883 research outputs found
Astrometry and photometry with HST-WFC3. I. Geometric distortion corrections of F225W, F275W, F336W bands of the UVIS-channel
An accurate geometric distortion solution for the Hubble Space Telescope
UVIS-channel of Wide Field Camera 3 is the first step towards its use for high
precision astrometry. In this work we present an average correction that
enables a relative astrometric accuracy of ~1 mas (in each axis for well
exposed stars) in three broad-band ultraviolet filters (F225W, F275W, and
F336W). More data and a better understanding of the instrument are required to
constrain the solution to a higher level of accuracy.Comment: 20 pages, 7 figures (3 in low resolution), 3 tables. Accepted for
publication in PASP on October 16 200
Ground-based CCD astrometry with wide field imagers. IV. An improved Geometric Distortion Correction for the Blue prime-focus Camera at the LBT
High precision astrometry requires an accurate geometric distortion solution.
In this work, we present an average correction for the Blue Camera of the Large
Binocular Telescope which enables a relative astrometric precision of ~15 mas
for the B_Bessel and V_Bessel broad-band filters. The result of this effort is
used in two companion papers: the first to measure the absolute proper motion
of the open cluster M67 with respect to the background galaxies; the second to
decontaminate the color-magnitude diagram of M67 from field objects, enabling
the study of the end of its white dwarf cooling sequence. Many other
applications might find this distortion correction useful.Comment: 13 pages, 11 figures, 4 tables. accepted for publication on Astronomy
and Astrophysic
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
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
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
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
A double stellar generation in the Globular Cluster NGC6656 (M 22). Two stellar groups with different iron and s-process element abundance
AIMS. In this paper we present the chemical abundance analysis from high
resolution UVES spectra of seventeen bright giant stars of the Globular Cluster
M~22. RESULTS. We obtained an average iron abundance of [Fe/H]=-1.76\pm0.02
(internal errors only) and an \alpha enhancement of 0.36\pm0.04 (internal
errors only). Na and O, and Al and O follow the well known anti-correlation
found in many other GCs. We identified two groups of stars with significantly
different abundances of the s-process elements Y, Zr and Ba. The relative
numbers of the two group members are very similar to the ratio of the stars in
the two SGBs of M22 recently found by Piotto (2009). Y and Ba abundances do not
correlate with Na, O and Al. The s-element rich stars are also richer in iron
and have higher Ca abundances. The results from high resolution spectra have
been further confirmed by lower resolution GIRAFFE spectra of fourteen
additional M22 stars. GIRAFFE spectra show also that the Eu -- a pure r-process
element -- abundance is not related to the iron content. We discuss the
chemical abundance pattern of M22 stars in the context of the multiple stellar
populations in GC scenario.Comment: 17 Pages, 21 figures, Accepted for publication in A&
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
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