278 research outputs found
Rico: An Accurate Cosmological Recombination Code
We present Rico, a code designed to compute the ionization fraction of the
Universe during the epoch of hydrogen and helium recombination with an
unprecedented combination of speed and accuracy. This is accomplished by
training the machine learning code Pico on the calculations of a multi-level
cosmological recombination code which self-consistently includes several
physical processes that were neglected previously. After training, Rico is used
to fit the free electron fraction as a function of the cosmological parameters.
While, for example at low redshifts (z<~900), much of the net change in the
ionization fraction can be captured by lowering the hydrogen fudge factor in
Recfast by about 3%, Rico provides a means of effectively using the accurate
ionization history of the full recombination code in the standard cosmological
parameter estimation framework without the need to add new or refined fudge
factors or functions to a simple recombination model. Within the new approach
presented here it is easy to update Rico whenever a more accurate full
recombination code becomes available. Once trained, Rico computes the
cosmological ionization history with negligible fitting error in ~10
milliseconds, a speed-up of at least 10^6 over the full recombination code that
was used here. Also Rico is able to reproduce the ionization history of the
full code to a level well below 0.1%, thereby ensuring that the theoretical
power spectra of CMB fluctuations can be computed to sufficient accuracy and
speed for analysis from upcoming CMB experiments like Planck. Furthermore it
will enable cross-checking different recombination codes across cosmological
parameter space, a comparison that will be very important in order to assure
the accurate interpretation of future cosmic microwave background data.Comment: 14 pages, 11 figures, submitted to PR
Variability of the NGC 1333 IRAS 4A Outflow: Molecular Hydrogen and Silicon Monoxide Images
The NGC 1333 region was observed in the H2 1-0 S(1) line. The H2 images cover
a 5' x 7' region around IRAS 4. Numerous H2 emission features were detected.
The northeast-southwest bipolar outflow driven by IRAS 4A was studied by
combining the H2 images with SiO maps published previously. The SiO-H2 outflows
are continuous on the southwestern side but show a gap on the northeastern
side. The southwestern outflow lobe curves smoothly, and the position angle
increases with the distance from the driving source. The base and the outer tip
of the northeastern outflow lobe are located at positions opposite to the
corresponding parts of the southwestern lobe. This point-symmetry suggests that
the outflow axis may be drifting or precessing clockwise in the plane of the
sky and that the cause of the axis drift may be intrinsic to the outflow
engine. The axis drift model is supported by the asymmetric lateral intensity
profile of the SiO outflow. The axis drift rate is about 0.011 deg yr-1. The
middle part of the northeastern outflow does not exactly follow the point
symmetry because of the superposition of two different kinds of directional
variability: the axis drift of the driving source and the deflection by a dense
core. The axis drift model provides a good explanation for the large deflection
angle of the northeastern outflow. Other H2 emission features around the IRAS 4
region are discussed briefly. Some of them are newly found outflows, and some
are associated with outflows already known before
VLBI imaging of a flare in the Crab Nebula: More than just a spot
We report on very long baseline interferometry (VLBI) observations of the
radio emission from the inner region of the Crab Nebula, made at 1.6 GHz and 5
GHz after a recent high-energy flare in this object. The 5 GHz data have
provided only upper limits of 0.4 milli-Jansky (mJy) on the flux density of the
pulsar and 0.4 mJy/beam on the brightness of the putative flaring region. The
1.6 GHz data have enabled imaging the inner regions of the nebula on scales of
up to ~40". The emission from the inner "wisps" is detected for the first time
with VLBI observations. A likely radio counterpart (designated "C1") of the
putative flaring region observed with Chandra and HST is detected in the radio
image, with an estimated flux density of \,mJy and a size of
0.2-0.6". Another compact feature ("C2") is also detected in the VLBI image
closer to the pulsar, with an estimated flux density of 0.4 +- 0.2 mJy and a
size smaller than 0{\farcs}2. Combined with the broad-band SED of the flare,
the radio properties of C1 yield a lower limit of ~0.5 mG for the magnetic
field and a total minimum energy of 1.2*10^41 ergs vested in the flare
(corresponding to using about 0.2% of the pulsar spin-down power). The 1.6 GHz
observations provide upper limits for the brightness (0.2 mJy/beam) and total
flux density (0.4 mJy) of the optical Knot 1 located at 0.6" from the pulsar.
The absolute position of the Crab pulsar is determined, and an estimate of the
pulsar proper motion is obtained.Comment: Astronomy & Astrophysics; accepted; 10 pages, 8 figure
One-dimensional collision carts computer model and its design ideas for productive experiential learning
We develop an Easy Java Simulation (EJS) model for students to experience the
physics of idealized one-dimensional collision carts. The physics model is
described and simulated by both continuous dynamics and discrete transition
during collision. In the field of designing computer simulations, we discuss
briefly three pedagogical considerations such as 1) consistent simulation world
view with pen paper representation, 2) data table, scientific graphs and
symbolic mathematical representations for ease of data collection and multiple
representational visualizations and 3) game for simple concept testing that can
further support learning. We also suggest using physical world setup to be
augmented complimentary with simulation while highlighting three advantages of
real collision carts equipment like tacit 3D experience, random errors in
measurement and conceptual significance of conservation of momentum applied to
just before and after collision. General feedback from the students has been
relatively positive, and we hope teachers will find the simulation useful in
their own classes. 2015 Resources added:
http://iwant2study.org/ospsg/index.php/interactive-resources/physics/02-newtonian-mechanics/02-dynamics/46-one-dimension-collision-js-model
http://iwant2study.org/ospsg/index.php/interactive-resources/physics/02-newtonian-mechanics/02-dynamics/195-elastic-collisionComment: 6 pages, 8 figures, 1 table, 1 L. K. Wee, Physics Education 47 (3),
301 (2012); ISSN 0031-912
The Propagation of Magneto-Centrifugally Launched Jets: I
We present simulations of the propagation of magnetized jets. This work
differs from previous studies in that the cross-sectional distributions of the
jets's state variables are derived from analytical models for
magneto-centrifugal launching. The source is a magnetized rotator whose
properties are specfied as boundary conditions. The jets in these simulations
are considerably more complex than the ``top-hat''constant density etc.
profiles used in previous work. We find that density and magnetic field
stratification (with radius) in the jet leads to new behavior including the
separation of an inner jet core from a low density collar. We find this {\it
jet within a jet} structure, along with the magnetic stresses, leads to
propagation behaviors not observed in previous simulation studies. Our
methodology allows us to compare MHD jets from different types of sources whose
properties could ultimately be derived from the behavior of the propagating
jets.Comment: 42 pages, accepted by the Ap
Can Protostellar Jets Drive Supersonic Turbulence in Molecular Clouds?
Jets and outflows from young stellar objects are proposed candidates to drive
supersonic turbulence in molecular clouds. Here, we present the results from
multi-dimensional jet simulations where we investigate in detail the energy and
momentum deposition from jets into their surrounding environment and quantify
the character of the excited turbulence with velocity probability density
functions. Our study include jet--clump interaction, transient jets, and
magnetised jets. We find that collimated supersonic jets do not excite
supersonic motions far from the vicinity of the jet. Supersonic fluctuations
are damped quickly and do not spread into the parent cloud. Instead subsonic,
non-compressional modes occupy most of the excited volume. This is a generic
feature which can not be fully circumvented by overdense jets or magnetic
fields. Nevertheless, jets are able to leave strong imprints in their cloud
structure and can disrupt dense clumps. Our results question the ability of
collimated jets to sustain supersonic turbulence in molecular clouds.Comment: 33 pages, 18 figures, accepted by ApJ, version with high resolution
figures at:
http://www.ita.uni-heidelberg.de/~banerjee/publications/jet_paper.pd
Assessing the levels of awareness among european citizens about the direct and indirect impacts of plastics on human health
There is an urgent need to assess European citizens' perspective regarding their plastic consumption and to evaluate their awareness of the direct and indirect effect of plastics on human health in order to influence current behavior trends. In this study, the evaluation has been cross-related with scientific facts, with the final aim of detecting the most recommendable paths in increasing human awareness, reducing plastic consumption, and consequently impacting human health. A statistical analysis of quantitative data, gathered from 1000 European citizens via an online survey in the period from May to June 2020, showed that a general awareness about the direct impact of plastic consumption and contamination (waste) on human health is high in Europe. However, only a few participants (from a higher educational group) were aware of the indirect negative effects that oil extraction and industrial production of plastic can have on human health. This finding calls for improved availability of this information to general public. Despite the participants' positive attitude toward active plastic reduction (61%), plastic consumption on a daily basis is still very high (86%). The most common current actions toward plastic reduction are plastic bag usage, reusage, or replacement with sustainable alternatives (e.g., textile bags) and selecting products with less plastic packaging. The participants showed important criticism toward the information available to the general public about plastics and health. This awareness is important since significant relation has been found between the available information and the participants' decisions on the actions they might undertake to reduce plastic consumption. The study clearly showed the willingness of the participants to take action, but they also requested to be strongly supported with joint efforts from government, policies, and marketing, defining it as the most successful way toward implementing these changes
Warping and Precession of Accretion Disks Around Magnetic Stars: Nonlinear Evolution
The inner region of the accretion disk around a magnetized star (T Tauri
star, white dwarf or neutron star) is subjected to magnetic torques that induce
warping and precession of the disk. These torques arise from the interaction
between the stellar field and the induced electric currents in the disk. We
carry out numerical simulations of the nonlinear evolution of warped, viscous
accretion disks driven by the magnetic torques. We show that the disk can
develop into a highly warped steady state in which the disk attains a fixed
(warped) shape and precesses rigidly. The warp is most pronounced at the disk
inner radius (near the magnetosphere boundary). As the system parameters (such
as accretion rate) change, the disk can switch between a completely flat state
(warping stable) and a highly warped state. The precession of warped disks may
be responsible for a variety of quasi-periodic oscillations or radiation flux
variabilities observed in many different systems, including young stellar
objects and X-ray binaries.Comment: 16 pages, 7 figures; extended parameter searches, changes in
discussion; accepted for publication in Ap
An effective mass theorem for the bidimensional electron gas in a strong magnetic field
We study the limiting behavior of a singularly perturbed
Schr\"odinger-Poisson system describing a 3-dimensional electron gas strongly
confined in the vicinity of a plane and subject to a strong uniform
magnetic field in the plane of the gas. The coupled effects of the confinement
and of the magnetic field induce fast oscillations in time that need to be
averaged out. We obtain at the limit a system of 2-dimensional Schr\"odinger
equations in the plane , coupled through an effective selfconsistent
electrical potential. In the direction perpendicular to the magnetic field, the
electron mass is modified by the field, as the result of an averaging of the
cyclotron motion. The main tools of the analysis are the adaptation of the
second order long-time averaging theory of ODEs to our PDEs context, and the
use of a Sobolev scale adapted to the confinement operator
A measurement of secondary cosmic microwave background anisotropies with two years of South Pole Telescope observations
We present the first three-frequency South Pole Telescope (SPT) cosmic
microwave background (CMB) power spectra. The band powers presented here cover
angular scales 2000 < ell < 9400 in frequency bands centered at 95, 150, and
220 GHz. At these frequencies and angular scales, a combination of the primary
CMB anisotropy, thermal and kinetic Sunyaev-Zel'dovich (SZ) effects, radio
galaxies, and cosmic infrared background (CIB) contributes to the signal. We
combine Planck and SPT data at 220 GHz to constrain the amplitude and shape of
the CIB power spectrum and find strong evidence for non-linear clustering. We
explore the SZ results using a variety of cosmological models for the CMB and
CIB anisotropies and find them to be robust with one exception: allowing for
spatial correlations between the thermal SZ effect and CIB significantly
degrades the SZ constraints. Neglecting this potential correlation, we find the
thermal SZ power at 150 GHz and ell = 3000 to be 3.65 +/- 0.69 muK^2, and set
an upper limit on the kinetic SZ power to be less than 2.8 muK^2 at 95%
confidence. When a correlation between the thermal SZ and CIB is allowed, we
constrain a linear combination of thermal and kinetic SZ power: D_{3000}^{tSZ}
+ 0.5 D_{3000}^{kSZ} = 4.60 +/- 0.63 muK^2, consistent with earlier
measurements. We use the measured thermal SZ power and an analytic, thermal SZ
model calibrated with simulations to determine sigma8 = 0.807 +/- 0.016.
Modeling uncertainties involving the astrophysics of the intracluster medium
rather than the statistical uncertainty in the measured band powers are the
dominant source of uncertainty on sigma8 . We also place an upper limit on the
kinetic SZ power produced by patchy reionization; a companion paper uses these
limits to constrain the reionization history of the Universe.Comment: 25 pages; 14 figures; Submitted to ApJ (Updated to reflect referee
comments
- âŠ