46,197 research outputs found
Space infrared telescope pointing control system. Infrared telescope tracking in the presence of target motion
The use of charge-coupled-devices, or CCD's, has been documented by a number of sources as an effective means of providing a measurement of spacecraft attitude with respect to the stars. A method exists of defocussing and interpolation of the resulting shape of a star image over a small subsection of a large CCD array. This yields an increase in the accuracy of the device by better than an order of magnitude over the case when the star image is focussed upon a single CCD pixel. This research examines the effect that image motion has upon the overall precision of this star sensor when applied to an orbiting infrared observatory. While CCD's collect energy within the visible spectrum of light, the targets of scientific interest may well have no appreciable visible emissions. Image motion has the effect of smearing the image of the star in the direction of motion during a particular sampling interval. The presence of image motion is incorporated into a Kalman filter for the system, and it is shown that the addition of a gyro command term is adequate to compensate for the effect of image motion in the measurement. The updated gyro model is included in this analysis, but has natural frequencies faster than the projected star tracker sample rate for dim stars. The system state equations are reduced by modelling gyro drift as a white noise process. There exists a tradeoff in selected star tracker sample time between the CCD, which has improved noise characteristics as sample time increases, and the gyro, which will potentially drift further between long attitude updates. A sample time which minimizes pointing estimation error exists for the random drift gyro model as well as for a random walk gyro model
Surface waves in protoplanetary disks induced by outbursts: Concentric rings in scattered light
Context: Vertically hydrostatic protoplanetary disk models are based on the
assumption that the main heating source, stellar irradiation, does not vary
much with time. However, it is known that accreting young stars are variable
sources of radiation. This is particularly evident for outbursting sources such
as EX Lupi and FU Orionis stars. Aim: We investigate how such outbursts affect
the vertical structure of the outer regions of the protoplanetary disk, in
particular their appearance in scattered light at optical and near-infrared
wavelengths. Methods: We employ the 3D FARGOCA radiation-hydrodynamics code, in
polar coordinates, to compute the time-dependent behavior of the axisymmetric
disk structure. The outbursting inner disk region is not included explicitly.
Instead, its luminosity is added to the stellar luminosity and is thus included
in the irradiation of the outer disk regions. For time snapshots of interest we
insert the density structure into the RADMC-3D radiative transfer code and
compute the appearance of the disk at optical/near-infrared wavelengths.
Results: We find that, depending on the amplitude of the outbursts, the
vertical structure of the disk can become highly dynamic, featuring circular
surface waves of considerable amplitude. These "hills" and "valleys" on the
disk's surface show up in the scattered light images as bright and dark
concentric rings. Initially these rings are small and act as standing waves,
but they subsequently lead to outward propagating waves, like the waves
produced by a stone thrown into a pond. These waves continue long after the
actual outburst has died out. Conclusions: We propose that some of the
multi-ringed structures seen in optical/infrared images of several
protoplanetary disks may have their origin in outbursts that occurred decades
or centuries ago.Comment: Accepted for publication in A&A Letter
Magnification as a Tool in Weak Lensing
Weak lensing surveys exploit measurements of galaxy ellipticities. These
measurements are subject to errors which degrade the cosmological information
that can be extracted from the surveys. Here we propose a way of using the
galaxy data themselves to calibrate the measurement errors. In particular, the
cosmic shear field, which causes the galaxies to appear elliptical, also
changes their sizes and fluxes. Information about the sizes and fluxes of the
galaxies can be added to the shape information to obtain more robust
information about the cosmic shear field. The net result will be tighter
constraints on cosmological parameters such as those which describe dark
energy.Comment: 4 pages, 2 figure
Recommended from our members
Situating multimodal learning analytics
The digital age has introduced a host of new challenges and opportunities for the learning sciences community. These challenges and opportunities are particularly abundant in multimodal learning analytics (MMLA), a research methodology that aims to extend work from Educational Data Mining (EDM) and Learning Analytics (LA) to multimodal learning environments by treating multimodal data. Recognizing the short-term opportunities and longterm challenges will help develop proof cases and identify grand challenges that will help propel the field forward. To support the field's growth, we use this paper to describe several ways that MMLA can potentially advance learning sciences research and touch upon key challenges that researchers who utilize MMLA have encountered over the past few years
The cranking formula and the spurious behaviour of the mass parameters
We discuss some aspects of the approach of the mass parameters by means of
the simple cranking model. In particular, it is well known that the numerical
application of this formula is often subject to ambiguities or contradictions.
It is found that these problems are induced by the presence of two derivatives
in the formula. To overcome these problems, we state a useful ansatz and we
develop a number of simple arguments which tend to justify the removal of these
terms. As soon as this is done, the formula becomes simpler and easier to
interpret. In this respect, it is shown how the shell effects affect the mass
parameters. A number of numerical tests help us in our conclusions.Comment: version 3 corrigendum of the ansatz of section V, corrigendum of the
legend of Fig3. Submission = text file + 5 figure
Toward an accurate mass function for precision cosmology
Cosmological surveys aim to use the evolution of the abundance of galaxy
clusters to accurately constrain the cosmological model. In the context of
LCDM, we show that it is possible to achieve the required percent level
accuracy in the halo mass function with gravity-only cosmological simulations,
and we provide simulation start and run parameter guidelines for doing so. Some
previous works have had sufficient statistical precision, but lacked robust
verification of absolute accuracy. Convergence tests of the mass function with,
for example, simulation start redshift can exhibit false convergence of the
mass function due to counteracting errors, potentially misleading one to infer
overly optimistic estimations of simulation accuracy. Percent level accuracy is
possible if initial condition particle mapping uses second order Lagrangian
Perturbation Theory, and if the start epoch is between 10 and 50 expansion
factors before the epoch of halo formation of interest. The mass function for
halos with fewer than ~1000 particles is highly sensitive to simulation
parameters and start redshift, implying a practical minimum mass resolution
limit due to mass discreteness. The narrow range in converged start redshift
suggests that it is not presently possible for a single simulation to capture
accurately the cluster mass function while also starting early enough to model
accurately the numbers of reionisation era galaxies, whose baryon feedback
processes may affect later cluster properties. Ultimately, to fully exploit
current and future cosmological surveys will require accurate modeling of
baryon physics and observable properties, a formidable challenge for which
accurate gravity-only simulations are just an initial step.Comment: revised in response to referee suggestions, MNRAS accepte
Implications of the isotope effects on the magnetization, magnetic torque and susceptibility
We analyze the magnetization, magnetic torque and susceptibility data of
La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal
3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that
on the anisotropy. Invoking the generic behavior of the anisotropy the doping
dependence of the isotope effects on the critical properties, including Tc,
correlation lengths and magnetic penetration depths are traced back to a change
of the mobile carrier concentration.Comment: 5 pages, 3 figure
A consistent interpretation of the low temperature magneto-transport in graphite using the Slonczewski--Weiss--McClure 3D band structure calculations
Magnetotransport of natural graphite and highly oriented pyrolytic graphite
(HOPG) has been measured at mK temperatures. Quantum oscillations for both
electron and hole carriers are observed with orbital angular momentum quantum
number up to . A remarkable agreement is obtained when comparing
the data and the predictions of the Slonczewski--Weiss--McClure tight binding
model for massive fermions. No evidence for Dirac fermions is observed in the
transport data which is dominated by the crossing of the Landau bands at the
Fermi level, corresponding to , which occurs away from the point
where Dirac fermions are expected.Comment: 3 figure
Dynamical Mass Measurements of Contaminated Galaxy Clusters Using Machine Learning
We study dynamical mass measurements of galaxy clusters contaminated by
interlopers and show that a modern machine learning (ML) algorithm can predict
masses by better than a factor of two compared to a standard scaling relation
approach. We create two mock catalogs from Multidark's publicly available
-body MDPL1 simulation, one with perfect galaxy cluster membership
information and the other where a simple cylindrical cut around the cluster
center allows interlopers to contaminate the clusters. In the standard
approach, we use a power-law scaling relation to infer cluster mass from galaxy
line-of-sight (LOS) velocity dispersion. Assuming perfect membership knowledge,
this unrealistic case produces a wide fractional mass error distribution, with
a width of . Interlopers introduce additional
scatter, significantly widening the error distribution further
(). We employ the support distribution machine (SDM)
class of algorithms to learn from distributions of data to predict single
values. Applied to distributions of galaxy observables such as LOS velocity and
projected distance from the cluster center, SDM yields better than a
factor-of-two improvement () for the contaminated
case. Remarkably, SDM applied to contaminated clusters is better able to
recover masses than even the scaling relation approach applied to
uncontaminated clusters. We show that the SDM method more accurately reproduces
the cluster mass function, making it a valuable tool for employing cluster
observations to evaluate cosmological models.Comment: 18 pages, 12 figures, accepted for publication at Ap
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