4,679 research outputs found
Image reconstruction in optical interferometry: Benchmarking the regularization
With the advent of infrared long-baseline interferometers with more than two
telescopes, both the size and the completeness of interferometric data sets
have significantly increased, allowing images based on models with no a priori
assumptions to be reconstructed. Our main objective is to analyze the multiple
parameters of the image reconstruction process with particular attention to the
regularization term and the study of their behavior in different situations.
The secondary goal is to derive practical rules for the users. Using the
Multi-aperture image Reconstruction Algorithm (MiRA), we performed multiple
systematic tests, analyzing 11 regularization terms commonly used. The tests
are made on different astrophysical objects, different (u,v) plane coverages
and several signal-to-noise ratios to determine the minimal configuration
needed to reconstruct an image. We establish a methodology and we introduce the
mean-square errors (MSE) to discuss the results. From the ~24000 simulations
performed for the benchmarking of image reconstruction with MiRA, we are able
to classify the different regularizations in the context of the observations.
We find typical values of the regularization weight. A minimal (u,v) coverage
is required to reconstruct an acceptable image, whereas no limits are found for
the studied values of the signal-to-noise ratio. We also show that
super-resolution can be achieved with increasing performance with the (u,v)
coverage filling. Using image reconstruction with a sufficient (u,v) coverage
is shown to be reliable. The choice of the main parameters of the
reconstruction is tightly constrained. We recommend that efforts to develop
interferometric infrastructures should first concentrate on the number of
telescopes to combine, and secondly on improving the accuracy and sensitivity
of the arrays.Comment: 15 pages, 16 figures; accepted in A&
SPI observations of the diffuse 60Fe emission in the Galaxy
Gamma-ray line emission from radioactive decay of 60Fe provides constraints
on nucleosynthesis in massive stars and supernovae. The spectrometer SPI on
board INTEGRAL has accumulated nearly three years of data on gamma-ray emission
from the Galactic plane. We have analyzed these data with suitable
instrumental-background models and sky distributions to produce high-resolution
spectra of Galactic emission. We detect the gamma-ray lines from 60Fe decay at
1173 and 1333 keV, obtaining an improvement over our earlier measurement of
both lines with now 4.9 sigma significance for the combination of the two
lines. The average flux per line is (4.4 \pm 0.9) \times 10^{-5} ph cm^{-2}
s^{-1} rad^{-1} for the inner Galaxy region. Deriving the Galactic 26Al
gamma-ray line flux with using the same set of observations and analysis
method, we determine the flux ratio of 60Fe/26Al gamma-rays as 0.148 \pm 0.06.
The current theoretical predictions are still consistent with our result.Comment: 10 pages, 7 figures, 2 tables, A&A in pres
Observing a column-dependent zeta in dense interstellar sources: the case of the Horsehead Nebula
Context: Observations of small carbon-bearing molecules such as CCH, C4H,
c-C3H2, and HCO in the Horsehead Nebula have shown these species to have higher
abundances towards the edge of the source than towards the center.
Aims: Given the determination of a wide range of values for zeta (s-1), the
total ionization rate of hydrogen atoms, and the proposal of a column-dependent
zeta(N_H), where N_H is the total column of hydrogen nuclei, we desire to
determine if the effects of zeta(N_H) in a single object with spatial variation
can be observable. We chose the Horsehead Nebula because of its geometry and
high density.
Method: We model the Horsehead Nebula as a near edge-on photon dominated
region (PDR), using several choices for zeta, both constant and as a function
of column. The column-dependent zeta functions are determined by a Monte Carlo
model of cosmic ray penetration, using a steep power-law spectrum and
accounting for ionization and magnetic field effects. We consider a case with
low-metal elemental abundances as well as a sulfur-rich case.
Results: We show that use of a column-dependent zeta(N_H) of 5(-15) s-1 at
the surface and 7.5(-16) s-1 at Av = 10 on balance improves agreement between
measured and theoretical molecular abundances, compared with constant values of
zeta.Comment: 12 pages, 6 figures, 5 tables, accepted in A&
Local scales on curves and surfaces
In this paper, we extend our previous work on the study of local scales of a
function to studying local scales on curves and surfaces. In the case of a
function f, the local scales of f at x is computed by measuring the deviation
of f from a linear function near x at different scales t's. In the case of a
d-dimensional surface E, the analogy is to measure the deviation of E from a
d-plane near x on E at various scale t's. We then apply the theory of singular
integral operators on E to show useful properties of local scales. We will also
show that the defined local scales are consistent in the sense that the number
of local scales are invariant under dilation
Milli-arcsecond images of the Herbig Ae star HD 163296
The very close environments of young stars are the hosts of fundamental
physical processes, such as planet formation, star-disk interactions, mass
accretion, and ejection. The complex morphological structure of these
environments has been confirmed by the now quite rich data sets obtained for a
few objects by near-infrared long-baseline interferometry. We gathered numerous
interferometric measurements for the young star HD163296 with various
interferometers (VLTI, IOTA, KeckI and CHARA), allowing for the first time an
image independent of any a priori model to be reconstructed. Using the
Multi-aperture image Reconstruction Algorithm (MiRA), we reconstruct images of
HD 163296 in the H and K bands. We compare these images with reconstructed
images obtained from simulated data using a physical model of the environment
of HD 163296. We obtain model-independent and -band images of the
surroundings of HD 163296. The images present several significant features that
we can relate to an inclined asymmetric flared disk around HD 163296 with the
strongest intensity at about 4-5 mas. Because of the incomplete spatial
frequency coverage, we cannot state whether each of them individually is
peculiar in any way. For the first time, milli-arcsecond images of the
environment of a young star are produced. These images confirm that the
morphology of the close environment of young stars is more complex than the
simple models used in the literature so far.Comment: 11 pages, 10 figures, accepted A&A pape
Testing Lorentz Invariance with Neutrinos from Ultrahigh Energy Cosmic Ray Interactions
We have previously shown that a very small amount of Lorentz invariance
violation (LIV), which suppresses photomeson interactions of ultrahigh energy
cosmic rays (UHECRs) with cosmic background radiation (CBR) photons, can
produce a spectrum of cosmic rays that is consistent with that currently
observed by the Pierre Auger Observatory (PAO) and HiRes experiments. Here, we
calculate the corresponding flux of high energy neutrinos generated by the
propagation of UHECR protons through the CBR in the presence of LIV. We find
that LIV produces a reduction in the flux of the highest energy neutrinos and a
reduction in the energy of the peak of the neutrino energy flux spectrum, both
depending on the strength of the LIV. Thus, observations of the UHE neutrino
spectrum provide a clear test for the existence and amount of LIV at the
highest energies. We further discuss the ability of current and future proposed
detectors make such observations.Comment: final version to appear in Astroparticle Physic
Complete Genome Sequences of Mycobacterium smegmatis Phages NihilNomen and Carlyle, Isolated in Las Vegas, Nevada
We present the complete genomes of the Mycobacterium smegmatis phages Carlyle and NihilNomen, isolated from soil in Las Vegas, Nevada. The phages were isolated and annotated by undergraduate students enrolled in the Phage Discovery course offered by the School of Life Sciences at the University of Nevada Las Vegas
The Dark Molecular Gas
The mass of molecular gas in an interstellar cloud is often measured using
line emission from low rotational levels of CO, which are sensitive to the CO
mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a
significant H_2 mass may lie outside the CO region, in the outer regions of the
molecular cloud where the gas phase carbon resides in C or C+. Here, H_2
self-shields or is shielded by dust from UV photodissociation, where as CO is
photodissociated. This H_2 gas is "dark" in molecular transitions because of
the absence of CO and other trace molecules, and because H_2 emits so weakly at
temperatures 10 K < T < 100 K typical of this molecular component. This
component has been indirectly observed through other tracers of mass such as
gamma rays produced in cosmic ray collisions with the gas and
far-infrared/submillimeter wavelength dust continuum radiation. In this paper
we theoretically model this dark mass and find that the fraction of the
molecular mass in this dark component is remarkably constant (~ 0.3 for average
visual extinction through the cloud with mean A_V ~ 8) and insensitive to the
incident ultraviolet radiation field strength, the internal density
distribution, and the mass of the molecular cloud as long as mean A_V, or
equivalently, the product of the average hydrogen nucleus column and the
metallicity through the cloud, is constant. We also find that the dark mass
fraction increases with decreasing mean A_V, since relatively more molecular
H_2 material lies outside the CO region in this case.Comment: 38 page, 11 figures, Accepted for Publication in ApJ, corrected
citation and typo in Appendix
On the Relationship Between Molecular Hydrogen and Carbon Monoxide Abundances in Molecular Clouds
The most usual tracer of molecular gas is line emission from CO. However, the
reliability of that tracer has long been questioned in environments different
from the Milky Way. We study the relationship between H2 and CO abundances
using a fully dynamical model of magnetized turbulence coupled to a chemical
network simplified to follow only the dominant pathways for H2 and CO formation
and destruction, and including photodissociation using a six-ray approximation.
We find that the abundance of H2 is primarily determined by the amount of time
available for its formation, which is proportional to the product of the
density and the metallicity, but insensitive to photodissociation.
Photodissociation only becomes important at extinctions under a few tenths of a
visual magnitude, in agreement with both observational and prior theoretical
work. On the other hand, CO forms quickly, within a dynamical time, but its
abundance depends primarily on photodissociation, with only a weak secondary
dependence on H2 abundance. As a result, there is a sharp cutoff in CO
abundance at mean visual extinctions A_V < 3. At lower values of A_V we find
that the ratio of H2 column density to CO emissivity X_CO is proportional to
A_V^(-3.5). This explains the discrepancy observed in low metallicity systems
between cloud masses derived from CO observations and other techniques such as
infrared emission. Our work predicts that CO-bright clouds in low metallicity
systems should be systematically larger or denser than Milky Way clouds, or
both. Our results further explain the narrow range of observed molecular cloud
column densities as a threshold effect, without requiring the assumption of
virial equilibrium.Comment: 16 pages, 11 figures. Updated to match version accepted by MNRA
Mott-Superfluid transition in bosonic ladders
We show that in a commensurate bosonic ladder, a quantum phase transition
occurs between a Mott insulator and a superfluid when interchain hopping
increases. We analyse the properties of such a transition as well as the
physical properties of the two phases. We discuss the physical consequences for
experimental systems such as Josephson Junction arrays.Comment: 4 pages, 2 figures, revtex
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