3,554 research outputs found
Evanescent field optical readout of graphene mechanical motion at room temperature
Graphene mechanical resonators have recently attracted considerable attention
for use in precision force and mass sensing applications. To date, readout of
their oscillatory motion has typically required cryogenic conditions to achieve
high sensitivity, restricting their range of applications. Here we report the
first demonstration of evanescent optical readout of graphene motion, using a
scheme which does not require cryogenic conditions and exhibits enhanced
sensitivity and bandwidth at room temperature. We utilise a high
microsphere to enable evanescent readout of a 70 m diameter graphene drum
resonator with a signal-to-noise ratio of greater than 25 dB, corresponding to
a transduction sensitivity of 2.6 m
. The sensitivity of force measurements using this
resonator is limited by the thermal noise driving the resonator, corresponding
to a force sensitivity of N
with a bandwidth of 35 kHz at room temperature (T = 300
K). Measurements on a 30 m graphene drum had sufficient sensitivity to
resolve the lowest three thermally driven mechanical resonances.Comment: Fixed formatting errors in bibliograph
Intracluster Red Giant Stars in the Virgo Cluster
We have used the WFPC2 camera of the Hubble Space Telescope to obtain deep
F814W images of a blank field in the Virgo Cluster located 41 arcmin northwest
of M87. We perform star counts in that field, and in another Virgo field
observed by Ferguson, Tanvir & von Hippel (1998), and show that, when compared
to the Hubble Deep Field North and South, the Virgo Cluster contains an excess
of objects with magnitudes I > 27. We attribute this excess to a population of
intracluster red-giant branch (IC-RGB) stars. By modeling the luminosity
function of these stars, we show that the tip of the Virgo RGB is at I = 27.31
+0.27/-0.17 and that the cluster contains a small, but significant, excess of
stars that are up to ~1 mag brighter than this tip. If this luminous component
is due entirely to stars on the asymptotic giant branch (AGB), it implies an
age for the population of > 2 Gyr; if foreground RGB stars contribute to the
luminous tail, then the derived age for the stars is older still. The
luminosity function also suggests that most of the intracluster stars are
moderately metal-rich (-0.8 < [Fe/H] <-0.2), a result consistent with that
expected from stars that have been tidally stripped from intermediate
luminosity galaxies. Additionally, a comparison with the planetary nebulae in
our field also supports this view, although the existence of a more metal-poor
population (from stripped dwarfs) cannot be ruled out. Our derived average
surface brightness, mu_I = 27.9 +0.3/-0.5 mag/arcsec^2 for Virgo's diffuse
component suggests that intracluster stars contribute 10% to 20% of the
cluster's total I-band luminosity.Comment: 21 pages, 8 figures included, accepted for publication in the
Astrophysical Journa
The Ital-FLAMES survey of the Sagittarius dwarf Spheroidal galaxy. I. Chemical abundances of bright RGB stars
We present iron and element (Mg, Ca, Ti) abundances for a sample of
15 Red Giant Branch stars belonging to the main body of the Sagittarius dwarf
Spheroidal galaxy. Abundances have been obtained from spectra collected using
the high resolution spectrograph FLAMES-UVES mounted at the VLT. Stars of our
sample have a mean metallicity of [Fe/H]=-0.410.20 with a metal poor tail
extending to [Fe/H]=-1.52. The element abundance ratios are slightly
subsolar for metallicities higher than [Fe/H]\gtsima-1, suggesting a slow star
formation rate. The [/Fe] of stars having [Fe/H]-1 are compatible to
what observed in Milky Way stars of comparable metallicity.Comment: 16 pages, 10 figures, 6 tables. Accepted for publication in A&A.
Minor changes in the tex
3D kinematics through the X-shaped Milky Way bulge
Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars.
Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities.
Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, −6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2 m at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines.
Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (μlcos (b),μb) = (0.30 ± 0.14, −0.43 ± 0.13), with a velocity dispersion of (σlcos(b),σb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster
AAOmega spectroscopy of 29 351 stars in fields centered on ten Galactic globular clusters
Galactic globular clusters have been pivotal in our understanding of many
astrophysical phenomena. Here we publish the extracted stellar parameters from
a recent large spectroscopic survey of ten globular clusters. A brief review of
the project is also presented. Stellar parameters have been extracted from
individual stellar spectra using both a modified version of the Radial Velocity
Experiment (RAVE) pipeline and a pipeline based on the parameter estimation
method of RAVE. We publish here all parameters extracted from both pipelines.
We calibrate the metallicity and convert this to [Fe/H] for each star and,
furthermore, we compare the velocities and velocity dispersions of the Galactic
stars in each field to the Besan\c{c}on Galaxy model. We find that the model
does not correspond well with the data, indicating that the model is probably
of little use for comparisons with pencil beam survey data such as this.Comment: 6 pages, 5 figures, 4 tables. Accepted for publication in A&A. Data
described in tables will be available on CDS (at
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/530/A31) once publishe
Evolutionary Toxicology: Population-Level Effects of Chronic Contaminant Exposure on the Marsh Frogs (Rana ridibunda) of Azerbaijan
We used molecular methods and population genetic analyses to study the effects of chronic contaminant exposure in marsh frogs from Sumgayit, Azerbaijan. Marsh frogs inhabiting wetlands in Sumgayit are exposed to complex mixtures of chemical contaminants, including petroleum products, pesticides, heavy metals, and many other industrial chemicals. Previous results documented elevated estimates of genetic damage in marsh frogs from the two most heavily contaminated sites. Based on mitochondrial DNA (mtDNA) control region sequence data, the Sumgayit region has reduced levels of genetic diversity, likely due to environmental degradation. The Sumgayit region also acts as an ecological sink, with levels of gene flow into the region exceeding gene flow out of the region. Additionally, localized mtDNA heteroplasmy and diversity patterns suggest that one of the most severely contaminated sites in Sumgayit is acting as a source of new mutations resulting from an increased mutation rate. This study provides an integrated method for assessing the cumulative population impacts of chronic contaminant exposure by studying both population genetic and evolutionary effects
Observations and Implications of the Star Formation History of the LMC
We present derivations of star formation histories based on color-magnitude
diagrams of three fields in the LMC from HST/WFPC2 observations. A significant
component of stars older than 4 Gyr is required to match the observed
color-magnitude diagrams. Models with a dispersion-free age-metallicity
relation are unable to reproduce the width of the observed main sequence;
models with a range of metallicity at a given age provide a much better fit.
Such models allow us to construct complete ``population boxes'' for the LMC
based entirely on color-magnitude diagrams; remarkably, these qualitatively
reproduce the age-metallicity relation observed in LMC clusters. We discuss
some of the uncertainties in deriving star formation histories. We find,
independently of the models, that the LMC bar field has a larger relative
component of older stars than the outer fields. The main implications suggested
by this study are: 1) the star formation history of field stars appears to
differ from the age distribution of clusters, 2) there is no obvious evidence
for bursty star formation, but our ability to measure bursts shorter in
duration than 25% of any given age is limited by the statistics of the
observed number of stars, 3) there may be some correlation of the star
formation rate with the last close passage of the LMC/SMC/Milky Way, but there
is no dramatic effect, and 4) the derived star formation history is probably
consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure
A wide-area view of the Phoenix dwarf galaxy from VLT/FORS imaging
We present results from a wide-area photometric survey of the Phoenix dwarf
galaxy, one of the rare dwarf irregular/ dwarf spheroidal transition type
galaxies (dTs) of the Local Group (LG). These objects offer the opportunity to
study the existence of possible evolutionary links between the late- and early-
type LG dwarf galaxies, since the properties of dTs suggest that they may be
dwarf irregulars in the process of transforming into dwarf spheroidals. Using
FORS at the VLT we have acquired VI photometry of Phoenix. The data reach a
S/N~10 just below the horizontal branch of the system and consist of a mosaic
of images that covers an area of 26' x 26' centered on the coordinates of the
optical center of the galaxy. Examination of the colour-magnitude diagram and
luminosity function revealed the presence of a bump above the red clump,
consistent with being a red giant branch bump. The deep photometry combined
with the large area covered allows us to put on a secure ground the
determination of the overall structural properties of the galaxy and to derive
the spatial distribution of stars in different evolutionary phases and age
ranges, from 0.1 Gyr to the oldest stars. The best-fitting profile to the
overall stellar population is a Sersic profile of Sersic radius R_S =
1.82'+-0.06' and m=0.83+-0.03. We confirm that the spatial distribution of
stars is found to become more and more centrally concentrated the younger the
stellar population, as reported in previous studies. This is similar to the
stellar population gradients found for close-by Milky Way dwarf spheroidal
galaxies. We quantify such spatial variations by analyzing the surface number
density profiles of stellar populations in different age ranges; [Abridged]Comment: 21 pages; 11 figures. Accepted for publication in MNRA
Stellar Populations in Three Outer Fields of the LMC
We present HST photometry for three fields in the outer disk of the LMC
extending approximately four magnitudes below the faintest main sequence
turnoff. We cannot detect any strongly significant differences in the stellar
populations of the three fields based on the morphologies of the
color-magnitude diagrams, the luminosity functions, and the relative numbers of
stars in different evolutionary stages. Our observations therefore suggest
similar star formation histories in these regions, although some variations are
certainly allowed. The fields are located in two regions of the LMC: one is in
the north-east field and two are located in the north-west. Under the
assumption of a common star formation history, we combine the three fields with
ground-based data at the same location as one of the fields to improve
statistics for the brightest stars. We compare this stellar population with
those predicted from several simple star formation histories suggested in the
literature, using a combination of the R-method of Bertelli et al (1992) and
comparisons with the observed luminosity function. The only model which we
consider that is not rejected by the observations is one in which the star
formation rate is roughly constant for most of the LMC's history and then
increases by a factor of three about 2 Gyr ago. Such a model has roughly equal
numbers of stars older and younger than 4 Gyr, and thus is not dominated by
young stars. This star formation history, combined with a closed box chemical
evolution model, is consistent with observations that the metallicity of the
LMC has doubled in the past 2 Gyr.Comment: 30 pages, includes 10 postscript figures. Figure 1 avaiable at
ftp://charon.nmsu.edu/pub/mgeha/LMC. Accepted for publication in Astronomical
Journa
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