993 research outputs found
Estimation of time delay by coherence analysis
Using coherence analysis (which is an extensively used method to study the
correlations in frequency domain, between two simultaneously measured signals)
we estimate the time delay between two signals. This method is suitable for
time delay estimation of narrow band coherence signals for which the
conventional methods cannot be reliably applied. We show by analysing coupled
R\"ossler attractors with a known delay, that the method yields satisfactory
results. Then, we apply this method to human pathologic tremor. The delay
between simultaneously measured traces of Electroencephalogram (EEG) and
Electromyogram (EMG) data of subjects with essential hand tremor is calculated.
We find that there is a delay of 11-27 milli-seconds () between the tremor
correlated parts (cortex) of the brain (EEG) and the trembling hand (EMG) which
is in agreement with the experimentally observed delay value of 15 for the
cortico-muscular conduction time. By surrogate analysis we calculate error-bars
of the estimated delay.Comment: 21 pages, 8 figures, elstart.cls file included. Accepted for
publication in Physica
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
1
Dynamic depletion in a Bose condensate via a sudden increase of the scattering length
We examine the time-dependent quantum depletion of a trapped Bose condensate
arising from a rapid increase of the scattering length. Our solution indicates
that a significant buildup of incoherent atoms can occur within a
characteristic time short compared with the harmonic trap period. We discuss
how the depletion density and the characteristic time depend on the physical
parameters of the condensate
Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: I. VLA Observations
We present a high sensitivity radio continuum survey at 6 and 1.3cm using
the Karl G. Jansky Very Large Array towards a sample of 58 high-mass star
forming regions. Our sample was chosen from dust clumps within infrared dark
clouds with and without IR sources (CMC-IRs, CMCs, respectively), and hot
molecular cores (HMCs), with no previous, or relatively weak radio continuum
detection at the mJy level. Due to the improvement in the continuum
sensitivity of the VLA, this survey achieved map rms levels of 3-10
Jy beam at sub-arcsecond angular resolution. We extracted 70
centimeter continuum sources associated with 1.2mm dust clumps. Most
sources are weak, compact, and are prime candidates for high-mass protostars.
Detection rates of radio sources associated with the mm dust clumps for CMCs,
CMC-IRs and HMCs are 6, 53 and 100, respectively. This result is
consistent with increasing high-mass star formation activity from CMCs to HMCs.
The radio sources located within HMCs and CMC-IRs occur close to the dust clump
centers with a median offset from it of 12,000AU and 4,000AU,
respectively. We calculated 5 - 25GHz spectral indices using power law fits
and obtain a median value of 0.5 (i.e., flux increasing with frequency),
suggestive of thermal emission from ionized jets. In this paper we describe the
sample, observations, and detections. The analysis and discussion will be
presented in Paper II.Comment: Accepted for publication in the ApJ
Stochastic differential equations for evolutionary dynamics with demographic noise and mutations
We present a general framework to describe the evolutionary dynamics of an
arbitrary number of types in finite populations based on stochastic
differential equations (SDE). For large, but finite populations this allows to
include demographic noise without requiring explicit simulations. Instead, the
population size only rescales the amplitude of the noise. Moreover, this
framework admits the inclusion of mutations between different types, provided
that mutation rates, , are not too small compared to the inverse
population size 1/N. This ensures that all types are almost always represented
in the population and that the occasional extinction of one type does not
result in an extended absence of that type. For this limits the use
of SDE's, but in this case there are well established alternative
approximations based on time scale separation. We illustrate our approach by a
Rock-Scissors-Paper game with mutations, where we demonstrate excellent
agreement with simulation based results for sufficiently large populations. In
the absence of mutations the excellent agreement extends to small population
sizes.Comment: 8 pages, 2 figures, accepted for publication in Physical Review
The extremely collimated bipolar H_2O jet from the NGC 1333-IRAS 4B protostar
We have performed observations of water maser emission towards a sample of
low-mass protostars, in order to investigate the properties of jets associated
with the earliest stages of star formation and their interaction with the
surrounding medium. The main aim is to measure the absolute positions and
proper motions of the H_2O spots in order to investigate the kinematics of the
region from where the jet is launched. We imaged the protostars in the nearby
region NGC 1333-IRAS 4 in the water maser line at 22.2 GHz by using the VLBA in
phase-reference mode at the milliarcsecond scale over four epochs, spaced by
one month to measure proper motions. Two protostars (A2 and B) were detected in
a highly variable H_2O maser emission, with an active phase shorter than four
weeks. The H_2O maps allow us to trace the fast jet driven by the B protostar:
we observed both the red- and blue-shifted lobes very close to the protostar,
=< 35 AU, moving away with projected velocities of ~10-50 km/s. The comparison
with the molecular outflow observed at larger scale suggests a jet precession
with a 18'/yr rate. By measuring the positional spread of the H_2O spots we
estimate a jet width of ~2 AU at a distance of ~12 AU from the driving
protostar.Comment: 9 pages, 8 figures, A&A accepte
A highly-collimated SiO jet in the HH212 protostellar outflow
We mapped the HH212 Class 0 outflow in SiO(2--1, 5--4) and continuum using
the PdBI in its extended configurations. The unprecedented angular resolution
(down to 0.34") allows accurate comparison with a new, deep H2 image obtained
at the VLT. The SiO emission is confined to a highly-collimated bipolar jet
(width 0.35") along the outflow axis. The jet can be traced down to within 500
AU of the protostar, in a region that is heavily obscured in H2 images. Where
both species are detected, SiO shows the same overall kinematics and structure
as H2, indicating that both molecules are tracing the same material. We find
that the high-velocity SiO gas near the protostar is not tracing a wide-angle
wind but is already confined to a flow inside a narrow cone of half-opening
angle < 6 deg.Comment: Astronomy and Astrophysics Letter, in pres
PdBI sub-arcsecond study of the SiO microjet in HH212 - Origin and collimation of Class 0 jets
The bipolar HH 212 outflow has been mapped in SiO using the extended
configuration of the Plateau de Bure Interferometer (PdBI), revealing a highly
collimated SiO jet closely associated with the H2 jet component. We study at
unprecedented resolution (0.34" across the jet axis) the properties of the
innermost SiO ``microjet'' within 1000 AU of this young Class 0 source, to
compare it with atomic microjets from more evolved sources and to constrain its
origin. The SiO channel maps are used to investigate the microjet collimation
and velocity structure. A large velocity gradient analysis is applied to SiO
(2-1), (5-4) and (8-7) data from the PdBI and the Submillimeter Array to
constrain the SiO opacity and abundance. The HH212 Class 0 microjet shows
striking similarities in collimation and energetic budget with atomic microjets
from T Tauri sources. Furthermore, the SiO lines appear optically thick, unlike
what is generally assumed. We infer T(kin) ~ 50-500 K and an SiO/H2 abundance
greater than 4 10(-8)-6 10(-5) for n(H2) = 10(7)-10(5) cm(-3), i.e. 0.05-90% of
the elemental silicon. This similar jet width, regardless of the presence of a
dense envelope, definitely rules out jet collimation by external pressure, and
favors a common MHD self-collimation (and possibly acceleration) process at all
stages of star formation. We propose that the more abundant SiO in Class 0 jets
could mainly result from rapid (less than 25 yrs) molecular synthesis at high
jet densities
Critical number of atoms in an attractive Bose-Einstein condensate on an optical plus harmonic traps
The stability of an attractive Bose-Einstein condensate on a joint
one-dimensional optical lattice and an axially-symmetric harmonic trap is
studied using the numerical solution of the time-dependent mean-field
Gross-Pitaevskii equation and the critical number of atoms for a stable
condensate is calculated. We also calculate this critical number of atoms in a
double-well potential which is always greater than that in an axially-symmetric
harmonic trap. The critical number of atoms in an optical trap can be made
smaller or larger than the corresponding number in the absence of the optical
trap by moving a node of the optical lattice potential along the axial
direction of the harmonic trap. This variation of the critical number of atoms
can be observed experimentally and compared with the present calculation.Comment: Latex with 7 eps figures, Accepted in Journal of Physics
VLA+PT Astrometry of 46 Radio Stars
We have used the Very Large Array (VLA), linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 46 radio stars in
the International Celestial Reference Frame (ICRF). Positions were obtained in
the ICRF directly through phase referencing of the stars to nearby ICRF quasars
whose positions are accurate at the 0.25 mas level. Radio star positions are
estimated to be accurate at the 10 mas level, with position errors approaching
a few milli-arcseconds for some of the stars observed. Our measured positions
were combined with previous measurements taken from as early as 1978 to obtain
proper motion estimates for all 46 stars with average uncertainties of ~1.7
mas/yr. We compared our radio star positions and proper motions with the
Hipparcos Catalogue data, and find consistency in the reference frames produced
by each data set on the 1-sigma level, with errors of ~2.7 mas per axis for the
reference frame orientation angles at our mean epoch of 2003.78. No significant
spin is found between our radio data frame and the Hipparcos Celestial
Reference Frame (HCRF) with largest rotation rates of +0.55 and -0.41 mas/yr
around the x and z axes, respectively, with 1-sigma errors of 0.36 mas/yr.
Thus, our results are consistent with a non-rotating Hipparcos frame with
respect to the ICRF.Comment: 29 pages, 8 figures, accepted for publication in the Astronomical
Journa
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