1,280 research outputs found
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
We determine the parallax and proper motion of the flaring, non-thermal radio
star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline
Array observations. Based on the parallax, we measure a distance of 389 +24/-21
parsecs to the source. Our measurement places the Orion Nebula Cluster
considerably closer than the canonical distance of 480 +/- 80 parsecs
determined by Genzel et al. (1981). A change of this magnitude in distance
lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We
briefly discuss two effects of this change--an increase in the age spread of
the pre-main sequence stars and better agreement between the zero-age
main-sequence and the temperatures and luminosities of massive stars.Comment: 10 pages, 4 figures, emulateapj, accepted to Ap
A Finite Element Study of Electromagnetic Riveting
Electromagnetic riveting, used in some aerospace assembly processes, involves rapid deformation, leading to the finished rivet configuration. Analysis of this process is described for the case of an aluminum rivet joining typical aluminum structural elements. The analysis is based on a finite element method that includes the effects of heating, due to rapid plastic deformation of the material, on the material properties. Useful details of material deformation and thermal history and the final rivet and structure configuration and states of stress and strain are obtained. These results have significant implications in the design, implementation, and improvement of practical fastening processes in the aerospace industry
Metamaterials for light rays: ray optics without wave-optical analog in the ray-optics limit
Volumes of sub-wavelength electromagnetic elements can act like homogeneous
materials: metamaterials. In analogy, sheets of optical elements such as prisms
can act ray-optically like homogeneous sheet materials. In this sense, such
sheets can be considered to be metamaterials for light rays (METATOYs).
METATOYs realize new and unusual transformations of the directions of
transmitted light rays. We study here, in the ray-optics and scalar-wave
limits, the wave-optical analog of such transformations, and we show that such
an analog does not always exist. Perhaps, this is the reason why many of the
ray-optical possibilities offered by METATOYs have never before been
considered.Comment: 10 pages, 3 figures, references update
Full-disc CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H conversion factor
Carbon monoxide (CO) provides crucial information about the molecular gas
properties of galaxies. While CO has been targeted extensively,
isotopologues such as CO have the advantage of being less optically
thick and observations have recently become accessible across full galaxy
discs. We present a comprehensive new dataset of CO(1-0) observations
with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from
the EMPIRE survey at a spatial resolution of 1.5kpc. CO(1-0) is
mapped out to and detected at high signal-to-noise throughout our
maps. We analyse the CO(1-0)-to-CO(1-0) ratio () as a
function of galactocentric radius and other parameters such as the
CO(2-1)-to-CO(1-0) intensity ratio, the 70-to-160m flux
density ratio, the star-formation rate surface density, the star-formation
efficiency, and the CO-to-H conversion factor. We find that varies by
a factor of 2 at most within and amongst galaxies, with a median value of 11
and larger variations in the galaxy centres than in the discs. We argue that
optical depth effects, most likely due to changes in the mixture of
diffuse/dense gas, are favored explanations for the observed variations,
while abundance changes may also be at play. We calculate a spatially-resolved
CO(1-0)-to-H conversion factor and find an average value of
cm (K.km/s) over our sample with a standard
deviation of a factor of 2. We find that CO(1-0) does not appear to be a
good predictor of the bulk molecular gas mass in normal galaxy discs due to the
presence of a large diffuse phase, but it may be a better tracer of the mass
than CO(1-0) in the galaxy centres where the fraction of dense gas is
larger.Comment: accepted for publication in MNRA
The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51
We present the first results from the EMPIRE survey, an IRAM large program
that is mapping tracers of high density molecular gas across the disks of nine
nearby star-forming galaxies. Here, we present new maps of the 3-mm transitions
of HCN, HCO+, and HNC across the whole disk of our pilot target, M51. As
expected, dense gas correlates with tracers of recent star formation, filling
the "luminosity gap" between Galactic cores and whole galaxies. In detail, we
show that both the fraction of gas that is dense, f_dense traced by HCN/CO, and
the rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend on
environment in the galaxy. The sense of the dependence is that high surface
density, high molecular gas fraction regions of the galaxy show high dense gas
fractions and low dense gas star formation efficiencies. This agrees with
recent results for individual pointings by Usero et al. 2015 but using unbiased
whole-galaxy maps. It also agrees qualitatively with the behavior observed
contrasting our own Solar Neighborhood with the central regions of the Milky
Way. The sense of the trends can be explained if the dense gas fraction tracks
interstellar pressure but star formation occurs only in regions of high density
contrast.Comment: 7 pages, 5 figures, ApJL accepte
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Cool dust heating and temperature mixing in nearby star-forming galaxies
Physical conditions of the interstellar medium in galaxies are closely linked
to the ambient radiation field and the heating of dust grains. In order to
characterize dust properties in galaxies over a wide range of physical
conditions, we present here the radial surface brightness profiles of the
entire sample of 61 galaxies from Key Insights into Nearby Galaxies:
Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the
characterization of the grain emissivities, dust temperatures, and interstellar
radiation fields responsible for heating the dust. After fitting the dust and
stellar radial profiles with exponential functions, we fit the far-infrared
spectral energy distribution (SED) in each annular region with
single-temperature modified black bodies using both variable (MBBV) and fixed
(MBBF) emissivity indices beta, as well as with physically motivated dust
models. Results show that while most SED parameters decrease with radius, the
emissivity index beta also decreases with radius in some galaxies, but in
others is increasing, or rising in the inner regions and falling in the outer
ones. Despite the fixed grain emissivity (average beta~ 2.1) of the
physically-motivated models, they are well able to accommodate flat spectral
slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated
with cooler temperatures, contrary to what would be expected from the usual
Tdust-beta degeneracy. This trend is related to variations in Umin since beta
and Umin are very closely linked over the entire range in Umin sampled by the
KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results
strongly suggest that the low apparent \beta values (flat slopes) in MBBV fits
are caused by temperature mixing along the line-of-sight, rather than by
intrinsic variations in grain properties. Abstract truncated for arXiv.Comment: 28 pages, 20 figures, accepted for publication in A&
Kondo resonance effect on persistent currents through a quantum dot in a mesoscopic ring
The persistent current through a quantum dot inserted in a mesoscopic ring of
length L is studied. A cluster representing the dot and its vicinity is exactly
diagonalized and embedded into the rest of the ring. The Kondo resonance
provides a new channel for the current to flow. It is shown that due to scaling
properties, the persistent current at the Kondo regime is enhanced relative to
the current flowing either when the dot is at resonance or along a perfect ring
of same length. In the Kondo regime the current scales as , unlike
the scaling of a perfect ring. We discuss the possibility of detection
of the Kondo effect by means of a persistent current measurement.Comment: 11 pages, 3 Postscript figure
Tracing the Bipolar Outflow from Orion Source I
Using CARMA, we imaged the 87 GHz SiO v=0 J=2-1 line toward Orion-KL with
0.45 arcsec angular resolution. The maps indicate that radio source I drives a
bipolar outflow into the surrounding molecular cloud along a NE--SW axis, in
agreement with the model of Greenhill et al. (2004). The extended high velocity
outflow from Orion-KL appears to be a continuation of this compact outflow.
High velocity gas extends farthest along a NW--SE axis, suggesting that the
outflow direction changes on time scales of a few hundred years.Comment: 4 pages, 4 figures; accepted for publication in Ap J Letter
Constraints on the Ultra-High Energy Neutrino Flux from Gamma-Ray Bursts from a Prototype Station of the Askaryan Radio Array
We report on a search for ultra-high-energy (UHE) neutrinos from gamma-ray
bursts (GRBs) in the data set collected by the Testbed station of the Askaryan
Radio Array (ARA) in 2011 and 2012. From 57 selected GRBs, we observed no
events that survive our cuts, which is consistent with 0.12 expected background
events. Using NeuCosmA as a numerical GRB reference emission model, we estimate
upper limits on the prompt UHE GRB neutrino fluence and quasi-diffuse flux from
to GeV. This is the first limit on the prompt UHE GRB
neutrino quasi-diffuse flux above GeV.Comment: 14 pages, 8 figures, Published in Astroparticle Physics Journa
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