588 research outputs found
Ground-Based CCD Astrometry with Wide Field Imagers. I. [Observations just a few years apart allow decontamination of field objects from members in two Globular clusters.]
This paper is the first of a series of papers in which we will apply the
methods we have developed for high-precision astrometry (and photometry) with
the Hubble Space Telescope to the case of wide-field ground-based images. In
particular, we adapt the software originally developed for WFPC2 to
ground-based, wide field images from the WFI at the ESO 2.2m telescope. In this
paper, we describe in details the new software, we characterize the WFI
geometric distortion, discuss the adopted local transformation approach for
proper-motion measurements, and apply the new technique to two-epoch archive
data of the two closest Galactic globular clusters: NGC 6121 (M4) and NGC 6397.
The results of this exercise are more than encouraging. We find that we can
achieve a precision of ~7 mas (in each coordinate) in a single exposure for a
well-exposed star, which allows a very good cluster-field separation in both
M4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1 years,
respectively.Comment: 1 pages, 17 figures, 1 table. Accepted for publication in A&A, on
April 15 2006. For high resolution version:
http://www.eso.org/~lbedin/WFI_method/ms.ps.g
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&
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
An upper limit to the secular variation of the gravitational constant from white dwarf stars
A variation of the gravitational constant over cosmological ages modifies the
main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art
stellar evolutionary code we compute the effects of a secularly varying G on
the main sequence ages and, employing white dwarf cooling ages computed taking
into account the effects of a running G, we place constraints on the rate of
variation of Newton's constant. This is done using the white dwarf luminosity
function and the distance of the well studied open Galactic cluster NGC 6791.
We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the
secular variation of the gravitational constant compares favorably with those
obtained using other stellar evolutionary properties, and can be easily
improved if deep images of the cluster allow to obtain an improved white dwarf
luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA
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)
Absolute proper motion of the Galactic open cluster M67
We derived the absolute proper motion (PM) of the old, solar-metallicity
Galactic open cluster M67 using observations collected with CFHT (1997) and
with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to
determine the absolute PM of the cluster. We find (mu_alpha
cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a
line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50
pc, we explore the influence of the Galactic potential, with and without the
bar and/or spiral arms, on the galactic orbit of the cluster.Comment: 7 pages, 5 figures, and 3 tables. Published in Astronomy and
Astrophysics, Volume 513, id.A51
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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