7,838 research outputs found
NGC 2362: a Template for Early Stellar Evolution
We present UBVRI photometry for the young open cluster NGC 2362. From
analysis of the appropriate color-color and color-magnitude diagrams we derive
the fundamental parameters of the NGC 2362 cluster to be: age = 5 (+1-2) Myr,
distance = 1480 pc, E(B-V)=0.10 mag. The cluster age was independently
determined for both high mass (2.1 - 36Msun) and low mass (0.7 - 1.2Msun) stars
with excellent agreement between the ages derived using post-main sequence and
pre-main sequence evolutionary tracks for the high and low mass stars
respectively. Analysis of this cluster's color-magnitude diagram reveals a well
defined pre-main sequence (covering DeltaV ~ 9 magnitudes in V and extending
from early A stars to near the hydrogen burning limit) which makes this cluster
an ideal laboratory for pre-main sequence evolution studies.Comment: 9 pages, 3 figures, to be published in ApJ
Global fixed point proof of time-dependent density-functional theory
We reformulate and generalize the uniqueness and existence proofs of
time-dependent density-functional theory. The central idea is to restate the
fundamental one-to-one correspondence between densities and potentials as a
global fixed point question for potentials on a given time-interval. We show
that the unique fixed point, i.e. the unique potential generating a given
density, is reached as the limiting point of an iterative procedure. The
one-to-one correspondence between densities and potentials is a straightforward
result provided that the response function of the divergence of the internal
forces is bounded. The existence, i.e. the v-representability of a density, can
be proven as well provided that the operator norms of the response functions of
the members of the iterative sequence of potentials have an upper bound. The
densities under consideration have second time-derivatives that are required to
satisfy a condition slightly weaker than being square-integrable. This approach
avoids the usual restrictions of Taylor-expandability in time of the uniqueness
theorem by Runge and Gross [Phys.Rev.Lett.52, 997 (1984)] and of the existence
theorem by van Leeuwen [Phys.Rev.Lett. 82, 3863 (1999)]. Owing to its
generality, the proof not only answers basic questions in density-functional
theory but also has potential implications in other fields of physics.Comment: 4 pages, 1 figur
Time correlations in a confined magnetized free-electron gas
The time-dependent pair correlation functions for a degenerate ideal quantum
gas of charged particles in a uniform magnetic field are studied on the basis
of equilibrium statistics. In particular, the influence of a flat hard wall on
the correlations is investigated, both for a perpendicular and a parallel
orientation of the wall with respect to the field. The coherent and incoherent
parts of the time-dependent structure function in position space are determined
from an expansion in terms of the eigenfunctions of the one-particle
Hamiltonian. For the bulk of the system, the intermediate scattering function
and the dynamical structure factor are derived by taking successive Fourier
transforms. In the vicinity of the wall the time-dependent coherent structure
function is found to decay faster than in the bulk. For coinciding positions
near the wall the form of the structure function turns out to be independent of
the orientation of the wall. Numerical results are shown to corroborate these
findings.Comment: 25 pages, 14 figures, to be published in Journal of Physics
Conserving approximations in time-dependent quantum transport: Initial correlations and memory effects
We study time-dependent quantum transport in a correlated model system by
means of time-propagation of the Kadanoff-Baym equations for the nonequilibrium
many-body Green function. We consider an initially contacted equilibrium system
of a correlated central region coupled to tight-binding leads. Subsequently a
time-dependent bias is switched on after which we follow in detail the
time-evolution of the system. Important features of the Kadanoff-Baym approach
are 1) the possibility of studying the ultrafast dynamics of transients and
other time-dependent regimes and 2) the inclusion of exchange and correlation
effects in a conserving approximation scheme. We find that initial correlation
and memory terms due to many-body interactions have a large effect on the
transient currents. Furthermore the value of the steady state current is found
to be strongly dependent on the approximation used to treat the electronic
interactions.Comment: 5 pages, 2 figure
Density-potential mappings in quantum dynamics
In a recent letter [Europhys. Lett. 95, 13001 (2011)] the question of whether
the density of a time-dependent quantum system determines its external
potential was reformulated as a fixed point problem. This idea was used to
generalize the existence and uniqueness theorems underlying time-dependent
density functional theory. In this work we extend this proof to allow for more
general norms and provide a numerical implementation of the fixed-point
iteration scheme. We focus on the one-dimensional case as it allows for a more
in-depth analysis using singular Sturm-Liouville theory and at the same time
provides an easy visualization of the numerical applications in space and time.
We give an explicit relation between the boundary conditions on the density and
the convergence properties of the fixed-point procedure via the spectral
properties of the associated Sturm-Liouville operator. We show precisely under
which conditions discrete and continuous spectra arise and give explicit
examples. These conditions are then used to show that in the most physically
relevant cases the fixed point procedure converges. This is further
demonstrated with an example.Comment: 20 pages, 8 figures, 3 table
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
Gaia on-board metrology: basic angle and best focus
The Gaia payload ensures maximum passive stability using a single material,
SiC, for most of its elements. Dedicated metrology instruments are, however,
required to carry out two functions: monitoring the basic angle and refocusing
the telescope. Two interferometers fed by the same laser are used to measure
the basic angle changes at the level of as (prad, micropixel), which is
the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors,
combined with an ad-hoc analysis of the scientific data are used to define and
reach the overall best-focus. In this contribution, the systems, data analysis,
procedures and performance achieved during commissioning are presentedComment: 18 pages, 14 figures. To appear in SPIE proceedings 9143-30. Space
Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wav
Extraction of Intrinsic Fluorescence from Single Fiber Fluorescence Measurements on a Turbid Medium: Experimental Validation
Abstract The detailed mechanisms associated with the influence of scattering and absorption properties on the fluorescence intensity sampled by a single optical fiber have recently been elucidated based on Monte Carlo simulated data. Here we develop an experimental single fiber fluorescence (SFF) spectroscopy setup and validate the Monte Carlo data and semi-empirical model equation that describes the SFF signal as a function of scattering. We present a calibration procedure that corrects the SFF signal for all system-related, wavelength dependent transmission efficiencies to yield an absolute value of intrinsic fluorescence. The validity of the Monte Carlo data and semi-empirical model is demonstrated using a set of fluorescent phantoms with varying concentrations of Intralipid to vary the scattering properties, yielding a wide range of reduced scattering coefficients (ÎŒâČs = 0â7 mm â1). We also introduce a small modification to the model to account for the case of ÎŒâČs = 0 mm â1 and show its relation to the experimental, simulated and theoretically calculated value of SFF intensity in the absence of scattering. Finally, we show that our method is also accurate in the presence of absorbers by performing measurements on phantoms containing red blood cells and correcting for their absorption properties
Biological modelling of the radiation dose escalation effect of regional hyperthermia in cervical cancer
Background Locoregional hyperthermia combined with radiotherapy significantly
improves locoregional control and overall survival for cervical tumors
compared to radiotherapy alone. In this study biological modelling is applied
to quantify the effect of radiosensitization for three cervical cancer
patients to evaluate the improvement in equivalent dose for the combination
treatment with radiotherapy and hyperthermia. Methods The Linear-Quadratic
(LQ) model extended with temperature-dependent LQ-parameters α and ÎČ was used
to model radiosensitization by hyperthermia and to calculate the conventional
radiation dose that is equivalent in biological effect to the combined
radiotherapy and hyperthermia treatment. External beam radiotherapy planning
was performed based on a prescription dose of 46Gy in 23 fractions of 2Gy.
Hyperthermia treatment using the AMC-4 system was simulated based on the
actual optimized system settings used during treatment. Results The simulated
hyperthermia treatments for the 3 patients yielded a T50 of 40.1 °C, 40.5 °C,
41.1 °C and a T90 of 39.2 °C, 39.7 °C, 40.4 °C, respectively. The combined
radiotherapy and hyperthermia treatment resulted in a D95 of 52.5Gy, 55.5Gy,
56.9Gy in the GTV, a dose escalation of 7.3â11.9Gy compared to radiotherapy
alone (D95â=â45.0â45.5Gy). Conclusions This study applied biological modelling
to evaluate radiosensitization by hyperthermia as a radiation-dose escalation
for cervical cancer patients. This model is very useful to compare the
effectiveness of different treatment schedules for combined radiotherapy and
hyperthermia treatments and to guide the design of clinical studies on dose
escalation using hyperthermia in a multi-modality setting
Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades
We show that the errors in the Hipparcos parallaxes towards the Pleiades and
the Hyades open clusters are spatially correlated over angular scales of 2 to 3
deg, with an amplitude of up to 2 mas. This correlation is stronger than
expected based on the analysis of the Hipparcos catalog. We predict the
parallaxes of individual cluster members, pi_pm, from their Hipparcos proper
motions, assuming that all cluster members have the same space velocity. We
compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are
significant spatial correlations in pi_Hip. We derive a distance modulus to the
Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This
value, agrees very well with the distance modulus of 5.60 +- 0.04 mag
determined using the main-sequence fitting technique, compared with the value
of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the
Pleiades members. We show that the difference between the main-sequence fitting
distance and the Hipparcos parallax distance can arise from spatially
correlated errors in the Hipparcos parallaxes of individual Pleiades members.
Although the Hipparcos parallax errors towards the Hyades are spatially
correlated in a manner similar to those of the Pleiades, the center of the
Hyades is located on a node of this spatial structure. Therefore, the parallax
errors cancel out when the average distance is estimated, leading to a mean
Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate
that these spatial correlations are also responsible for the discrepant
distances that are inferred using the mean Hipparcos parallaxes to some open
clusters. Finally, we note that our conclusions are based on a purely geometric
method and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for
publication in Ap
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