203 research outputs found
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
Effects of age, sex, and anthropometric factors on nerve conduction measures
Associations among measures of median, ulnar, and sural nerve conduction and age, skin temperature, sex, and anthropometric factors were evaluated in a population of 105 healthy, asymptomatic adults without occupational exposure to highly repetitive or forceful hand exertions. Height was negatively associated with sensory amplitude in all nerves tested ( P < 0.001), and positively associated with median and ulnar sensory distal latencies ( P < 0.01) and sural latency ( P < 0.001). Index finger circumference was negatively associated with median and ulnar sensory amplitudes ( P < 0.05). Sex, in isolation from highly correlated anthropometric factors such as height, was not found to be a significant predictor of median or ulnar nerve conduction measures. Equations using age, height, and finger circumference for prediction of normal values are presented. Failure to adjust normal nerve conduction values for these factors decreases the diagnostic specificity and sensitivity of the described measures, and may result in misclassification of individuals. © 1992 John Wiley & Sons, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50152/1/880151007_ftp.pd
The HST Key Project on the Extragalactic Distance Scale. XXII. The Discovery of Cepheids in NGC 1326-A
We report on the detection of Cepheids and the first distance measurement to
the spiral galaxy NGC 1326-A, a member of the Fornax cluster of galaxies. We
have employed data obtained with the Wide Field and Planetary Camera 2 on board
the Hubble Space Telescope. Over a 49 day interval, a total of twelve V-band
(F555W) and eight I-band (F814W) epochs of observation were obtained. Two
photometric reduction packages, ALLFRAME and DoPHOT, have been employed to
obtain photometry measures from the three Wide Field CCDs. Variability analysis
yields a total of 17 Cepheids in common with both photometry datasets, with
periods ranging between 10 and 50 days. Of these 14 Cepheids with high-quality
lightcurves are used to fit the V and I period-luminosity relations and derive
apparent distance moduli, assuming a Large Magellanic Cloud distance modulus
(m-M) (LMC) = 18.50 +- 0.10 mag and color excess E(B-V) = 0.10 mag. Assuming
A(V)/E(V-I) = 2.45, the DoPHOT data yield a true distance modulus to NGC 1326-A
of (m-M)_0 = 31.36 +- 0.17 (random) +- 0.13 (systematic) mag, corresponding to
a distance of 18.7 \pm 1.5 (random) \pm 1.2 (systematic) Mpc. The derived
distance to NGC 1326-A is in good agreement with the distance derived
previously to NGC 1365, another spiral galaxy member of the Fornax cluster.
However the distances to both galaxies are significantly lower than to NGC
1425, a third Cepheid calibrator in the outer parts of the cluster.Comment: 33 pages A gzipped tar file containing 12 figures can be obtained
from http://www.ipac.caltech.edu/H0kp/n1326a/n1326a.htm
Dopamine [beta]-hydroxylation in diabetes and diabetic autonomic neuropathy
Dopamine [beta]-hydroxylase (DBH) has been studied in healthy subjects and diabetic patients, some of whom had autonomic neuropathy. DBH was assayed by a recently described in vivo tritium release method (10,11). Dopamine specifically labeled in the [beta] position, [[beta]-3H]DA, was given intravenously, and the time course of tritiated water (THO) release into the total body water was measure as an index of DBH activity in sympathetic neurons. We simultaneously assayed [3H]norepinephrine metabolite excretion.The DBH activity of patients with autonomic neuropathy as a group did not differ significantly from that of diabetics without neuropathy or healthy controls. Our data do not support the suggestion (7) that a deficit in DBH activity is responsible for sympathetic neuronal dysfunction in diabetic autonomic neuropathy. This syndrome thus differs from idiopathic orthostatic hypotension in which condition there is an impairment in the ability of sympathetic neurons to hydroxylate dopamine (9).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24287/1/0000553.pd
Constraining the False Positive Rate for Kepler Planet Candidates with Multi-Color Photometry from the GTC
Using the OSIRIS instrument installed on the 10.4-m Gran Telescopio Canarias
(GTC) we acquired multi-color transit photometry of four small (Rp < 5 R_Earth)
short-period (P < 6 days) planet candidates recently identified by the Kepler
space mission. These observations are part of a program to constrain the false
positive rate for small, short-period Kepler planet candidates. Since planetary
transits should be largely achromatic when observed at different wavelengths
(excluding the small color changes due to stellar limb darkening), we use the
observed transit color to identify candidates as either false positives (e.g.,
a blend with a stellar eclipsing binary either in the background/foreground or
bound to the target star) or validated planets. Our results include the
identification of KOI 225.01 and KOI 1187.01 as false positives and the
tentative validation of KOI 420.01 and KOI 526.01 as planets. The probability
of identifying two false positives out of a sample of four targets is less than
1%, assuming an overall false positive rate for Kepler planet candidates of 10%
(as estimated by Morton & Johnson 2011). Therefore, these results suggest a
higher false positive rate for the small, short-period Kepler planet candidates
than has been theoretically predicted by other studies which consider the
Kepler planet candidate sample as a whole. Furthermore, our results are
consistent with a recent Doppler study of short-period giant Kepler planet
candidates (Santerne et al. 2012). We also investigate how the false positive
rate for our sample varies with different planetary and stellar properties. Our
results suggest that the false positive rate varies significantly with orbital
period and is largest at the shortest orbital periods (P < 3 days), where there
is a corresponding rise in the number of detached eclipsing binary stars...
(truncated)Comment: 13 pages, 12 figures, 3 tables; revised for MNRA
The HST Key Project on the Extragalactic Distance Scale XVII. The Cepheid Distance to NGC 4725
The distance to NGC 4725 has been derived from Cepheid variables, as part of
the Hubble Space Telescope Key Project on the Extragalactic Distance Scale.
Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and
Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered,
with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud
distance modulus and extinction of 18.50+/-0.10 mag and E(V-I)=0.13 mag,
respectively, a true reddening-corrected distance modulus (based on an analysis
employing the ALLFRAME software package) of 30.50 +/- 0.16 (random) +/- 0.17
(systematic) mag was determined for NGC 4725. The corresponding of distance of
12.6 +/- 1.0 (random) +/- 1.0 (systematic) Mpc is in excellent agreement with
that found with an independent analysis based upon the DoPHOT photometry
package. With a foreground reddening of only E(V-I)=0.02, the inferred
intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the
most highly-reddened, encountered by the HST Key Project, to date.Comment: To be published in The Astrophysical Journal, Vol. 512 (1999). 34
pages, LaTeX, 9 jpg figure
The Open Cluster NGC 7789: I. Radial Velocities for Giant Stars
A total of 597 radial-velocity observations for 112 stars in the ~1.6 Gyr old
open cluster NGC 7789 have been obtained since 1979 with the radial velocity
spectrometer at the Dominion Astrophysical Observatory. The mean cluster radial
velocity is -54.9 +/- 0.12 km/s and the dispersion is 0.86 km/s, from 50
constant-velocity stars selected as members from this radial-velocity study and
the proper motion study of McNamara and Solomon (1981). Twenty-five stars (32%)
among 78 members are possible radial-velocity variable stars, but no orbits are
determined because of the sparse sampling. Seventeen stars are radial-velocity
non-members, while membership estimates of six stars are uncertain.
There is a hint that the observed velocity dispersion falls off at large
radius. This may due to the inclusion of long-period binaries preferentially in
the central area of the cluster. The known radial-velocity variables also seem
to be more concentrated toward the center than members with constant velocity.
Although this is significant at only the 85% level, when combined with similar
result of Raboud and Mermilliod (1994) for three other clusters, the data
strongly support the conclusion that mass segregation is being detected.Comment: 16 pages (including 3 figures) and 3 table
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