132 research outputs found
Automated supervised classification of variable stars I. Methodology
The fast classification of new variable stars is an important step in making
them available for further research. Selection of science targets from large
databases is much more efficient if they have been classified first. Defining
the classes in terms of physical parameters is also important to get an
unbiased statistical view on the variability mechanisms and the borders of
instability strips. Our goal is twofold: provide an overview of the stellar
variability classes that are presently known, in terms of some relevant stellar
parameters; use the class descriptions obtained as the basis for an automated
`supervised classification' of large databases. Such automated classification
will compare and assign new objects to a set of pre-defined variability
training classes. For every variability class, a literature search was
performed to find as many well-known member stars as possible, or a
considerable subset if too many were present. Next, we searched on-line and
private databases for their light curves in the visible band and performed
period analysis and harmonic fitting. The derived light curve parameters are
used to describe the classes and define the training classifiers. We compared
the performance of different classifiers in terms of percentage of correct
identification, of confusion among classes and of computation time. We describe
how well the classes can be separated using the proposed set of parameters and
how future improvements can be made, based on new large databases such as the
light curves to be assembled by the CoRoT and Kepler space missions.Comment: This paper has been accepted for publication in Astronomy and
Astrophysics (reference AA/2007/7638) Number of pages: 27 Number of figures:
1
How semiregular are irregular variables?
We investigate the question whether there is a real difference in the light
change between stars classified as semiregular (SRV) or irregular (Lb)
variables by analysing photometric light curves of 12 representatives of each
class. Using Fourier analysis we try to find a periodic signal in each light
curve and determine the S/N of this signal. For all stars, independent of their
variability class we detect a period above the significance threshold. No
difference in the measured S/N between the two classes could be found. We
propose that the Lb stars can be seen as an extension of the SRVs towards
shorter periods and smaller amplitudes. This is in agreement with findings from
other quantities which also showed no marked difference between the two
classes.Comment: 7 pages, accepted for publication by A
Global stellar variability study in the field-of-view of the Kepler satellite
We present the results of an automated variability analysis of the Kepler
public data measured in the first quarter (Q1) of the mission. In total, about
150 000 light curves have been analysed to detect stellar variability, and to
identify new members of known variability classes. We also focus on the
detection of variables present in eclipsing binary systems, given the important
constraints on stellar fundamental parameters they can provide. The methodology
we use here is based on the automated variability classification pipeline which
was previously developed for and applied successfully to the CoRoT exofield
database and to the limited subset of a few thousand Kepler asteroseismology
light curves. We use a Fourier decomposition of the light curves to describe
their variability behaviour and use the resulting parameters to perform a
supervised classification. Several improvements have been made, including a
separate extractor method to detect the presence of eclipses when other
variability is present in the light curves. We also included two new
variability classes compared to previous work: variables showing signs of
rotational modulation and of activity. Statistics are given on the number of
variables and the number of good candidates per class. A comparison is made
with results obtained for the CoRoT exoplanet data. We present some special
discoveries, including variable stars in eclipsing binary systems. Many new
candidate non-radial pulsators are found, mainly Delta Sct and Gamma Dor stars.
We have studied those samples in more detail by using 2MASS colours. The full
classification results are made available as an online catalogue.Comment: 15 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysics on 09/02/201
Automated supervised classification of variable stars II. Application to the OGLE database
We aim to extend and test the classifiers presented in a previous work
against an independent dataset. We complement the assessment of the validity of
the classifiers by applying them to the set of OGLE light curves treated as
variable objects of unknown class. The results are compared to published
classification results based on the so-called extractor methods.Two
complementary analyses are carried out in parallel. In both cases, the original
time series of OGLE observations of the Galactic bulge and Magellanic Clouds
are processed in order to identify and characterize the frequency components.
In the first approach, the classifiers are applied to the data and the results
analyzed in terms of systematic errors and differences between the definition
samples in the training set and in the extractor rules. In the second approach,
the original classifiers are extended with colour information and, again,
applied to OGLE light curves. We have constructed a classification system that
can process huge amounts of time series in negligible time and provide reliable
samples of the main variability classes. We have evaluated its strengths and
weaknesses and provide potential users of the classifier with a detailed
description of its characteristics to aid in the interpretation of
classification results. Finally, we apply the classifiers to obtain object
samples of classes not previously studied in the OGLE database and analyse the
results. We pay specific attention to the B-stars in the samples, as their
pulsations are strongly dependent on metallicity.Comment: 42 pages, 39 figures. Accepted for publication in Astronomy and
Astrophysic
The Many-faceted Light Curves of Young Disk-bearing Stars in Upper Sco ââ Oph Observed by K2 Campaign 2
The K2 Mission has photometrically monitored thousands of stars at high precision and cadence in a series of ~80-day campaigns focused on sections of the ecliptic plane. During its second campaign, K2 targeted over 1000 young stellar objects (YSOs) in the ~1â3 Myr Ï Ophiuchus and 5â10 Myr Upper Scorpius regions. From this set, we have carefully vetted photometry from WISE and Spitzer to identify those YSOs with infrared excess indicative of primordial circumstellar disks. We present here the resulting comprehensive sample of 288 young disk-bearing stars from B through M spectral types and analysis of their associated K2 light curves. Using statistics of periodicity and symmetry, we categorize each light curve into eight different variability classes, notably including "dippers" (fading events), "bursters" (brightening events), stochastic, and quasi-periodic types. Nearly all (96%) of disk-bearing YSOs are identified as variable at 30-minute cadence with the sub-1% precision of K2. Combining our variability classifications with (circum)stellar properties, we find that the bursters, stochastic sources, and the largest amplitude quasi-periodic stars have larger infrared colors, and hence stronger circumstellar disks. They also tend to have larger Hα equivalent widths, indicative of higher accretion rates. The dippers, on the other hand, cluster toward moderate infrared colors and low Hα. Using resolved disk observations, we further find that the latter favor high inclinations, except for a few notable exceptions with close to face-on disks. These observations support the idea that YSO time-domain properties are dependent on several factors, including accretion rate and view angle
Ground-based observations of the beta Cephei CoRoT main target HD 180642: abundance analysis and mode identification
The known beta Cephei star HD 180642 was observed by the CoRoT satellite in
2007. From the very high-precision light curve, its pulsation frequency
spectrum could be derived for the first time (Degroote and collaborators). In
this paper, we obtain additional constraints for forthcoming asteroseismic
modeling of the target. Our results are based on both extensive ground-based
multicolour photometry and high-resolution spectroscopy. We determine T_eff =
24 500+-1000 K and log g = 3.45+-0.15 dex from spectroscopy. The derived
chemical abundances are consistent with those for B stars in the solar
neighbourhood, except for a mild nitrogen excess. A metallicity Z =
0.0099+-0.0016 is obtained. Three modes are detected in photometry. The degree
l is unambiguously identified for two of them: l = 0 and l = 3 for the
frequencies 5.48694 1/d and 0.30818 1/d, respectively. The radial mode is
non-linear and highly dominant with an amplitude in the U-filter about 15 times
larger than the strongest of the other modes. For the third frequency of
7.36673 1/d found in photometry, two possibilities remain: l = 0 or 3. In the
radial velocities, the dominant radial mode presents a so-called stillstand but
no clear evidence of the existence of shocks is observed. Four low-amplitude
modes are found in spectroscopy and one of them, with frequency 8.4079 1/d, is
identified as (l,m)=(3,2). Based on this mode identification, we finally deduce
an equatorial rotational velocity of 38+-15 km/s.Comment: Accepted for publication in Astronomy and Astrophysic
An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite
We present space-based photometry of the transiting exoplanetary system HD
209458 obtained with the MOST (Microvariablity and Oscillations of STars)
satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses.
The HD 209458 photometry was obtained in MOST's lower-precision Direct Imaging
mode, which is used for targets in the brightness range . We
describe the photometric reduction techniques for this mode of observing, in
particular the corrections for stray Earthshine. We do not detect the secondary
eclipse in the MOST data, to a limit in depth of 0.053 mmag (1 \sigma). We set
a 1 \sigma upper limit on the planet-star flux ratio of 4.88 x 10^-5
corresponding to a geometric albedo upper limit in the MOST bandpass (400 to
700 nm) of 0.25. The corresponding numbers at the 3 \sigma level are 1.34 x
10^-4 and 0.68 respectively. HD 209458b is half as bright as Jupiter in the
MOST bandpass. This low geometric albedo value is an important constraint for
theoretical models of the HD209458b atmosphere, in particular ruling out the
presence of reflective clouds. A second MOST campaign on HD 209458 is expected
to be sensitive to an exoplanet albedo as low as 0.13 (1 sigma), if the star
does not become more intrinsically variable in the meantime.Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical
Journal (July 2006, v645n1
Using a multiperiodic linear programming model and a simulation programme for competing field crops and energy orchards
Considering the land use of Hungary, there is a need to develop a rationale land use in which, beside the less-favoured areas, the use of set-aside areas are also permitted. There are several opportunities to utilize the less-favoured areas. We prepared a multiperiodic linear programming model in order to model the crop structure, in which field crops with woody energy orchards were also competed. After having each field and orchard technology compiled, we set the dynamic simulation model, that we prepared in MS Excel. After running the model we analyzed the shadow prices of the constraints and the marginal cost of variables. Considering the result of the analysis and the professional information we made a sensitivity analysis, which gave a basis to create new decision variants. The results of linear programming model were compared with those of Monte Cralo simulationâs, where we managed the enterprisesâ profit contribution as probability variable with normal distribution in the course of modelling
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