490 research outputs found
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
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&
Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4
We report new metallicities for stars of Galactic globular cluster M4 using
the largest number of stars ever observed at high spectral resolution in any
cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT
FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a
small wavelength range, and often have very low signal-to-noise ratios. We
attacked this dataset by reconsidering the whole method of abundance analysis
of large stellar samples from beginning to end. We developed a new algorithm
that automatically determines the atmospheric parameters of a star. Nearly all
data preparation steps for spectroscopic analyses are processed on the
syntheses, not the observed spectra. For 322 Red Giant Branch stars with we obtain a nearly constant metallicity, ( = 0.02). No difference in the metallicity at the level of
is observed between the two RGB sequences identified by
\cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars we
obtain ( = 0.09) after fixing the
microturbulent velocity. These values are consistent with previous studies that
have performed detailed analyses of brighter RGB stars at higher spectroscopic
resolution and wavelength coverage. It is not clear if the small mean
metallicity difference between brighter and fainter M4 members is real or is
the result of the low signal-to-noise characteristics of the fainter stars. The
strength of our approach is shown by recovering a metallicity close to a single
value for more than two thousand stars, using a dataset that is non-optimal for
atmospheric analyses. This technique is particularly suitable for noisy data
taken in difficult observing conditions.Comment: 17 pages, 20 figures, 3 tables. Accepted for publication in The
Astronomical Journa
Multiple stellar populations in Magellanic Cloud clusters. II. Evidence also in the young NGC1844?
We use HST observations to study the LMC's young cluster NGC1844. We estimate
the fraction and the mass-ratio distribution of photometric binaries and report
that the main sequence presents an intrinsic breadth which can not be explained
in terms of photometric errors only, and is unlikely due to differential
reddening. We attempt some interpretation of this feature, including stellar
rotation, binary stars, and the presence of multiple stellar populations with
different age, metallicity, helium, or C+N+O abundance. Although we exclude
age, helium, and C+N+O variations to be responsible of the main-sequence spread
none of the other interpretations is conclusive.Comment: 9 Pages, 11 figures, accepted for publication in A&A
The age of 47Tuc from self-consistent isochrone fits to colour-magnitude diagrams and the eclipsing member V69
Our aim is to derive a self-consistent age, distance and composition for the
globular cluster Tucanae (Tuc; NGC104). First, we reevaluate the
reddening towards the cluster resulting in a nominal as
the best estimate. The of the components of the eclipsing binary
member V69 is found to be K from both photometric and spectroscopic
evidence. This yields a true distance modulus (random)(systematic) to Tuc when combined with existing measurements of
V69 radii and luminosity ratio. We then present a new completely
self-consistent isochrone fitting method to ground based and
cluster colour-magnitude diagrams and the eclipsing binary member V69. The
analysis suggests that the composition of V69, and by extension one of the
populations of Tuc, is given by [Fe/H], [O/Fe], and
on the solar abundance scale of Asplund, Grevesse & Sauval.
However, this depends on the accuracy of the model scale which is
50-75 K cooler than our best estimate but within measurement uncertainties. Our
best estimate of the age of Tuc is 11.8 Gyr, with firm () lower
and upper limits of 10.4 and 13.4 Gyr, respectively, in satisfactory agreement
with the age derived from the white dwarf cooling sequence if our determination
of the distance modulus is adopted.Comment: 19 pages, 8 figures, accepted for publication in MNRA
Multiple stellar populations in Galactic globular clusters: observational evidence
An increasing number of both photometric and spectroscopic observations over
the last years have shown the existence of distinct sub-populations in many
Galactic globular clusters and shattered the paradigm of globulars hosting
single, simple stellar populations.
These multiple populations manifest themselves in a split of different
evolutionary sequences in the cluster color-magnitude diagrams and in
star-to-star abundance variations. In this paper we will summarize the
observational scenario.Comment: 6 pages, 3 figures, Proceedings SF2A 201
The State-of-the-Art HST Astro-Photometric Analysis of the core of \omega Centauri. III. The Main Sequence's Multiple Populations Galore
We take advantage of the exquisite quality of the Hubble Space Telescope
26-filter astro-photometric catalog of the core of Omega Cen presented in the
first paper of this series and the empirical differential-reddening correction
presented in the second paper in order to distill the main sequence into its
constituent populations. To this end, we restrict ourselves to the five most
useful filters: the magic "trio" of F275W, F336W, and F438W, along with F606W
and F814W. We develop a strategy for identifying color systems where different
populations stand out most distinctly, then we isolate those populations and
examine them in other filters where their sub-populations also come to light.
In this way, we have identified at least 15 sub-populations, each of which has
a distinctive fiducial curve through our 5-dimensional photometric space. We
confirm the MSa to be split into two subcomponents, and find that both the bMS
and the rMS are split into three subcomponents. Moreover, we have discovered
two additional MS groups: the MSd (which has three subcomponents) shares
similar properties with the bMS, and the MSe (which has four subcomponents),
has properties more similar to those of the rMS. We examine the fiducial curves
together and use synthetic spectra to infer relative heavy-element,
light-element, and Helium abundances for the populations. Our findings show
that the stellar populations and star formation history of Omega Cen are even
more complex than inferred previously. Finally, we provide as a supplement to
the original catalog a list that identifies for each star which population it
most likely is associated with.Comment: 22 pages, 17 figures (most in lower res), 2 tables, accepted for
publication in Ap
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)
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