240 research outputs found
NELIOTA: The wide-field, high-cadence lunar monitoring system at the prime focus of the Kryoneri telescope
We present the technical specifications and first results of the ESA-funded,
lunar monitoring project "NELIOTA" (NEO Lunar Impacts and Optical TrAnsients)
at the National Observatory of Athens, which aims to determine the
size-frequency distribution of small Near-Earth Objects (NEOs) via detection of
impact flashes on the surface of the Moon. For the purposes of this project a
twin camera instrument was specially designed and installed at the 1.2 m
Kryoneri telescope utilizing the fast-frame capabilities of scientific
Complementary Metal-Oxide Semiconductor detectors (sCMOS). The system provides
a wide field-of-view (17.0' 14.4') and simultaneous observations in
two photometric bands (R and I), reaching limiting magnitudes of 18.7 mag in 10
sec in both bands at a 2.5 signal-to-noise level. This makes it a unique
instrument that can be used for the detection of NEO impacts on the Moon, as
well as for any astronomy projects that demand high-cadence multicolor
observations. The wide field-of-view ensures that a large portion of the Moon
is observed, while the simultaneous, high-cadence, monitoring in two
photometric bands makes possible, for the first time, the determination of the
temperatures of the impacts on the Moon's surface and the validation of the
impact flashes from a single site. Considering the varying background level on
the Moon's surface we demonstrate that the NELIOTA system can detect NEO impact
flashes at a 2.5 signal-to-noise level of ~12.4 mag in the I-band and R-band
for observations made at low lunar phases ~0.1. We report 31 NEO impact flashes
detected during the first year of the NELIOTA campaign. The faintest flash was
at 11.24 mag in the R-band (about two magnitudes fainter than ever observed
before) at lunar phase 0.32. Our observations suggest a detection rate of events .Comment: Accepted for publication in A&
Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds
The star complexes (large scale star forming regions) of NGC 6822 were traced
and mapped and their size distribution was compared with the size distribution
of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial
distributions of different age stellar populations were compared with each
other. The star complexes of NGC 6822 were determined by using the isopleths,
based on star counts, of the young stars of the galaxy, using a statistical
cutoff limit in density. In order to map them and determine their geometrical
properties, an ellipse was fitted to every distinct region satisfying this
minimum limit. The Kolmogorov-Smirnov statistical test was used to study
possible patterns in their size distribution. Isopleths were also used to study
the stellar populations of NGC 6822. The star complexes of NGC 6822 were
detected and a list of their positions and sizes was produced. Indications of
hierarchical star formation, in terms of spatial distribution, time evolution
and preferable sizes were found in NGC 6822 and the MCs. The spatial
distribution of the various age stellar populations has indicated traces of an
interaction in NGC 6822, dated before 350 +/- 50 Myr.Comment: 10 pages, 7 figures, accepted by A&A; minor typeface correction
Comparative performance of selected variability detection techniques in photometric time series
Photometric measurements are prone to systematic errors presenting a
challenge to low-amplitude variability detection. In search for a
general-purpose variability detection technique able to recover a broad range
of variability types including currently unknown ones, we test 18 statistical
characteristics quantifying scatter and/or correlation between brightness
measurements. We compare their performance in identifying variable objects in
seven time series data sets obtained with telescopes ranging in size from a
telephoto lens to 1m-class and probing variability on time-scales from minutes
to decades. The test data sets together include lightcurves of 127539 objects,
among them 1251 variable stars of various types and represent a range of
observing conditions often found in ground-based variability surveys. The real
data are complemented by simulations. We propose a combination of two indices
that together recover a broad range of variability types from photometric data
characterized by a wide variety of sampling patterns, photometric accuracies,
and percentages of outlier measurements. The first index is the interquartile
range (IQR) of magnitude measurements, sensitive to variability irrespective of
a time-scale and resistant to outliers. It can be complemented by the ratio of
the lightcurve variance to the mean square successive difference, 1/h, which is
efficient in detecting variability on time-scales longer than the typical time
interval between observations. Variable objects have larger 1/h and/or IQR
values than non-variable objects of similar brightness. Another approach to
variability detection is to combine many variability indices using principal
component analysis. We present 124 previously unknown variable stars found in
the test data.Comment: 29 pages, 8 figures, 7 tables; accepted to MNRAS; for additional
plots, see http://scan.sai.msu.ru/~kirx/var_idx_paper
Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST
We present the results of our investigation on the phenomenon of mass
segregation in young star clusters in the Magellanic Clouds. HST/WFPC2
observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and
NGC 330 in the Small Magellanic Cloud have been used for the application of
diagnostic tools for mass segregation: i) the radial density profiles of the
clusters for various mass groups and ii) their mass functions (MFs) at various
radii around their centres. All four clusters are found to be mass segregated,
but each one in a different manner. Specifically not all the clusters in the
sample show the same dependence of their density profiles on the selected
magnitude range, with NGC 1818 giving evidence of a strong such relation and
NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any
significant signature of mass segregation in its density profiles. The MFs
radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100
and NGC 2004, while for NGC 330 it gives only indications. An investigation on
the constraints introduced by the application of both diagnostic tools is
presented. We also discuss the problems related to the construction of a
reliable MF for a cluster and their impact on the investigation of the
phenomenon of mass segregation. We find that the MFs of these clusters as they
were constructed with two methods, are comparable to Salpeter's IMF. A
discussion is given on the dynamical status of the clusters and a test is
applied on the equipartition among several mass groups in them. Both showed
that the observed mass segregation in the clusters is of primordial nature.Comment: A&A Accepted, 20 pages, 9 Figures, Version with language errors and
typos correcte
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