898 research outputs found
Role of land cover changes for atmospheric CO2 increase and climate change during the last 150 years
OH (1720 MHz) Masers: A Multiwavelength Study of the Interaction between the W51C Supernova Remnant and the W51B Star Forming Region
We present a comprehensive view of the W51B HII region complex and the W51C
supernova remnant (SNR) using new radio observations from the VLA, VLBA,
MERLIN, JCMT, and CSO along with archival data from Spitzer, ROSAT, ASCA, and
Chandra. Our VLA data include the first 400 cm (74 MHz) continuum image of W51
at high resolution (88 arcsec). The 400 cm image shows non-thermal emission
surrounding the G49.2-0.3 HII region, and a compact source of non-thermal
emission (W51B_NT) coincident with the previously-identified OH (1720 MHz)
maser spots, non-thermal 21 and 90 cm emission, and a hard X-ray source.
W51B_NT falls within the region of high likelihood for the position of TeV
gamma-ray emission. Using the VLBA three OH (1720 MHz) maser spots are detected
in the vicinity of W51B_NT with sizes of 60 to 300 AU and Zeeman effect
magnetic field strengths of 1.5 to 2.2 mG. The multiwavelength data demonstrate
that the northern end of the W51B HII region complex has been partly enveloped
by the advancing W51C SNR and this interaction explains the presence of W51B_NT
and the OH masers. This interaction also appears in the thermal molecular gas
which partially encircles W51B_NT and exhibits narrow pre-shock (DeltaV 5 km/s)
and broad post-shock (DeltaV 20 km/s) velocity components. RADEX radiative
transfer modeling of these two components yield physical conditions consistent
with the passage of a non-dissociative C-type shock. Confirmation of the
W51B/W51C interaction provides additional evidence in favor of this region
being one of the best candidates for hadronic particle acceleration known thus
far.Comment: Accepted to Ap
spectra in elementary cellular automata and fractal signals
We systematically compute the power spectra of the one-dimensional elementary
cellular automata introduced by Wolfram. On the one hand our analysis reveals
that one automaton displays spectra though considered as trivial, and on
the other hand that various automata classified as chaotic/complex display no
spectra. We model the results generalizing the recently investigated
Sierpinski signal to a class of fractal signals that are tailored to produce
spectra. From the widespread occurrence of (elementary) cellular
automata patterns in chemistry, physics and computer sciences, there are
various candidates to show spectra similar to our results.Comment: 4 pages (3 figs included
The Nature of the Nuclear H2O Masers of NGC 1068: Reverberation and Evidence for a Rotating Disk Geometry
We report new (1995) Very Large Array observations and (1984 - 1999)
Effelsberg 100m monitoring observations of the 22 GHz H2O maser spectrum of the
Seyfert 2 galaxy NGC 1068. The sensitive VLA observations provide a
registration of the 22 GHz continuum emission and the location of the maser
spots with an accuracy of ~ 5 mas. Within the monitoring data, we find evidence
that the nuclear masers vary coherently on time-scales of months to years, much
more rapidly than the dynamical time-scale. We argue that the nuclear masers
are responding in reverberation to a central power source, presumably the
central engine. Between October and November 1997, we detected a simultaneous
flare of the blue-shifted and red-shifted satellite maser lines. Reverberation
in a rotating disk naturally explains the simultaneous flaring. There is also
evidence that near-infrared emission from dust grains associated with the maser
disk also responds to the central engine. We present a model in which an X-ray
flare results in both the loss of maser signal in 1990 and the peak of the
near-infrared light curve in 1994. In support of a rotating disk geometry for
the nuclear masers, we find no evidence for centripetal accelerations of the
redshifted nuclear masers; the limits are +/- 0.006 km/s/year, implying that
the masers are located within 2 degrees of the kinematic line-of-nodes. We also
searched for high velocity maser emission like that observed in NGC 4258. In
both VLA and Effelsberg spectra, we detect no high velocity lines between +/-
350 km/s to +/- 850 km/s relative to systemic, arguing that masers only lie
outside a radius of ~ 0.6 pc (1.9 light years) from the central engine
(assuming a distance of 14.4 Mpc).Comment: 62 pages, 19 figure
Probing MHD Shocks with high-J CO observations: W28F
Context. Observing supernova remnants (SNRs) and modelling the shocks they
are associated with is the best way to quantify the energy SNRs re-distribute
back into the Interstellar Medium (ISM). Aims. We present comparisons of shock
models with CO observations in the F knot of the W28 supernova remnant. These
comparisons constitute a valuable tool to constrain both the shock
characteristics and pre-shock conditions. Methods. New CO observations from the
shocked regions with the APEX and SOFIA telescopes are presented and combined.
The integrated intensities are compared to the outputs of a grid of models,
which were combined from an MHD shock code that calculates the dynamical and
chemical structure of these regions, and a radiative transfer module based on
the 'large velocity gradient' (LVG) approximation. Results. We base our
modelling method on the higher J CO transitions, which unambiguously trace the
passage of a shock wave. We provide fits for the blue- and red-lobe components
of the observed shocks. We find that only stationary, C-type shock models can
reproduce the observed levels of CO emission. Our best models are found for a
pre-shock density of 104 cm-3, with the magnetic field strength varying between
45 and 100 {\mu}G, and a higher shock velocity for the so-called blue shock
(\sim25 km s-1) than for the red one (\sim20 km s-1). Our models also
satisfactorily account for the pure rotational H2 emission that is observed
with Spitzer.Comment: 8 pages, 6 figures, 1 table, accepted for A&A SOFIA/GREAT Special
Issu
Triggering up states in all-to-all coupled neurons
Slow-wave sleep in mammalians is characterized by a change of large-scale
cortical activity currently paraphrased as cortical Up/Down states. A recent
experiment demonstrated a bistable collective behaviour in ferret slices, with
the remarkable property that the Up states can be switched on and off with
pulses, or excitations, of same polarity; whereby the effect of the second
pulse significantly depends on the time interval between the pulses. Here we
present a simple time discrete model of a neural network that exhibits this
type of behaviour, as well as quantitatively reproduces the time-dependence
found in the experiments.Comment: epl Europhysics Letters, accepted (2010
Memory difference control of unknown unstable fixed points: Drifting parameter conditions and delayed measurement
Difference control schemes for controlling unstable fixed points become
important if the exact position of the fixed point is unavailable or moving due
to drifting parameters. We propose a memory difference control method for
stabilization of a priori unknown unstable fixed points by introducing a memory
term. If the amplitude of the control applied in the previous time step is
added to the present control signal, fixed points with arbitrary Lyapunov
numbers can be controlled. This method is also extended to compensate arbitrary
time steps of measurement delay. We show that our method stabilizes orbits of
the Chua circuit where ordinary difference control fails.Comment: 5 pages, 8 figures. See also chao-dyn/9810029 (Phys. Rev. E 70,
056225) and nlin.CD/0204031 (Phys. Rev. E 70, 046205
High resolution radio observations of the colliding-wind binary WR140
Milli-arcsecond resolution Very Long Baseline Array (VLBA) observations of
the archetype WR+O star colliding-wind binary (CWB) system WR140 are presented
for 23 epochs between orbital phases 0.74 and 0.97. At 8.4 GHz, the emission in
the wind-collision region (WCR) is clearly resolved as a bow-shaped arc that
rotates as the orbit progresses. We interpret this rotation as due to the O
star moving from SE to approximately E of the WR star, which leads to solutions
for the orbit inclination of 122+/-5 deg, the longitude of the ascending node
of 353+/-3 deg, and an orbit semi-major axis of 9.0+/-0.5 mas. The distance to
WR140 is determined to be 1.85+/-0.16 kpc, which requires the O star to be a
supergiant. The inclination implies the mass of the WR and O star to be 20+/-4
and 54+/-10 solar masses respectively. We determine a wind-momentum ratio of
0.22, with an expected half-opening angle for the WCR of 63 deg, consistent
with 65+/-10 deg derived from the VLBA observations. Total flux measurements
from Very Large Array (VLA) observations show the radio emission from WR140 is
very closely the same from one orbit to the next, pointing strongly toward
emission, absorption and cooling mechanism(s) that are controlled largely by
the orbital motion. The synchrotron spectra evolve dramatically through the
orbital phases observed, exhibiting both optically thin and optically thick
emission. We discuss a number of absorption and cooling mechanisms that may
determine the evolution of the synchrotron spectrum with orbital phase.Comment: Accepted by ApJ, to appear in v623, April 20, 2005. 14 pages, 13
figs, requires emulateapj.cls. A version with full resolution figs can be
obtained from http://www.drao.nrc.ca/~smd/preprint/wr140_data.pd
Early Universe Quantum Processes in BEC Collapse Experiments
We show that in the collapse of a Bose-Einstein condensate (BEC) {For an
excellent introduction to BEC theory, see C. Pethick and H. Smith,
Bose-Einstein condensation in dilute gases (Cambridge University Press,
Cambridge, England, 2002)} certain processes involved and mechanisms at work
share a common origin with corresponding quantum field processes in the early
universe such as particle creation, structure formation and spinodal
instability. Phenomena associated with the controlled BEC collapse observed in
the experiment of Donley et al E. Donley et. al., Nature 412, 295 (2001)(they
call it `Bose-Nova', see also J. Chin, J. Vogels and W. Ketterle, Phys. Rev.
Lett. 90, 160405 (2003)) such as the appearance of bursts and jets can be
explained as a consequence of the squeezing and amplification of quantum
fluctuations above the condensate by the dynamics of the condensate. Using the
physical insight gained in depicting these cosmological processes, our analysis
of the changing amplitude and particle contents of quantum excitations in these
BEC dynamics provides excellent quantitative fits with the experimental data on
the scaling behavior of the collapse time and the amount of particles emitted
in the jets. Because of the coherence properties of BEC and the high degree of
control and measurement precision in atomic and optical systems, we see great
potential in the design of tabletop experiments for testing out general ideas
and specific (quantum field) processes in the early universe, thus opening up
the possibility for implementing `laboratory cosmology'.Comment: 7 pages, 3 figures. Invited Talk presented at the Peyresq Meetings of
Gravitation and Cosmology, 200
Subarcsecond mid-infrared and radio observations of the W3 IRS5 protocluster
Observations at mid-infrared (4.8-17.65 um) and radio (0.7-1.3 cm)
wavelengths are used to constrain the structure of the high-mass star-forming
region W3 IRS5 on 0.1'' (200 AU) scales. Two bright mid-infrared sources are
detected, as well as diffuse emission. The bright sources have associated
compact radio emission and probably are young high-mass stars.
The measured sizes and estimated temperatures indicate that these sources
together can supply the observed far-infrared luminosity. However, an optically
thick radio source with a possible mid-infrared counterpart may also contribute
significant luminosity; if so, it must be extremely deeply embedded.
The infrared colour temperatures of 350-390 K and low radio brightness
suggest gravitational confinement of the H II regions and ongoing accretion at
a rate of a few 10^-8 M0/yr or more. Variations in the accretion rate would
explain the observed radio variability. The low estimated foreground extinction
suggests the existence of a cavity around the central stars, perhaps blown by
stellar winds.
At least three radio sources without mid-infrared counterparts appear to show
proper motions of ~100 km/s, and may be deeply embedded young runaway OB stars,
but more likely are clumps in the ambient material which are shock-ionized by
the OB star winds.Comment: Accepted by A&A; 14 pages, 7 b/w figure
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