822 research outputs found
College or Career?
As would-be students weigh one of the most important decisions of their lives, society in general and higher education in particular are sending mixed messages about affordability
Childhood Risk Factors in Dually Diagnosed Homeless Adults
Although the negative long term effects of specific childhood risk factors - sexual and physical abuse, parental mental illness and substance abuse, and out of home placement - have been recognized, most studies have focused on just one of these risks. This article examines the prevalence of these five childhood risk factors among dually diagnosed (mental illness and substance abusing) homeless adults in rehabilitation programs. It further assesses the impact of each risk factor individually and in combinations of two on the social functioning skills and rehabilitation progress of these multiply disadvantaged clients
Photometric Recovery of Crowded Stellar Fields Observed with HST/WFPC2 and the Effects of Confusion Noise on the Extragalactic Distance Scale
We explore the limits of photometric reductions of crowded stellar fields
observed with the Wide Field and Planetary Camera 2 on board the Hubble Space
Telescope. Two photometric procedures, based on the DoPHOT and DAOPHOT/ALLFRAME
programs are tested, and the effects of crowding, complex sky background and
cosmic-ray contamination are discussed using an extensive set of artificial
star simulations. As a specific application of the results presented in this
paper, we assess the magnitude of photometric biases on programs aimed at
finding Cepheids and determining distances. We find that while the photometry
in individual images can be biased too bright by up to 0.2 mag in the most
crowded fields due to confusion noise, the effects on distance measurements
based on Cepheid variables are insignificant, less than 0.02 mag (1% in
distance) even in the most problematic cases. This result, which is at odds
with claims recently surfaced in the literature, is due to the strict criteria
applied in the selection of the variable stars, and the photometric cross
checks made possible by the availability of multiple exposures in different
filters which characterizes Cepheid observations.Comment: Accepted for publication in PASP. 41 pages, 18 figures, 8 tables. The
figures included with this submission are very low quality bitmap postscript,
please see http://www.astro.ucla.edu/~laura/pub.htm for the full size image
The Cepheid Period-Luminosity Relation at Mid-Infrared Wavelengths: II. Second-Epoch LMC Data
We present revised and improved mid-infrared Period-Luminosity (PL) relations
for Large Magellanic Cloud (LMC) Cepheids based on double-epoch data of 70
Cepheids observed by Spitzer at 3.6, 4.5, 5.8 and 8.0um. The observed scatter
at all wavelengths is found to decrease from +/-0.17 mag to +/-0.14 mag, which
is fully consistent with the prediction that the total scatter is made up of
roughly equal contributions from random sampling of the light curve and
nearly-uniform samplings of stars across the instability strip. It is
calculated that the Cepheids in this sample have a full amplitude of about 0.4
mag and that their fully-sampled, time-averaged magnitudes should eventually
reveal mid-infrared PL relations that each have intrinsic scatter at most at
the +/-0.12 mag level, and as low as +/-0.08 mag after correcting for the tilt
of the LMC.Comment: 7 pages, 6 figures, 1 data tabl
The Carnegie Hubble Program
We present an overview of and preliminary results from an ongoing
comprehensive program that has a goal of determining the Hubble constant to a
systematic accuracy of 2%. As part of this program, we are currently obtaining
3.6 micron data using the Infrared Array Camera (IRAC) on Spitzer, and the
program is designed to include JWST in the future. We demonstrate that the
mid-infrared period-luminosity relation for Cepheids at 3.6 microns is the most
accurate means of measuring Cepheid distances to date. At 3.6 microns, it is
possible to minimize the known remaining systematic uncertainties in the
Cepheid extragalactic distance scale. We discuss the advantages of 3.6 micron
observations in minimizing systematic effects in the Cepheid calibration of the
Hubble constant including the absolute zero point, extinction corrections, and
the effects of metallicity on the colors and magnitudes of Cepheids. We are
undertaking three independent tests of the sensitivity of the mid-IR Cepheid
Leavitt Law to metallicity, which when combined will allow a robust constraint
on the effect. Finally, we are providing a new mid-IR Tully-Fisher relation for
spiral galaxies
The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths: III. Cepheids in NGC 6822
We present the first application of mid-infrared Period-Luminosity relations
to the determination of a Cepheid distance beyond the Magellanic Clouds. Using
archival IRAC imaging data on NGC 6822 from Spitzer we were able to measure
single-epoch magnitudes for sixteen long-period (10 to 100-day) Cepheids at
3.6um, fourteen at 4.5um, ten at 5.8um and four at 8.0um. The measured slopes
and the observed scatter both conform to the relations previously measured for
the Large Magellanic Cloud Cepheids, and fitting to those relations gives
apparent distance moduli of mod{3.6} = 23.57 +/- 0.06, mod{4.5} = 23.55 +/-
0.07, mod{5.8} = 23.60 +/- 0.09 and mod{8.0} = 23.51 +/-0.08 mag. A
multi-wavelength fit to the new IRAC moduli, and previously published BVRIJHK
moduli, allows for a final correction for interstellar reddening and gives a
true distance modulus of 23.49 +/- 0.03 mag with E(B-V) = 0.26 mag,
corresponding to a metric distance of 500 +/-8 kpc.Comment: Accepted to ApJ, 2 figure
The Cepheid Period-Luminosity Relation at Mid-Infrared Wavelengths: I. First-Epoch LMC Data
We present the first mid-infrared Period-Luminosity (PL) relations for Large
Magellanic Cloud (LMC) Cepheids. Single-epoch observations of 70 Cepheids were
extracted from Spitzer IRAC observations at 3.6, 4.5, 5.8 and 8.0 microns,
serendipitously obtained during the SAGE (Surveying the Agents of a Galaxy's
Evolution) imaging survey of the LMC. All four mid-infrared PL relations have
nearly identical slopes over the period range 6 - 88 days, with a small scatter
of only +/-0.16 mag independent of period for all four of these wavelengths. We
emphasize that differential reddening is not contributing significantly to the
observed scatter, given the nearly two orders of magnitude reduced sensitivity
of the mid-IR to extinction compared to the optical. Future observations,
filling in the light curves for these Cepheids, should noticeably reduce the
residual scatter. These attributes alone suggest that mid-infrared PL relations
will provide a practical means of significantly improving the accuracy of
Cepheid distances to nearby galaxies.Comment: 19 pages, 4 figures, 1 table, Accepted for publication in the
Astrophysical Journa
ADHD and Brain Anatomy: What Do Academic Textbooks Used in the Netherlands Tell Students?
Studies of brain size of children classified with ADHD appear to reveal smaller brains when compared to ‘normal’ children. Yet, what does this mean? Even with the use of rigorously screened case and control groups, these studies show only small, average group differences between children with and without an ADHD classification. However, academic textbooks used in the Netherlands often portray individual children with an ADHD classification as having a different, malfunctioning brain that necessitates medical intervention. This conceptualisation of ADHD might serve professional interests, but not necessarily the interests of children
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