763 research outputs found
A PSF-based approach to Kepler/K2 data. I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158
Kepler and K2 data analysis reported in the literature is mostly based on
aperture photometry. Because of Kepler's large, undersampled pixels and the
presence of nearby sources, aperture photometry is not always the ideal way to
obtain high-precision photometry and, because of this, the data set has not
been fully exploited so far. We present a new method that builds on our
experience with undersampled HST images. The method involves a point-spread
function (PSF) neighbour-subtraction and was specifically developed to exploit
the huge potential offered by the K2 "super-stamps" covering the core of dense
star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed
during the K2 Campaign 0. We present our PSF modeling and demonstrate that, by
using a high-angular-resolution input star list from the Asiago Schmidt
telescope as the basis for PSF neighbour subtraction, we are able to reach
magnitudes as faint as Kp~24 with a photometric precision of 10% over 6.5
hours, even in the densest regions. At the bright end, our photometric
precision reaches ~30 parts-per-million. Our method leads to a considerable
level of improvement at the faint magnitudes (Kp>15.5) with respect to the
classical aperture photometry. This improvement is more significant in crowded
regions. We also extracted raw light curves of ~60,000 stars and detrended them
for systematic effects induced by spacecraft motion and other artifacts that
harms K2 photometric precision. We present a list of 2133 variables.Comment: 27 pages (included appendix), 2 tables, 25 figures (5 in low
resolution). Accepted for publication in MNRAS on November 05, 2015. Online
materials will be available on the Journal website soo
The Puzzling White Dwarf Cooling Sequence in NGC6791: A Simple Solution
In this paper we demonstrate that the puzzling bright peak in the luminosity
function of the white dwarf (WD) cooling sequence of NGC6791 can be naturally
accounted for if ~34% of the observed WDs are WD+WD binary systems.Comment: 12 pages, 3 figures. Accepted (April 9th 2008) on ApJ Lette
A large stellar evolution database for population synthesis studies: VI. White dwarf cooling sequences
We present a new set of cooling models and isochrones for both H- and
He-atmosphere white dwarfs, incorporating accurate boundary conditions from
detailed model atmosphere calculations, and carbon-oxygen chemical abundance
profiles based on updated stellar evolution calculations from the BaSTI stellar
evolution archive - a theoretical data center for the Virtual Observatory. We
discuss and quantify the uncertainties in the cooling times predicted by the
models, arising from the treatment of mixing during the central H- and
He-burning phases, number of thermal pulses experienced by the progenitors,
progenitor metallicity and the reaction rate. The
largest sources of uncertainty turn out to be related to the treatment of
convection during the last stages of the progenitor central He-burning phase,
and the reaction rate. We compare our new models
to previous calculations performed with the same stellar evolution code, and
discuss their application to the estimate of the age of the solar neighborhood,
and the interpretation of the observed number ratios between H- and
He-atmosphere white dwarfs. The new white dwarf sequences and an extensive set
of white dwarf isochrones that cover a large range of ages and progenitor
metallicities are made publicly available at the official BaSTI website.Comment: 34 pages, 13 figures, The Astrophysical Journal, in pres
Search for giant planets in M67 IV: survey results
We present the results of a seven-year-long radial velocity survey of a
sample of 88 main-sequence and evolved stars to reveal signatures of
Jupiter-mass planets in the solar-age and solar-metallicity open cluster M67.
We aim at studying the frequency of giant planets in this cluster with respect
to the field stars. In addition, our sample is also ideal to perform a
long-term study to compare the chemical composition of stars with and without
giant planets in detail. We analyzed precise radial velocity (RV) measurements
obtained with five different instruments. We conducted Monte Carlo simulations
to estimate the occurrence rate of giant planets in our radial velocity survey.
All the planets previously announced in this RV campaign with their properties
are summarized here: 3 hot Jupiters around the main-sequence stars YBP1194,
YBP1514, and YBP401, and 1 giant planet around the evolved star S364. Two
additional planet candidates around the stars YBP778 and S978 are also analyzed
in the present work. We discuss stars that exhibit large RV variability or
trends individually. For 2 additional stars, long-term trends are compatible
with new binary candidates or substellar objects, which increases the total
number of binary candidates detected in our campaign to 14. Based on the
Doppler-detected planets discovered in this survey, we find an occurrence of
giant planets of ~18.0%(+12.0/-8.0%) in the selected period-mass range. This
frequency is slightly higher but consistent within the errors with the estimate
for the field stars, which leads to the general conclusion that open cluster
and field statistics agree. However, we find that the rate of hot Jupiters in
the cluster (~5.7%(+5.5/-3.0%)) is substantially higher than in the field.Comment: Accepted by A&
Accurate Internal Proper Motions of Globular Clusters
We have undertaken a long term program to measure high precision proper
motions of nearby Galactic globular cluster (GC) stars using multi-epoch
observations with the WFPC2 and the ACS cameras on-board the Hubble Space
Telescope. The proper motions are used to study the internal cluster
kinematics, and to obtain accurate cluster distances. In this paper, we also
show how the proper motions of the field stars projected in the direction of
the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
A Gaia DR2 view of white dwarfs in the Hyades
We have exploited the very precise parallaxes, proper motions, and photometry of Gaia Data Release 2 to study white dwarf members of the Hyades star cluster. Gaia photometry and parallaxes for the eight DA white dwarfs confirmed members have been then used to compute absolute magnitudes and colours. These were compared to three independent sets of white dwarf evolutionary tracks, to derive cooling times and white dwarf (final) masses. All sets of models provide the same mass values, with only small differences in the cooling ages. The precision in the derived masses and cooling ages is typically 1-3 per cent. Our derived masses are generally consistent with spectroscopic estimates from the literature, whilst cooling ages are generally larger. The recent estimate of the cluster age from the Gaia Data Release 2 main-sequence turn offcolour-magnitude diagram (790 Myr) has been employed to derive progenitor (initial) masses. We find a slope of the initial-final mass relation for the Hyades white dwarfs (masses between ~0.67 and ~0.84 Mâ) steeper than that derived for the same mass range from global estimates - averaged over the whole spectrum of white dwarf masses - irrespectively of the cooling models adopted. However, when considering the error in this age estimate (-100+160Myr), a definitive conclusion on this issue cannot be reached yet. The lower limit of 690 Myr (closer to the classical Hyades age of 600-650 Myr) would provide a slope of the initial-final mass relation closer to the global determinations. We also find hints of an intrinsic spread of the cluster initial-final mass relation for the cluster. © 2018 The Author(s)
The bottom of the white dwarf cooling sequence in the old open cluster NGC 2158
We use 10 orbits of Advanced Camera for Surveys observations to reach the end
of the white dwarf cooling sequence in the solar-metallicity open cluster NGC
2158. Our photometry and completeness tests show that the end falls at
magnitude m_F606W = 27.5 +/- 0.15, which implies an age between ~1.8 and ~2.0
Gyr, consistent with the age of 1.9 +/- 0.2 Gyr obtained from fits to the
main-sequence turn-off. The faintest white dwarfs show a clear turn toward
bluer colors, as predicted by theoretical isochrones.Comment: 12 pages, 4 figures (2 in low resolution, and 1 bonus for
astro-ph-only). ApJ Letter accepted on December 1, 200
SiC(0001): a surface Mott-Hubbard insulator
We present ab-initio electronic structure calculations for the Si-terminated
SiC(0001) surface. While local density approximation
(LDA) calculations predict a metallic ground state with a half-filled narrow
band, Coulomb effects, included by the spin-polarized LDA+U method, result in a
magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the
experimental value of 2.0 eV. The calculated value of the inter-site exchange
parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except
at very low temperatures. The observed Si 2p surface core level doublet can
naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure
Hubble Space Telescope observations of the Kepler-field cluster NGC 6819 - I. The bottom of the white dwarf cooling sequence
We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050 ± 0.075. This implies an age of 2.25 ± 0.20 Gyr, consistent with the age of 2.25 ± 0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initialâfinal-mass relations have a minor impact the WD age estimate, at the level of âŒ0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size âŒ70 arcminÂČ) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.JA and IRK acknowledge support from STScI grant GO-11688 and
GO-12669. PB was supported in part by the NSERC Canada and
by the Fund FRQ-NT (Québec)
Hubble Space Telescope astrometry of the closest brown dwarf binary system -- I. Overview and improved orbit
Located at ~2pc, the L7.5+T0.5 dwarfs system WISE J104915.57-531906.1
(Luhman16AB) is the third closest system known to Earth, making it a key
benchmark for detailed investigation of brown dwarf atmospheric properties,
thermal evolution, multiplicity, and planet-hosting frequency. In the first
study of this series -- based on a multi-cycle Hubble Space Telescope (HST)
program -- we provide an overview of the project and present improved estimates
of positions, proper motions, annual parallax, mass ratio, and the current best
assessment of the orbital parameters of the A-B pair. Our HST observations
encompass the apparent periastron of the binary at 220.5+/-0.2 mas at epoch
2016.402. Although our data seem to be inconsistent with recent ground-based
astrometric measurements, we also exclude the presence of third bodies down to
Neptune masses and periods longer than a year.Comment: 19 pages, 9 figures, 9 tables. Accepted for publication in MNRAS on
2017 May
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