160 research outputs found
The first search for variable stars in the open cluster NGC 6253 and its surrounding field
This work presents the first high-precision variability survey in the field
of the intermediate-age, metal--rich open cluster NGC 6253. Clusters of this
type are benchmarks for stellar evolution models. Continuous photometric
monitoring of the cluster and its surrounding field was performed over a time
span of ten nights using the Wide Field Imager mounted at the ESO-MPI 2.2m
telescope. High-quality timeseries, each composed of about 800 datapoints, were
obtained for 250,000 stars using ISIS and DAOPHOT packages. Candidate members
were selected by using the colour-magnitude diagrams and
period-luminosity-colour relations. Membership probabilities based on the
proper motions were also used. The membership of all the variables discovered
within a radius of 8 arcmin from the centre is discussed by comparing the
incidence of the classes in the cluster direction and in the surrounding field.
We discovered 595 variables and we also characterized most of them providing
their variability classes, periods, and amplitudes. The sample is complete for
short periods: we classified 20 pulsating variables, 225 contact systems, 99
eclipsing systems (22 Beta Lyr type, 59 Beta Per type, 18 RS CVn type), and 77
rotational variables. The time-baseline hampered the precise characterization
of 173 variables with periods longer than 4-5 days. Moreover, we found a
cataclysmic system undergoing an outburst of about 2.5 mag. We propose a list
of 35 variable stars (8 contact systems, 2 eclipsing systems, 15 rotational
variables, 9 long-period variables and the cataclysmic variable) as probable
members of NGC 6253.Comment: Astronomy and Astrophysics, in press. Extended version with tables
and light curve
SPOTS: The Search for Planets Orbiting Two Stars: II. First constraints on the frequency of sub-stellar companions on wide circumbinary orbits
A large number of direct imaging surveys for exoplanets have been performed
in recent years, yielding the first directly imaged planets and providing
constraints on the prevalence and distribution of wide planetary systems.
However, like most of the radial velocity ones, these surveys generally focus
on single stars, hence binaries and higher-order multiples have not been
studied to the same level of scrutiny. This motivated the SPOTS (Search for
Planets Orbiting Two Stars) survey, which is an ongoing direct imaging study of
a large sample of close binaries, started with VLT/NACO and now continuing with
VLT/SPHERE. To complement this survey, we have identified the close binary
targets in 24 published direct imaging surveys. Here we present our statistical
analysis of this combined body of data. We analysed a sample of 117 tight
binary systems, using a combined Monte Carlo and Bayesian approach to derive
the expected values of the frequency of companions, for different values of the
companion's semi-major axis. Our analysis suggest that the frequency of
sub-stellar companions in wide orbit is moderately low (13% with a
best value of 6% at 95% confidence level) and not significantly different
between single stars and tight binaries. One implication of this result is that
the very high frequency of circumbinary planets in wide orbits around
post-common envelope binaries, implied by eclipse timing (up to 90% according
to Zorotovic & Schreiber 2013), can not be uniquely due to planets formed
before the common-envelope phase (first generation planets), supporting instead
the second generation planet formation or a non-Keplerian origin of the timing
variations.Comment: 21 pages, 3 figure
Frame Combination Techniques for Ultra High-Contrast Imaging
We summarize here an experimental frame combination pipeline we developed for
ultra high-contrast imaging with systems like the upcoming VLT SPHERE
instrument. The pipeline combines strategies from the Drizzle technique, the
Spitzer IRACproc package, and homegrown codes, to combine image sets that may
include a rotating field of view and arbitrary shifts between frames. The
pipeline is meant to be robust at dealing with data that may contain non-ideal
effects like sub-pixel pointing errors, missing data points, non-symmetrical
noise sources, arbitrary geometric distortions, and rapidly changing point
spread functions. We summarize in this document individual steps and
strategies, as well as results from preliminary tests and simulations.Comment: 9 pages, 4 figures, SPIE conference pape
Abundances for metal-poor stars with accurate parallaxes I. Basic data
We present element-to-element abundance ratios measured from high dispersion
spectra for 150 field subdwarfs and early subgiants with accurate Hipparcos
parallaxes (errors <20%). For 50 stars new spectra were obtained with the UVES
on Kueyen (VLT UT2), the McDonald 2.7m telescope, and SARG at TNG.
Additionally, literature equivalent widths were taken from the works by Nissen
& Schuster, Fulbright, and Prochaska et al. to complement our data. The whole
sample includes both thick disk and halo stars (and a few thin disk stars);
most stars have metallicities in the range -2<[Fe/H]<-0.6. We found our data,
that of Nissen & Schuster, and that of Prochaska to be of comparable quality;
results from Fulbright scatter a bit more, but they are still of very good
quality and are extremely useful due to the large size of his sample. The
results of the present analysis will be used in forthcoming papers to discuss
the chemical properties of the dissipational collapse and accretion components
of our Galaxy.Comment: 43 pages, 8 encapsulated figures, 13 tables. Table 1 only available
in electronic form at CDS or from first author. Accepted for publication in
A&
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The SPHERE view of multiple star formation
While a large fraction of the stars are in multiple systems, our understanding of the processes leading to the formation of these systems is still inadequate. Given the large theoretical uncertainties, observation plays a basic role. Here we discuss the contribution of high contrast imaging, and more specifically of the SPHERE instrument at the ESO Very Large Telescope, in this area. SPHERE nicely complements other instruments such as Gaia or ALMAâin detecting and characterizing systems near the peak of the binary distribution with separation and allows to capture snapshots of binary formation within disks that are invaluable for the understanding of disk fragmentation
Detection of solar-like oscillations in the G5 subgiant mu-Herculis
A clear detection of excess of power, providing a substantial evidence for
solar-like oscillations in the G5 subgiant \muher{}, is presented. This star
was observed over seven nights with the SARG echelle spectrograph operating
with the 3.6-m Italian TNG Telescope, using an iodine absorption cell as a
velocity reference. A clear excess of power centered at 1.2 mHz, with peak
amplitudes of about 0.9 \ms in the amplitude spectrum is present. Fitting the
asymptotic relation to the power spectrum, a mode identification for the
modes in the frequency range 900-1600 \muHz is derived. The
most likely value for the large separation turns out to be 56.5 \muHz,
consistent with theoretical expectations. The mean amplitude per mode ()
at peak power results to be , almost three times larger than
the solar one.Comment: 8 pages, 6 figures, ApJ to appea
SPOTS: The Search for Planets Orbiting Two Stars. I. Survey description and first observations
Direct imaging surveys for exoplanets commonly exclude binary stars from
their target lists, leaving a large part of the overall planet demography
unexplored. To address this gap in our understanding of planet formation and
evolution, we have launched the first direct imaging survey dedicated to
circumbinary planets: SPOTS, the Search for Planets Orbiting Two Stars. In this
paper, we discuss the theoretical context, scientific merit, and technical
feasibility of such observations, describe the target sample and observational
strategy of our survey, and report on the first results from our pilot survey
of 26 targets with the VLT NaCo facility. While we have not found any confirmed
substellar companions to date, a number of promising candidate companions
remain to be tested for common proper motion in upcoming follow-up
observations. We also report on the astrometry of the three resolved binaries
in our target sample. This pilot survey constitutes a successful proof of
concept for our survey strategy and paves the way for a second stage of
exploratory observations with VLT SPHERE.Comment: 12 pages, 4 figures, 3 tables. Accepted for publication in A&
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