1,296 research outputs found
Time-resolved multicolour photometry of bright B-type variable stars in Scorpius
The first two of a total of six nano-satellites that will constitute the
BRITE-Constellation space photometry mission have recently been launched
successfully. In preparation for this project, we carried out time-resolved
colour photometry in a field that is an excellent candidate for BRITE
measurements from space. We acquired 117 h of Stromgren uvy data during 19
nights. Our targets comprised the Beta Cephei stars Kappa and Lambda Sco, the
eclipsing binary Mu 1 Sco, and the variable super/hypergiant Zeta 1 Sco. For
Kappa Sco, a photometric mode identification in combination with results from
the spectroscopic literature suggests a dominant (l, m) = (1, -1) Beta
Cephei-type pulsation mode of the primary star. The longer period of the star
may be a rotational variation or a g-mode pulsation. For Lambda Sco, we recover
the known dominant Beta Cephei pulsation, a longer-period variation, and
observed part of an eclipse. Lack of ultraviolet data precludes mode
identification for this star. We noticed that the spectroscopic orbital
ephemeris of the closer pair in this triple system is inconsistent with eclipse
timings and propose a refined value for the orbital period of the closer pair
of 5.95189 +/- 0.00003 d. We also argue that the components of the Lambda Sco
system are some 30% more massive than previously thought. The binary light
curve solution of Mu 1 Sco requires inclusion of the irradiation effect to
explain the u light curve, and the system could show additional low amplitude
variations on top of the orbital light changes. Zeta 1 Sco shows long-term
variability on a time scale of at least two weeks that we prefer to interpret
in terms of a variable wind or strange mode pulsations.Comment: 7 pages, 7 figures, 3 Tables, accepted by A&
Efficient analysis in planet transit surveys
With the growing number of projects dedicated to the search for extrasolar
planets via transits, there is a need to develop fast, automatic, robust
methods with a statistical background in order to efficiently do the analysis.
We propose a modified analysis of variance (AoV) test particularly suitable for
the detection of planetary transits in stellar light curves. We show how
savings of labor by a factor of over 10 could be achieved by the careful
organization of computations. Basing on solid analytical statistical
formulation, we discuss performance of our and other methods for different
signal-to-noise and number of observations.Comment: 7 pages, to be published in MNRAS, downloadable software from
http://www.camk.edu.pl/~alex/#softwar
Power density spectrum of NGC 5548 and the nature of its variability
We derive power density spectra in the optical and X-ray band in the
timescale range from several years down to a day. We suggest that the
optical power density spectrum consists of two separate components: long
timescale variations and short timescale variations, with the dividing
timescale around 100 days. The shape of the short timescale component is
similar to X-ray power density spectrum which is consistent with the
interpretation of short timescale optical variations being caused by X-ray
reprocessing. We show that the observed optical long timescale variability is
consistent with thermal pulsations of the accretion disc.Comment: Accepted for publication in The Monthly Notices of the Royal
Astronomical Societ
Cube-Cut: Vertebral Body Segmentation in MRI-Data through Cubic-Shaped Divergences
In this article, we present a graph-based method using a cubic template for
volumetric segmentation of vertebrae in magnetic resonance imaging (MRI)
acquisitions. The user can define the degree of deviation from a regular cube
via a smoothness value Delta. The Cube-Cut algorithm generates a directed graph
with two terminal nodes (s-t-network), where the nodes of the graph correspond
to a cubic-shaped subset of the image's voxels. The weightings of the graph's
terminal edges, which connect every node with a virtual source s or a virtual
sink t, represent the affinity of a voxel to the vertebra (source) and to the
background (sink). Furthermore, a set of infinite weighted and non-terminal
edges implements the smoothness term. After graph construction, a minimal
s-t-cut is calculated within polynomial computation time, which splits the
nodes into two disjoint units. Subsequently, the segmentation result is
determined out of the source-set. A quantitative evaluation of a C++
implementation of the algorithm resulted in an average Dice Similarity
Coefficient (DSC) of 81.33% and a running time of less than a minute.Comment: 23 figures, 2 tables, 43 references, PLoS ONE 9(4): e9338
Assessing statistical significance of periodogram peaks
The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is
used routinely in many branches of astronomy to search for periodicities in
observational data. The problem of assessing statistical significance of
candidate periodicities for different periodograms is considered. Based on
results in extreme value theory, improved analytic estimations of false alarm
probabilities are given. They include an upper limit to the false alarm
probability (or a lower limit to the significance). These estimations are
tested numerically in order to establish regions of their practical
applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA
The Lick Planet Search: Detectability and Mass Thresholds
We analyse 11 years of precise radial velocities for 76 solar type stars from
the Lick survey. Eight stars in this sample have previously reported
planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses
(MJ). For the stars without a detected companion, we place upper limits on
possible companion mass. For most stars, we can exclude companions with m sin i
> 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU.
We use our results to interpret the observed masses and orbital radii of
planetary-mass companions. For example, we show that the finite duration of the
observations makes detection of Jupiter mass companions more and more difficult
for orbital radii beyond 3 AU. Thus it is possible that the majority of solar
type stars harbor Jupiter-mass companions much like our own, and if so these
companions should be detectable in a few years.
To search for periodicities, we adopt a "floating-mean" periodogram, which
improves on the traditional Lomb-Scargle periodogram by accounting for
statistical fluctuations in the mean of a sampled sinusoid. We discuss in
detail the normalization of the periodogram, an issue which has been of some
debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11
figures
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