART, have been investigated largely in isolation. We show that these models exist along a spectrum, revealing previously unseen connections between these approaches. This paper introduces a rigorous formalization for the additive tree, an empirically validated learning technique for creating a single decision tree, and shows that this method can produce models equivalent to CART or gradient boosted stumps at the extremes by varying a single parameter. Although the additive tree is designed primarily to provide both the model interpretability and predictive performance needed for high-stakes applications like medicine, it also can produce decision trees represented by hybrid models between CART and boosted stumps that can outperform either of these approaches
A toolkit of mechanism and context independent widgets
Most human-computer interfaces are designed to run on a static platform (e.g. a workstation with a monitor) in a static environment (e.g. an office). However, with mobile devices becoming ubiquitous and capable of running applications similar to those found on static devices, it is no longer valid to design static interfaces. This paper describes a user-interface architecture which allows interactors to be flexible about the way they are presented. This flexibility is defined by the different input and output mechanisms used. An interactor may use different mechanisms depending upon their suitability in the current context, user preference and the resources available for presentation using that mechanism
Expert-Augmented Machine Learning
Machine Learning is proving invaluable across disciplines. However, its
success is often limited by the quality and quantity of available data, while
its adoption by the level of trust that models afford users. Human vs. machine
performance is commonly compared empirically to decide whether a certain task
should be performed by a computer or an expert. In reality, the optimal
learning strategy may involve combining the complementary strengths of man and
machine. Here we present Expert-Augmented Machine Learning (EAML), an automated
method that guides the extraction of expert knowledge and its integration into
machine-learned models. We use a large dataset of intensive care patient data
to predict mortality and show that we can extract expert knowledge using an
online platform, help reveal hidden confounders, improve generalizability on a
different population and learn using less data. EAML presents a novel framework
for high performance and dependable machine learning in critical applications
Quasar Candidates in the Hubble Deep Field
We focus on the search for unresolved faint quasars and AGN in the crude
combine images using a multicolor imaging analysis that has proven very
successful in recent years. Quasar selection was carried out both in multicolor
space and in "profile space," defined as the multi-parameter space formed by
the radial profiles of the objects in the different images. By combining the
dither frames available for each filter, we were able to obtain well-sampled
radial profiles of the objects and measure their deviation from that of a
stellar source. We also generated synthetic quasar spectra in the range 1.0 < z
< 5.5 and computed expected quasar colors. We determined that the data are 90%
complete for point sources at 26.2, 28.0, 27.8, 26.8 in the F300W, F450W, F606W
and F814W filters, respectively. We find 41 compact objects in the HDF: 8
pointlike objects with colors consistent with quasars or stars, 18 stars, and
15 slightly resolved objects, 12 of which have colors consistent with quasars
or stars. We estimate the upper limit of unresolved and slightly resolved
quasars/AGNs with V < 27.0 and z < 3.5 to be 20 objects (16,200 per deg^2). We
find good agreement among authors on the number of stars and the lack of quasar
candidates with z > 3.5. We find more quasar candidates than previous work
because of our more extensive modeling and use of all of the available color
information. (abridged)Comment: We have clarified our discussion and conclusions, added some
references and removed the appendix, which is now available from the first
author. 37 pages including 10 embedded postscript figures and 6 tables. To
appear in the Feb. 99 issue of A
The Indo-U.S. Library of Coude Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9m Coud\'e Feed telescope
at Kitt Peak National Observatory. This telescope feeds the coud\'e
spectrograph of the 2.1m telescope. The spectra have been obtained with the #5
camera of the coud\'e spectrograph and a Loral 3K X 1K CCD. Two gratings have
been used to provide spectral coverage from 3460 \AA to 9464 \AA, at a
resolution of 1\AA FWHM and at an original dispersion of 0.44 \AA/pixel.
For 885 stars we have complete spectra over the entire 3460 \AA to 9464 \AA
wavelength region (neglecting small gaps of 50 \AA), and partial spectral
coverage for the remaining stars. The 1273 stars have been selected to provide
broad coverage of the atmospheric parameters T, log g, and [Fe/H], as
well as spectral type. The goal of the project is to provide a comprehensive
library of stellar spectra for use in the automated classification of stellar
and galaxy spectra and in galaxy population synthesis. In this paper we discuss
the characteristics of the spectral library, viz., details of the observations,
data reduction procedures, and selection of stars. We also present a few
illustrations of the quality and information available in the spectra. The
first version of the complete spectral library is now publicly available from
the National Optical Astronomy Observatory (NOAO) via FTP and HTTP.Comment: 18 pages, 6 figures, 4 table
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