4,761 research outputs found
The primary cosmic ray spectrum above 10 to the 19th power eV
Progress on a re-evaluation of the spectrum of cosmic rays determined with the Haverah Park shower array is described. Particular attention is paid to the reality of some giant showers
A new array for the study of ultra high energy gamma-ray sources
The design and operation of a 32 x 1 10 to the 15th power sq m array of scintillation detectors for the detection of 10 to the 15th power eV cosmic rays is described with an expected angular resolution of 1 deg, thus improving the present signal/background ratio for gamma ray sources. Data are recorded on a hybrid CAMAC, an in-house system which uses a laser and Pockel-Cell arrangement to routinely calibrate the timing stability of the detectors
Young starless cores embedded in the magnetically dominated Pipe Nebula
The Pipe Nebula is a massive, nearby dark molecular cloud with a low
star-formation efficiency which makes it a good laboratory to study the very
early stages of the star formation process. The Pipe Nebula is largely
filamentary, and appears to be threaded by a uniform magnetic field at scales
of few parsecs, perpendicular to its main axis. The field is only locally
perturbed in a few regions, such as the only active cluster forming core B59.
The aim of this study is to investigate primordial conditions in low-mass
pre-stellar cores and how they relate to the local magnetic field in the cloud.
We used the IRAM 30-m telescope to carry out a continuum and molecular survey
at 3 and 1 mm of early- and late-time molecules toward four selected starless
cores inside the Pipe Nebula. We found that the dust continuum emission maps
trace better the densest regions than previous 2MASS extinction maps, while
2MASS extinction maps trace better the diffuse gas. The properties of the cores
derived from dust emission show average radii of ~0.09 pc, densities of
~1.3x10^5 cm^-3, and core masses of ~2.5 M_sun. Our results confirm that the
Pipe Nebula starless cores studied are in a very early evolutionary stage, and
present a very young chemistry with different properties that allow us to
propose an evolutionary sequence. All of the cores present early-time molecular
emission, with CS detections toward all the sample. Two of them, Cores 40 and
109, present strong late-time molecular emission. There seems to be a
correlation between the chemical evolutionary stage of the cores and the local
magnetic properties that suggests that the evolution of the cores is ruled by a
local competition between the magnetic energy and other mechanisms, such as
turbulence.Comment: Accepted for publication in ApJ. 15 pages, 5 figures, 9 table
Diffractive Contribution to the Elasticity and the Nucleonic Flux in the Atmosphere
We calculate the average elasticity considering non-diffractive and single
diffractive interactions and perform an analysis of the cosmic-ray flux by
means of an analytical solution for the nucleonic diffusion equation. We show
that the diffractive contribution is important for the adequate description of
the nucleonic and hadronic fluxes in the atmosphere.Comment: 10 pages, latex, 2 figures (uuencoded PostScript
Detailed Interstellar Polarimetric Properties of the Pipe Nebula at Core Scales
We use R-band CCD linear polarimetry collected for about 12000 background
field stars in 46 fields of view toward the Pipe nebula to investigate the
properties of the polarization across this dark cloud. Based on archival 2MASS
data we estimate that the surveyed areas present total visual extinctions in
the range 0.6 < Av < 4.6. While the observed polarizations show a well ordered
large scale pattern, with polarization vectors almost perpendicularly aligned
to the cloud's long axis, at core scales one see details that are
characteristics of each core. Although many observed stars present degree of
polarization which are unusual for the common interstellar medium, our analysis
suggests that the dust grains constituting the diffuse parts of the Pipe nebula
seem to have the same properties as the normal Galactic interstellar medium.
Estimates of the second-order structure function of the polarization angles
suggest that most of the Pipe nebula is magnetically dominated and that
turbulence is sub-Alvenic. The Pipe nebula is certainly an interesting region
where to investigate the processes prevailing during the initial phases of low
mass stellar formation.Comment: 20 pages, 23 figures, Accepted for The Astrophysical Journa
Measurement of the cosmic ray hadron spectrum up to 30 TeV at mountain altitude: the primary proton spectrum
The flux of cosmic ray hadrons at the atmospheric depth of 820 g/cm^2 has
been measured by means of the EAS-TOP hadron calorimeter (Campo Imperatore,
National Gran Sasso Laboratories, 2005 m a.s.l.). The hadron spectrum is well
described by a single power law : S(E_h) = (2.25 +- 0.21 +- 0.34(sys))
10^(-7)(E_h/1000)^(-2.79 +- 0.05) m^(-2) s^(-1) sr^(-1) GeV^(-1) over the
energy range 30 GeV-30 TeV. The procedure and the accuracy of the measurement
are discussed. The primary proton spectrum is derived from the data by using
the CORSIKA/QGSJET code to compute the local hadron flux as a function of the
primary proton spectrum and to calculate and subtract the heavy nuclei
contribution (basing on direct measurements). Over a wide energy range E_0 =
0.5-50 TeV its best fit is given by a single power law : S(E_0) = (9.8 +- 1.1
+- 1.6(sys)) 10^(-5) (E_0/1000)^(-2.80 +- 0.06) m^(-2) s^(-1) sr^(-1) GeV^(-1).
The validity of the CORSIKA/QGSJET code for such application has been checked
using the EAS-TOP and KASCADE experimental data by reproducing the ratio of the
measured hadron fluxes at the two experimental depths (820 and 1030 g/cm^2
respectively) at better than 10% in the considered energy range.Comment: 16 pages, 9 figures, accepted for publication in Astroparticle
Physic
Direct measurement of antiferromagnetic domain fluctuations
Measurements of magnetic noise emanating from ferromagnets due to domain
motion were first carried out nearly 100 years ago and have underpinned much
science and technology. Antiferromagnets, which carry no net external magnetic
dipole moment, yet have a periodic arrangement of the electron spins extending
over macroscopic distances, should also display magnetic noise, but this must
be sampled at spatial wavelengths of order several interatomic spacings, rather
than the macroscopic scales characteristic of ferromagnets. Here we present the
first direct measurement of the fluctuations in the nanometre-scale spin-
(charge-) density wave superstructure associated with antiferromagnetism in
elemental Chromium. The technique used is X-ray Photon Correlation
Spectroscopy, where coherent x-ray diffraction produces a speckle pattern that
serves as a "fingerprint" of a particular magnetic domain configuration. The
temporal evolution of the patterns corresponds to domain walls advancing and
retreating over micron distances. While the domain wall motion is thermally
activated at temperatures above 100K, it is not so at lower temperatures, and
indeed has a rate which saturates at a finite value - consistent with quantum
fluctuations - on cooling below 40K. Our work is important because it provides
an important new measurement tool for antiferromagnetic domain engineering as
well as revealing a fundamental new fact about spin dynamics in the simplest
antiferromagnet.Comment: 19 pages, 4 figure
Retrieval and Validation of Carbon Dioxide, Methane, and Water Vapor for the Canary Islands IR-Laser Occultation Experiment
The first ground-based experiment to prove the concept of a novel space-based observation technique for microwave and infrared-laser occultation between low-Earthorbit satellites was performed in the Canary Islands between La Palma and Tenerife. For two nights from 21 to 22 July 2011 the experiment delivered the infrared-laser differential transmission principle for the measurement of greenhouse gases (GHGs) in the free atmosphere. Such global and long-term stable measurements of GHGs, accompanied also by measurements of thermodynamic parameters and line-of-sight wind in a self-calibrating way, have become very important for climate change monitoring. The experiment delivered promising initial data for demonstrating the new observation concept by retrieving volume mixing ratios of GHGs along a ~ 144 km signal path at altitudes of ~ 2.4 km. Here, we present a detailed analysis of the measurements, following a recent publication that introduced the experiment\u27s technical setup and first results for an example retrieval of CO2. We present the observational and validation data sets, the latter simultaneously measured at the transmitter and receiver sites; the measurement data handling; and the differential transmission retrieval procedure. We also determine the individual and combined uncertainties influencing the results and present the retrieval results for 12CO2, 13CO2, C18OO, H2O and CH4. The new method is found to have a reliable basis for monitoring of greenhouse gases such as CO2, CH4, and H2O in the free atmosphere
Phylogenetic Relationships in Pterodroma Petrels Are Obscured by Recent Secondary Contact and Hybridization
The classification of petrels (Pterodroma spp.) from Round
Island, near Mauritius in the Indian Ocean, has confounded researchers since
their discovery in 1948. In this study we investigate the relationships between
Round Island petrels and their closest relatives using evidence from
mitochondrial DNA sequence data and ectoparasites. Far from providing clear
delimitation of species boundaries, our results reveal that hybridization among
species on Round Island has led to genetic leakage between populations from
different ocean basins. The most common species on the island,
Pterodroma arminjoniana, appears to be hybridizing with two
rarer species (P. heraldica and P. neglecta),
subverting the reproductive isolation of all three and allowing gene flow.
P. heraldica and P. neglecta breed
sympatrically in the Pacific Ocean, where P. arminjoniana is
absent, but no record of hybridization between these two exists and they remain
phenotypically distinct. The breakdown of species boundaries in Round Island
petrels followed environmental change (deforestation and changes in species
composition due to hunting) within their overlapping ranges. Such multi-species
interactions have implications not only for conservation, but also for our
understanding of the processes of evolutionary diversification and
speciation
Optical polarimetry toward the Pipe nebula: Revealing the importance of the magnetic field
Magnetic fields are proposed to play an important role in the formation and
support of self-gravitating clouds and the formation and evolution of
protostars in such clouds. We use R-band linear polarimetry collected for about
12000 stars in 46 fields with lines of sight toward the Pipe nebula to
investigate the properties of the polarization across this dark cloud complex.
Mean polarization vectors show that the magnetic field is locally perpendicular
to the large filamentary structure of the Pipe nebula (the `stem'), indicating
that the global collapse may have been driven by ambipolar diffusion. The
polarization properties clearly change along the Pipe nebula. The northwestern
end of the nebula (B59 region) is found to have a low degree of polarization
and high dispersion in polarization position angle, while at the other extreme
of the cloud (the `bowl') we found mean degrees of polarization as high as
15% and a low dispersion in polarization position angle. The plane of
the sky magnetic field strength was estimated to vary from about 17 G in
the B59 region to about 65 G in the bowl. We propose that three distinct
regions exist, which may be related to different evolutionary states of the
cloud; this idea is supported by both the polarization properties across the
Pipe and the estimated mass-to-flux ratio that varies between approximately
super-critical toward the B59 region and sub-critical inside the bowl. The
three regions that we identify are: the B59 region, which is currently forming
stars; the stem, which appears to be at an earlier stage of star formation
where material has been through a collapsing phase but not yet given birth to
stars; and the bowl, which represents the earliest stage of the cloud in which
the collapsing phase and cloud fragmentation has already started.Comment: 4 pages, 2 figure
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