312 research outputs found
Metallicity Calibration and Photometric Parallax Estimation: I. UBV photometry
We present metallicity and photometric parallax calibrations for the F and G
type dwarfs with photometric, astrometric and spectroscopic data. The sample
consists of 168 dwarf stars covering the colour, iron abundance and absolute
magnitude intervals mag, dex and
mag, respectively. The means and standard deviations of the
metallicity and absolute magnitude residuals are small, i.e.
and dex, and and mag, respectively, which indicate
accurate metallicity and photometric parallax estimations.Comment: 13 pages, 11 figures and 2 tables, accepted for publication in
Astrophysics and Space Scienc
Local Stellar Kinematics from RAVE data - VII. Metallicity Gradients from Red Clump Stars
We investigate the Milky Way Galaxy's radial and vertical metallicity
gradients using a sample of 47,406 red clump stars from the RAVE DR4. This
sample is more than twice the size of the largest sample in the literature
investigating radial and vertical metallicity gradients. The absolute magnitude
of Groenewegen (2008) is used to determine distances to our sample stars. The
resulting distances agree with the RAVE DR4 distances Binney et al. (2014) of
the same stars. Our photometric method also provides distances to 6185 stars
that are not assigned a distance in RAVE DR4. The metallicity gradients are
calculated with their current orbital positions ( and ) and with
their orbital properties (mean Galactocentric distance, and ),
as a function of the distance to the Galactic plane:
d[Fe/H]/d- dex/kpc for kpc and
d[Fe/H]/d- dex/kpc for kpc. This
reaffirms the radial metallicity gradient in the thin disc but highlights that
gradients are sensitive to the selection effects caused by the difference
between and . The radial gradient is flat in the distance
interval 0.5-1 kpc from the plane and then becomes positive greater than 1 kpc
from the plane. The radial metallicity gradients are also eccentricity
dependent. We showed that d[Fe/H]/d-, -,
- and - dex/kpc for , ,
and sub-samples, respectively, in the distance
interval kpc. Similar trend is found for vertical
metallicity gradients. Both the radial and vertical metallicity gradients are
found to become shallower as the eccentricity of the sample increases. These
findings can be used to constrain different formation scenarios of the thick
and thin discs.Comment: 18 pages, including 16 figures and 6 tables, accepted for publication
in PAS
A New Absolute Magnitude Calibration for Red Clump Stars
We present an M_V absolute magnitude calibration including the B-V colour and
[Fe/H] metallicity for the red clump stars in the globular and open clusters
with a wide range of metallicities: M_V =
0.627(0.104)(B-V)o+0.046(0.043)[Fe/H]+0.262(0.111). The calibration equation is
valid in the ranges 0.42<(B-V)o<1.20 mag, -1.55<[Fe/H]<+0.40 dex and
0.43<M_V<1.03 mag. We found that the consistencies in the comparisons of the
distances estimated from the calibration equation in this study both with the
distances obtained from trigonometric parallaxes and spectrophotometric
analysis demonstrate that reliable precise absolute magnitudes for the clump
giants can be estimated from the calibration formula.Comment: 19 pages, 5 figures and 1 table, accepted for publication in New
Astronom
Metallicity and absolute magnitude calibrations for F-G type main-sequence stars in the Gaia era
In this study, photometric metallicity and absolute magnitude calibrations
were derived using F-G spectral type main-sequence stars in the Solar
neighbourhood with precise spectroscopic, photometric and Gaia astrometric data
for UBV photometry. The sample consists of 504 main-sequence stars covering the
temperature, surface gravity and colour index intervals
K, (cgs) and mag, respectively. Stars with
relative trigonometric parallax errors were
preferred from Gaia DR2 data for the estimation of their absolute
magnitudes. In order to obtain calibrations, and colour
indices of stars were preferred and a multi-variable second order equation was
used. Calibrations are valid for main-sequence stars in the metallicity and
absolute magnitude ranges dex and mag,
respectively. The mean value and standard deviation of the differences between
original and estimated values for the metal abundance and absolute magnitude
are dex and mag, respectively. In this work, it has been shown that
more precise iron abundance and absolute magnitude values were obtained with
the new calibrations, compared to previous calibrations in the literature.Comment: 14 pages, 10 figures and 4 tables, accepted for publication in
Astrophysics and Space Scienc
Sensor fusion of camera, GPS and IMU using fuzzy adaptive multiple motion models
A tracking system that will be used for augmented reality applications has two main requirements: accuracy and frame rate. The first requirement is related to the performance of the pose estimation algorithm and how accurately the tracking system can find the position and orientation of the user in the environment. Accuracy problems of current tracking devices, considering that they are low-cost devices, cause static errors during this motion estimation process. The second requirement is related to dynamic errors (the end-to-end system delay, occurring because of the delay in estimating the motion of the user and displaying images based on this estimate). This paper investigates combining the vision-based estimates with measurements from other sensors, GPS and IMU, in order to improve the tracking accuracy in outdoor environments. The idea of using Fuzzy Adaptive Multiple Models was investigated using a novel fuzzy rule-based approach to decide on the model that results in improved accuracy and faster convergence for the fusion filter. Results show that the developed tracking system is more accurate than a conventional GPS–IMU fusion approach due to additional estimates from a camera and fuzzy motion models. The paper also presents an application in cultural heritage context running at modest frame rates due to the design of the fusion algorithm
A comprehensive study of the open cluster NGC 6866
We present CCD photometry of the field of the open cluster NGC 6866.
Structural parameters of the cluster are determined utilizing the stellar
density profile of the stars in the field. We calculate the probabilities of
the stars being a physical member of the cluster using their astrometric data
and perform further analyses using only the most probable members. The
reddening and metallicity of the cluster were determined by independent
methods. The LAMOST spectra and the ultraviolet excess of the F and G type
main-sequence stars in the cluster indicate that the metallicity of the cluster
is about the solar value. We estimated the reddening
mag using the vs two-colour diagram. The distance modula, the
distance and the age of NGC 6866 were derived as mag,
pc and Myr, respectively, by fitting
colour-magnitude diagrams of the cluster with the PARSEC isochrones. The
Galactic orbit of NGC 6866 indicates that the cluster is orbiting in a slightly
eccentric orbit with . The mass function slope was
derived by using the most probable members of the cluster.Comment: 14 pages, including 16 figures and 7 tables, accepted for publication
in MNRAS. Table 4 in the manuscript will be published electronicall
Diagnostic value of combined serum biomarkers for the evaluation of liver fibrosis in chronic hepatitis C infection: A multicenter, noninterventional, observational study
Background/Aims: The hepatitis C virus (HCV) infection is important cause of chronic hepatitis. Liver biopsy is considered the gold standard for assessment of fibrosis but this procedure is an invasive procedure. We aimed to evaluate the diagnostic efficiency of non-invasive serum biomarkers, separately and in combinations, on liver fibrosis in treatment-naive chronic hepatitis C (CHC) patients. Materials and Methods: Two hundred and sixteen treatment-naive CHC patients were enrolled from 32 locations across Turkey in this open-labelled, non-interventional prospective observational study. FibroTest®, aspartate aminotransferase-to-platelet ratio index(APRI), aspartate aminotransferase and alanine aminotransferase ratio (AAR), fibrosis index based on four factors (FIB-4), Age-platelet(AP) index and Forns index were measured and compared with Metavir scores got from liver biopsies. Results: Data from 182 patients with baseline liver biopsy were suitable for analysis. One hundred and twenty patients (65.9%) had F0-F1 fibrosis and 62 patients (34.1%) had F2-F4 fibrosis. APRI 0.732 area under the curve(AUC) indicated advanced fibrosis with 69% sensitivity and 77% specificity. FIB-4 0.732 AUC and FibroTest 0.715 AUC indicated advanced fibrosis with 69% and 78.4% sensitivity, and 75% and 71.4% specificity, respectively. The combined use of tests also led to an increase in AUC and specificity. Combinations of FibroTest with APRI and/or FIB-4, and FIB-4 with APRI were optimal for the evaluation of liver fibrosis. Conclusion: Fibrotest, FIB-4, APRI, AP index and Forns index exhibit good diagnostic performance for determining liver fibrosis in CHC patients, and the use of at least two tests together will increase their diagnostic value still further. © Copyright 2018 by The Turkish Society of Gastroenterology
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