142 research outputs found
Logic operations demonstrated with localized vibrations in a micromechanical cantilever array
A method is presented for realizing logic operations in a micromechanical cantilever array based on the timed application of a lattice disturbance to control the properties of intrinsic localized modes (ILMs). The application of a specific inhomogeneous field destroys a driver-locked ILM, while the same operation can create an ILM if initially no-ILM exists. Logic states \1" and \0" correspond to \present" or \absent" ILM
The Sources of HCN and CH3OH and the Rotational Temperature in Comet 103P/Hartley 2 from Time-Resolved Millimeter Spectroscopy
One of the least understood properties of comets is the compositional
structure of their nuclei, which can either be homogeneous or heterogeneous.
The nucleus structure can be conveniently studied at millimeter wavelengths,
using velocity-resolved spectral time series of the emission lines, obtained
simultaneously for multiple molecules as the body rotates. Using this
technique, we investigated the sources of CH3OH and HCN in comet 103P/Hartley
2, the target of NASA's EPOXI mission, which had an exceptionally favorable
apparition in late 2010. Our monitoring with the IRAM 30 m telescope shows
short-term variability of the spectral lines caused by nucleus rotation. The
varying production rates generate changes in brightness by a factor of 4 for
HCN and by a factor of 2 for CH3OH, and they are remarkably well correlated in
time. With the addition of the velocity information from the line profiles, we
identify the main sources of outgassing: two jets, oppositely directed in a
radial sense, and icy grains, injected into the coma primarily through one of
the jets. The mixing ratio of CH3OH and HCN is dramatically different in the
two jets, which evidently shows large-scale chemical heterogeneity of the
nucleus. We propose a network of identities linking the two jets with
morphological features reported elsewhere, and postulate that the chemical
heterogeneity may result from thermal evolution. The model-dependent average
production rates are 3.5x10**26 molec/s for CH3OH and 1.25x10**25 molec/s for
HCN, and their ratio of 28 is rather high but not abnormal. The rotational
temperature from CH3OH varied strongly, presumably due to nucleus rotation,
with the average value being 47 K.Comment: Published in ApJ 756, 80 (2012). Supplementary materials available at
http://www.its.caltech.edu/~mdrahus/103p_paperII.htm
A Proteomic View of an Important Human Pathogen – Towards the Quantification of the Entire Staphylococcus aureus Proteome
The genome sequence is the “blue-print of life,” but proteomics provides the link to the actual physiology of living cells. Because of their low complexity bacteria are excellent model systems to identify the entire protein assembly of a living organism. Here we show that the majority of proteins expressed in growing and non-growing cells of the human pathogen Staphylococcus aureus can be identified and even quantified by a metabolic labeling proteomic approach. S. aureus has been selected as model for this proteomic study, because it poses a major risk to our health care system by combining high pathogenicity with an increasing frequency of multiple antibiotic resistance, thus requiring the development of new anti-staphylococcal therapy strategies. Since such strategies will likely have to target extracellular and surface-exposed virulence factors as well as staphylococcal survival and adaptation capabilities, we decided to combine four subproteomic fractions: cytosolic proteins, membrane-bound proteins, cell surface-associated and extracellular proteins, to comprehensively cover the entire proteome of S. aureus. This quantitative proteomics approach integrating data ranging from gene expression to subcellular localization in growing and non-growing cells is a proof of principle for whole-cell physiological proteomics that can now be extended to address physiological questions in infection-relevant settings. Importantly, with more than 1700 identified proteins (and 1450 quantified proteins) corresponding to a coverage of about three-quarters of the expressed proteins, our model study represents the most comprehensive quantification of a bacterial proteome reported to date. It thus paves the way towards a new level in understanding of cell physiology and pathophysiology of S. aureus and related pathogenic bacteria, opening new avenues for infection-related research on this crucial pathogen
COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies: I. Relations between H2, HI, stellar content and structural properties
We are conducting COLD GASS, a legacy survey for molecular gas in nearby
galaxies. Using the IRAM 30m telescope, we measure the CO(1-0) line in a sample
of ~350 nearby (D=100-200 Mpc), massive galaxies (log(M*/Msun)>10.0). The
sample is selected purely according to stellar mass, and therefore provides an
unbiased view of molecular gas in these systems. By combining the IRAM data
with SDSS photometry and spectroscopy, GALEX imaging and high-quality Arecibo
HI data, we investigate the partition of condensed baryons between stars,
atomic gas and molecular gas in 0.1-10L* galaxies. In this paper, we present CO
luminosities and molecular hydrogen masses for the first 222 galaxies. The
overall CO detection rate is 54%, but our survey also uncovers the existence of
sharp thresholds in galaxy structural parameters such as stellar mass surface
density and concentration index, below which all galaxies have a measurable
cold gas component but above which the detection rate of the CO line drops
suddenly. The mean molecular gas fraction MH2/M* of the CO detections is
0.066+/-0.039, and this fraction does not depend on stellar mass, but is a
strong function of NUV-r colour. Through stacking, we set a firm upper limit of
MH2/M*=0.0016+/-0.0005 for red galaxies with NUV-r>5.0. The average
molecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with
significant scatter from one galaxy to the next. The existence of strong
detection thresholds in both the HI and CO lines suggests that "quenching"
processes have occurred in these systems. Intriguingly, atomic gas strongly
dominates in the minority of galaxies with significant cold gas that lie above
these thresholds. This suggests that some re-accretion of gas may still be
possible following the quenching event.Comment: Accepted for publications in MNRAS. 32 pages, 25 figure
Measuring and Understanding the Universe
Revolutionary advances in both theory and technology have launched cosmology
into its most exciting period of discovery yet. Unanticipated components of the
universe have been identified, promising ideas for understanding the basic
features of the universe are being tested, and deep connections between physics
on the smallest scales and on the largest scales are being revealed.Comment: 39 pages, 11 figures, 1 table, accepted for publication in Reviews of
Modern Physics Colloqui
PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES)
Within the framework of the HERM33ES key project, we are studying the star
forming interstellar medium in the nearby, metal-poor spiral galaxy M33,
exploiting the high resolution and sensitivity of Herschel. We use PACS and
SPIRE maps at 100, 160, 250, 350, and 500 micron wavelength, to study the
variation of the spectral energy distributions (SEDs) with galacto-centric
distance. Detailed SED modeling is performed using azimuthally averaged fluxes
in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance.
Simple isothermal and two-component grey body models, with fixed dust
emissivity index, are fitted to the SEDs between 24 and 500 micron using also
MIPS/Spitzer data, to derive first estimates of the dust physical conditions.
The far-infrared and submillimeter maps reveal the branched, knotted spiral
structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500
micron). Two component fits to the SEDs agree better than isothermal models
with the observed, total and radially averaged flux densities. The two
component model, with beta fixed at 1.5, best fits the global and the radial
SEDs. The cold dust component clearly dominates; the relative mass of the warm
component is less than 0.3% for all the fits. The temperature of the warm
component is not well constrained and is found to be about 60K plus/minus 10K.
The temperature of the cold component drops significantly from about 24K in the
inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting
model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher
than the solar value by a factor of 1.5 and is roughly in agreement with the
subsolar metallicity of M33.Comment: 5 pages, 3 figures, accepted for publication in the A&A Herschel
Special Issu
COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: II. The non-universality of the Molecular Gas Depletion Timescale
We study the relation between molecular gas and star formation in a
volume-limited sample of 222 galaxies from the COLD GASS survey, with
measurements of the CO(1-0) line from the IRAM 30m telescope. The galaxies are
at redshifts 0.025<z<0.05 and have stellar masses in the range
10.0<log(M*/Msun)<11.5. The IRAM measurements are complemented by deep Arecibo
HI observations and homogeneous SDSS and GALEX photometry. A reference sample
that includes both UV and far-IR data is used to calibrate our estimates of
star formation rates from the seven optical/UV bands. The mean molecular gas
depletion timescale, tdep(H2), for all the galaxies in our sample is 1 Gyr,
however tdep(H2) increases by a factor of 6 from a value of ~0.5 Gyr for
galaxies with stellar masses of 10^10 Msun to ~3 Gyr for galaxies with masses
of a few times 10^11 Msun. In contrast, the atomic gas depletion timescale
remains contant at a value of around 3 Gyr. This implies that in high mass
galaxies, molecular and atomic gas depletion timescales are comparable, but in
low mass galaxies, molecular gas is being consumed much more quickly than
atomic gas. The strongest dependences of tdep(H2) are on the stellar mass of
the galaxy (parameterized as log tdep(H2)= (0.36+/-0.07)(log M* -
10.70)+(9.03+/-0.99)), and on the specific star formation rate. A single
tdep(H2) versus sSFR relation is able to fit both "normal" star-forming
galaxies in our COLD GASS sample, as well as more extreme starburst galaxies
(LIRGs and ULIRGs), which have tdep(H2) < 10^8 yr. Normal galaxies at z=1-2 are
displaced with respect to the local galaxy population in the tdep(H2) versus
sSFR plane and have molecular gas depletion times that are a factor of 3-5
times longer at a given value of sSFR due to their significantly larger gas
fractions.Comment: Accepted for publication in MNRAS. 19 pages, 11 figure
Separation of Gravitational-Wave and Cosmic-Shear Contributions to Cosmic Microwave Background Polarization
Inflationary gravitational waves (GW) contribute to the curl component in the
polarization of the CMB. Cosmic shear--gravitational lensing of the CMB--
converts a fraction of the dominant gradient polarization to the curl
component. Higher-order correlations can be used to map the cosmic shear and
subtract this contribution to the curl. Arcminute resolution will be required
to pursue GW amplitudes smaller than those accessible by Planck. The finite
cutoff in CMB power at small scales leads to a minimum detectable GW amplitude
corresponding to an inflation energy near 10^15 GeV.Comment: 4 pages, 2 figures; Final version accepted for publication in Phys.
Rev. Lett. (Figure 1 updated to account for a numerical correction and minor
changes to the text
Neural network-based emulation of interstellar medium models
The interpretation of observations of atomic and molecular tracers in the
galactic and extragalactic interstellar medium (ISM) requires comparisons with
state-of-the-art astrophysical models to infer some physical conditions.
Usually, ISM models are too time-consuming for such inference procedures, as
they call for numerous model evaluations. As a result, they are often replaced
by an interpolation of a grid of precomputed models.
We propose a new general method to derive faster, lighter, and more accurate
approximations of the model from a grid of precomputed models.
These emulators are defined with artificial neural networks (ANNs) designed
and trained to address the specificities inherent in ISM models. Indeed, such
models often predict many observables (e.g., line intensities) from just a few
input physical parameters and can yield outliers due to numerical instabilities
or physical bistabilities. We propose applying five strategies to address these
characteristics: 1) an outlier removal procedure; 2) a clustering method that
yields homogeneous subsets of lines that are simpler to predict with different
ANNs; 3) a dimension reduction technique that enables to adequately size the
network architecture; 4) the physical inputs are augmented with a polynomial
transform to ease the learning of nonlinearities; and 5) a dense architecture
to ease the learning of simple relations.
We compare the proposed ANNs with standard classes of interpolation methods
to emulate the Meudon PDR code, a representative ISM numerical model.
Combinations of the proposed strategies outperform all interpolation methods by
a factor of 2 on the average error, reaching 4.5% on the Meudon PDR code. These
networks are also 1000 times faster than accurate interpolation methods and
require ten to forty times less memory.
This work will enable efficient inferences on wide-field multiline
observations of the ISM
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