302 research outputs found
The radio emission from the Galaxy at 22 MHz
We present maps of the 22MHz radio emission between declinations -28d and
+80d, covering ~73% of the sky, derived from observations with the 22MHz
radiotelescope at the Dominion Radio Astrophysical Observatory (DRAO). The
resolution of the telescopt (EWxNS) is 1.1d x 1.7d secant(zenith angle). The
maps show the large scale features of the emission from the Galaxy including
the thick non-thermal disk, the North Polar Spur (NPS) and absorption due to
discrete HII regions and to an extended band of thermal electrons within 40d of
the Galactic centre. We give the flux densities of nine extended supernova
remnants shown on the maps
Robot life: simulation and participation in the study of evolution and social behavior.
This paper explores the case of using robots to simulate evolution, in particular the case of Hamilton's Law. The uses of robots raises several questions that this paper seeks to address. The first concerns the role of the robots in biological research: do they simulate something (life, evolution, sociality) or do they participate in something? The second question concerns the physicality of the robots: what difference does embodiment make to the role of the robot in these experiments. Thirdly, how do life, embodiment and social behavior relate in contemporary biology and why is it possible for robots to illuminate this relation? These questions are provoked by a strange similarity that has not been noted before: between the problem of simulation in philosophy of science, and Deleuze's reading of Plato on the relationship of ideas, copies and simulacra
Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2-216
We present 1420 MHz polarization images of a 2.5 X 2.5 degree region around
the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian
Galactic Plane Survey (CGPS). An arc of low polarized intensity appears
prominently in the north-east portion of the visible disk of Sh 2-216,
coincident with the optically identified interaction region between the PN and
the interstellar medium (ISM). The arc contains structural variations down to
the ~1 arcminute resolution limit in both polarized intensity and polarization
angle. Several polarization-angle "knots" appear along the arc. By comparison
of the polarization angles at the centers of the knots and the mean
polarization angle outside Sh 2-216, we estimate the rotation measure (RM)
through the knots to be -43 +/- 10 rad/m^2. Using this estimate for the RM and
an estimate of the electron density in the shell of Sh 2-216, we derive a
line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 microG.
We believe it more likely the observed magnetic field is interstellar than
stellar, though we cannot completely dismiss the latter possibility. We
interpret our observations via a simple model which describes the ISM magnetic
field around Sh 2-216, and comment on the potential use of old PNe as probes of
the magnetized ISM.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
A molecular shell with star formation toward the supernova remnant G349.7+0.2
A field of ~38'x38' around the supernova remnant (SNR) G349.7+0.2 has been
surveyed in the CO J=1-0 transition with the 12 Meter Telescope of the NRAO,
using the On-The-Fly technique. The resolution of the observations is 54". We
have found that this remnant is interacting with a small CO cloud which, in
turn, is part of a much larger molecular complex, which we call the ``Large CO
Shell''. The Large CO Shell has a diameter of about 100 pc, an H_2 mass of
930,000 solar masses, and a density of 35 cm-3. We investigate the origin of
this structure and suggest that an old supernova explosion ocurred about 4
million years ago, as a suitable hypothesis. Analyzing the interaction between
G349.7+0.2 and the Large CO Shell, it is possible to determine that the shock
front currently driven into the molecular gas is a non-dissociative shock
(C-type), in agreement with the presence of OH 1720 MHz masers. The positional
and kinematical coincidence among one of the CO clouds that constitute the
Large CO Shell, an IRAS point-like source and an ultracompact H II region,
indicate the presence of a recently formed star. We suggest that the formation
of this star was triggered during the expansion of the Large CO Shell, and
suggest the possibility that the same expansion also created the progenitor
star of G349.7+0.2. The Large CO Shell would then be one of the few
observational examples of supernova-induced star formation.Comment: accepted in Astronomical Journal, corrected typo in the abstract (in
first line, 38' instead of 38"
DA495 - an aging pulsar wind nebula
We present a radio continuum study of the pulsar wind nebula (PWN) DA 495
(G65.7+1.2), including images of total intensity and linear polarization from
408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations
with the Effelsberg 100-m Radio Telescope. Removal of flux density
contributions from a superimposed \ion{H}{2} region and from compact
extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz,
with a spectral index below the break and
above it (). The
spectral break is more than three times lower in frequency than the lowest
break detected in any other PWN. The break in the spectrum is likely the result
of synchrotron cooling, and DA 495, at an age of 20,000 yr, may have
evolved from an object similar to the Vela X nebula, with a similarly energetic
pulsar. We find a magnetic field of 1.3 mG inside the nebula. After
correcting for the resulting high internal rotation measure, the magnetic field
structure is quite simple, resembling the inner part of a dipole field
projected onto the plane of the sky, although a toroidal component is likely
also present. The dipole field axis, which should be parallel to the spin axis
of the putative pulsar, lies at an angle of {\sim}50\degr east of the North
Celestial Pole and is pointing away from us towards the south-west. The upper
limit for the radio surface brightness of any shell-type supernova remnant
emission around DA 495 is OAWatt
m Hz sr (assuming a radio spectral index of ), lower than the faintest shell-type remnant known to date.Comment: 25 pages, accepted by Ap
Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy
We present radio continuum and polarization images of the North Polar Spur
(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the
Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization
angle versus wavelength squared over 2048 frequency channels from 1280 to 1750
MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM
map with a published Faraday depth map of the entire Galaxy in this direction,
we derive the Faraday depth introduced by the NPS and the Galactic interstellar
medium (ISM) in front of and behind the NPS. The Faraday depth contributed by
the NPS is close to zero, indicating that the NPS is an emitting only feature.
The Faraday depth caused by the ISM in front of the NPS is consistent with zero
at b>50 degree, implying that this part of the NPS is local at a distance of
approximately several hundred parsecs. The Faraday depth contributed by the ISM
behind the NPS gradually increases with Galactic latitude up to b=44 degree,
and decreases at higher Galactic latitudes. This implies that either the part
of the NPS at b<44 degree is distant or the NPS is local but there is a sign
change of the large-scale magnetic field. If the NPS is local, there is then no
evidence for a large-scale anti-symmetry pattern in the Faraday depth of the
Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes
b>50 degree can be explained by including a coherent vertical magnetic field.Comment: 9 pages, 8 figures, accepted for publication in ApJ. Some figures
have been degraded to reduce sizes, for a high resolution version, see
http://physics.usyd.edu.au/~xhsun/ms_nps.pd
Origin of TeV Galactic Cosmic Rays
We consider a possibility of identification of sources of cosmic rays (CR) of
the energy above 1 TeV via observation of degree-scale extended gamma-ray
emission which traces the locations of recent sources in the Galaxy. Such
emission in the energy band above 100 GeV is produced by CR nuclei and
electrons released by the sources and spreading into the interstellar medium.
We use the data from the Fermi gamma-ray telescope to locate the degree-scale
100 GeV gamma-ray sources. We find that the number of such sources and their
overall power match to those expected when CRs injection events happen every
~100 yr in portions of ~1e50 erg. We find that most of the sources are
associated to pulsars with spin down age less than ~30 kyr and hence to the
recent supernova explosions. This supports the hypothesis of supernova origin
of Galactic CRs. We notice that the degree-scale extended emission does not
surround shell-like supernova remnants without pulsars. Based on this
observation, we argue that the presence of the pulsar is essential for the CR
acceleration process. We expect that a significant fraction of the degree-scale
sources should be detectable as extended sources with km3-scale neutrino
detectors.Comment: 14 pages, 14 figures, accepted for publication in Phys.Rev.
The Cygnus X region XXII. A probable HAeBe star with a giant bipolar outflow in DR 16
From medium-resolution radio images, DR 16 was suspected to be a large
cometary nebula. To test this suggestion we obtained a higher resolution
(15\arcsec) VLA continuum map. We also analyzed data from the Canadian Galactic
Plane Survey in continuum, H I line, and IR. These data were supplemented by
published near-infrared (J, H, K) stellar photometric results and MSX 8.28
micrometer data. We suggest that DR 16 is the diffuse H II region of an ongoing
star formation site at a distance of about 3 kpc. The complicated radio picture
arises from the superposition of diffuse H II with the remains of a giant
bipolar outflow. The outflow was generated by a probable Herbig AeBe star, and
the lobes are the remnants of its working surfaces. Additional ring-like
features are discussed. DR 16 is part of a larger volume of space in the local
spiral arm where star formation is an ongoing process.Comment: 15 pages, 14 figures, accepted by A&
A Highly Ordered Faraday-Rotation Structure in the Interstellar Medium
We describe a Faraday-rotation structure in the Interstellar Medium detected
through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane
Survey (CGPS). The structure, at l=91.8, b=-2.5, has an extent of ~2 degree,
within which polarization angle varies smoothly over a range of ~100 degree.
Polarized intensity also varies smoothly, showing a central peak within an
outer shell. This region is in sharp contrast to its surroundings, where
low-level chaotic polarization structure occurs on arcminute scales. The
Faraday-rotation structure has no counterpart in radio total intensity, and is
unrelated to known objects along the line of sight, which include a Lynds
Bright Nebula, LBN 416, and the star cluster M39 (NGC7092). It is interpreted
as a smooth enhancement of electron density. The absence of a counterpart,
either in optical emission or in total intensity, establishes a lower limit to
its distance. An upper limit is determined by the strong beam depolarization in
this direction. At a probable distance of 350 +/- 50 pc, the size of the object
is 10 pc, the enhancement of electron density is 1.7 cm-3, and the mass of
ionized gas is 23 M_sun. It has a very smooth internal magnetic field of
strength 3 microG, slightly enhanced above the ambient field. G91.8-2.5 is the
second such object to be discovered in the CGPS, and it seems likely that such
structures are common in the Magneto-Ionic Medium.Comment: 16 pages, 5 figures, ApJ accepte
In search of lost time: age and the promise of induced pluripotent stem cell models of the brain
This paper explores the promise of induced pluripotent stem cells as a model system for the study of neurodegenerative diseases of Alzheimerâs, Parkinsonâs and other diseases of the aging brain. Research in these areas, as in neuroscience more broadly, has struggled with the imperfect mapping between human and animal brains. The paper argues that the contemporary promise of induced pluripotent stem cells for research is established through their potential to resolve problems of translation, bridging laboratory and clinical contexts by acting as a model of ârealâ patient bodies. However, the paper shows how this promise is contested and renewed through a rearticulation of the relationship between neurodegeneration, aging and the qualities of âyoungâ and âagedâ bodies. This not only results in the introduction of new qualities and attributes to the model system, but also a re-imagining of how aging features within both late and early-onset neurological diseases.The research was funded through the NIHR Cambridge Biomedical Research Centre, a partnership between the University of Cambridge and Cambridge University Hospitals NHS Foundation Trust
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