1,157 research outputs found
Canonical Transformation Path to Gauge Theories of Gravity
In this paper, the generic part of the gauge theory of gravity is derived,
based merely on the action principle and on the general principle of
relativity. We apply the canonical transformation framework to formulate
geometrodynamics as a gauge theory. The starting point of our paper is
constituted by the general De~Donder-Weyl Hamiltonian of a system of scalar and
vector fields, which is supposed to be form-invariant under (global) Lorentz
transformations. Following the reasoning of gauge theories, the corresponding
locally form-invariant system is worked out by means of canonical
transformations. The canonical transformation approach ensures by construction
that the form of the action functional is maintained. We thus encounter amended
Hamiltonian systems which are form-invariant under arbitrary spacetime
transformations. This amended system complies with the general principle of
relativity and describes both, the dynamics of the given physical system's
fields and their coupling to those quantities which describe the dynamics of
the spacetime geometry. In this way, it is unambiguously determined how spin-0
and spin-1 fields couple to the dynamics of spacetime.
A term that describes the dynamics of the free gauge fields must finally be
added to the amended Hamiltonian, as common to all gauge theories, to allow for
a dynamic spacetime geometry. The choice of this "dynamics Hamiltonian" is
outside of the scope of gauge theory as presented in this paper. It accounts
for the remaining indefiniteness of any gauge theory of gravity and must be
chosen "by hand" on the basis of physical reasoning. The final Hamiltonian of
the gauge theory of gravity is shown to be at least quadratic in the conjugate
momenta of the gauge fields -- this is beyond the Einstein-Hilbert theory of
General Relativity.Comment: 16 page
Engaging Parents and Caregivers in Substance Use Disorder Prevention and Recovery
Fred Muench the President of the national organization, Partnership for Drug-Free Kids and Center on Addiction, will present on family-based interventions to engage parents and caregivers, as well as data on the resources the Partnership for Drug-Free Kids offers for families addressing every aspect of substance use and addiction, from prevention to recovery. Learn about the support and guidance offered through the latest science-based information designed to empower parents and caregivers. Fred will also discuss how the merger with the Center on Addiction now provides the research necessary to advance effective prevention and treatment strategies advocating for lifesaving policy changes with state and local partners. Through these initiatives the Partnership and Center on Addiction aspire to change the national conversation around addiction so that no one will feel alone or ashamed to seek help, and everyone can have access to the care they need and deserve.
Learning Objectives: Become familiar with the resources for Substance Use Disorder NLM and partner organizations offer such as MedlinePlus, Drug Information Portal and Pillbox. Learn the latest research on parenting and caregiver interventions for substance use prevention and intervention. Learn about how the Partnership is infusing, best-in-class science with compassion from prevention to sustained recovery, to support families caring for a child or loved one struggling with substance use. Learn about the work the Partnership is doing with health care systems to provide effective addiction treatment, research and practices that work, and promote accessibility and affordability of care for everyone
The spatial distribution of substellar objects in IC348 and the Orion Trapezium Cluster
Aims: Some theoretical scenarios suggest the formation of brown dwarfs as
ejected stellar embryos in star-forming clusters. Such a formation mechanism
can result in different spatial distributions of stars and substellar objects.
We aim to investigate the spatial structure of stellar and substellar objects
in two well sampled and nearby embedded clusters, namely IC348 and the Orion
Trapezium Cluster (OTC) to test this hypothesis. Methods:Deep near-infrared
K-band data complete enough to sample the substellar population in IC348 and
OTC are obtained from the literature. The spatial distribution of the K-band
point sources is analysed using the Minimum Spanning Tree (MST) method. The Q
parameter and the spanning trees are evaluated for stellar and substellar
objects as a function of cluster core radius R. Results: The stellar
population in both IC348 and OTC display a clustered distribution whereas the
substellar population is distributed homogeneously in space within twice the
cluster core radius. Although the substellar objects do not appear to be bound
by the cluster potential well, they are still within the limits of the cluster
and not significantly displaced from their birth sites. Conclusions: The
spatially homogeneous distribution of substellar objects is best explained by
assuming higher initial velocities, distributed in a random manner and going
through multiple interactions. The overall spatial coincidence of these objects
with the cluster locations can be understood if these objects are nevertheless
travelling slowly enough so as to feel the gravitational effect of the cluster.
The observations support the formation of substellar objects as ``ejected
stellar embryos''. Higher ejection velocities are necessary but net spatial
displacements may not be necessary to explain the observational data.Comment: 4 pages. Accepted by A&A Letter
The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure
In this paper we present the results of a systematic investigation of an
entire population of starless dust cores within a single molecular cloud.
Analysis of extinction data shows the cores to be dense objects characterized
by a narrow range of density. Analysis of C18O and NH3 molecular-line
observations reveals very narrow lines. The non-thermal velocity dispersions
measured in both these tracers are found to be subsonic for the large majority
of the cores and show no correlation with core mass (or size). Thermal pressure
is thus the dominate source of internal gas pressure and support for most of
the core population. The total internal gas pressures of the cores are found to
be roughly independent of core mass over the entire range of the core mass
function (CMF) indicating that the cores are in pressure equilibrium with an
external source of pressure. This external pressure is most likely provided by
the weight of the surrounding Pipe cloud within which the cores are embedded.
Most of the cores appear to be pressure confined, gravitationally unbound
entities whose nature, structure and future evolution are determined by only a
few physical factors which include self-gravity, the fundamental processes of
thermal physics and the simple requirement of pressure equilibrium with the
surrounding environment. The observed core properties likely constitute the
initial conditions for star formation in dense gas. The entire core population
is found to be characterized by a single critical Bonnor-Ebert mass. This mass
coincides with the characteristic mass of the Pipe CMF indicating that most
cores formed in the cloud are near critical stability. This suggests that the
mass function of cores (and the IMF) has its origin in the physical process of
thermal fragmentation in a pressurized medium.Comment: To appear in the Astrophysical Journa
Young Brown Dwarfs in the Core of the W3 Main Star-Forming Region
We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR
observations with the Subaru telescope, to search for very low mass young
stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey
covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20
mag in the JHK bands. The survey is sensitive enough to provide unprecedented
details in W3 IRS 5 region and reveals a census of the stellar population down
to objects below the hydrogen-burning limit. We construct JHK color-color (CC)
and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low
luminosity YSOs and to estimate their masses. Based on these CC and CM
diagrams, we identified a rich population of embedded YSO candidates with
infrared excesses (Class I and Class II), associated with the W3 Main region. A
large number of red sources (H-K > 2) have also been detected around W3 Main.
We argue that these red stars are most probably pre-main-sequence (PMS) stars
with intrinsic color excesses. Based on the comparison between theoretical
evolutionary models of very low-mass PMS objects with the observed CM diagram,
we find there exists a substantial substellar population in the observed
region. The mass function (MF) does not show the presence of cutoff and sharp
turnover around the substellar limit, at least at the hydrogen-burning limit.
Furthermore, the MF slope indicates that the number ratio of young brown dwarfs
and hydrogen-burning stars in the W3 Main is probably higher than those in
Trapezium and IC 348. The presence of mass segregation, in the sense that
relatively massive YSOs lie near the cluster center, is seen. The estimated
dynamical evolution time indicates that the observed mass segregation in the W3
Main may be the imprint of the star formation process.Comment: 39 pages, 15 figures. Accepted for publication in the Astrophysical
Journa
Phase-Field Simulation and Design of a Ferroelectric Nano-Generator
We study the behavior of ferroelectric material (BaTiO3) for the design of a nano-generator to convert mechanical
into electrical energy. The investigations consider an electro-mechanical phase-field model with polarization as
state variable. This widely accepted model has its origins in the work of and is fully developed by Landis and
coworkers. We use a finite element model to simulate tetragonal regions of ferroelectric material sputtered on
substrate. Different geometries as well as various mechanical and electrical boundary conditions are considered.
The model parameters are normalized to achieve better computational conditions within the stiffness matrix.
The major objective of this contribution is the fundamental understanding of domain switching caused by a
cyclic electrical field. The corresponding hysteresis loops of the overall polarization cannot be achieved by using
a two-dimensional model because the domain topologies evolve in three dimensions. The three-dimensional
nature of the domain structure evolution is even true for flat regions or thin films. We show some examples of
three-dimensional domain topologies, which are able to break energetically unfavorable symmetries. Finally, the
computational model of a tetragonal nano-generator with dimensions 10 x 60 x 10 nm is presented. The specific
ratio of height to width and the mounting on substrate is essential for its performance and principle of energy
harvesting. We discuss the challenges and scopes of such a system.Aerospace Engineerin
The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit
We present a new census of the Orion Nebula Cluster (ONC) over a large field
of view (>30'x30'), significantly increasing the known population of stellar
and substellar cluster members with precisely determined properties. We develop
and exploit a technique to determine stellar effective temperatures from
optical colors, nearly doubling the previously available number of objects with
effective temperature determinations in this benchmark cluster. Our technique
utilizes colors from deep photometry in the I-band and in two medium-band
filters at lambda~753 and 770nm, which accurately measure the depth of a
molecular feature present in the spectra of cool stars. From these colors we
can derive effective temperatures with a precision corresponding to better than
one-half spectral subtype, and importantly this precision is independent of the
extinction to the individual stars. Also, because this technique utilizes only
photometry redward of 750nm, the results are only mildly sensitive to optical
veiling produced by accretion. Completing our census with previously available
data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign
masses and ages down to 0.02 solar masses. At faint luminosities, we detect a
large population of background sources which is easily separated in our
photometry from the bona fide cluster members. The resulting initial mass
function of the cluster has good completeness well into the substellar mass
range, and we find that it declines steeply with decreasing mass. This suggests
a deficiency of newly formed brown dwarfs in the cluster compared to the
Galactic disk population.Comment: 16 pages, 18 figures. Accepted for publication in The Astrophysical
Journa
The nature of the dense core population in the Pipe Nebula: A survey of NH3, CCS, and HC5N molecular line emission
Recent extinction studies of the Pipe Nebula (d=130 pc) reveal many cores
spanning a range in mass from 0.2 to 20.4 Msun. These dense cores were
identified via their high extinction and comprise a starless population in a
very early stage of development. Here we present a survey of NH3 (1,1), NH3
(2,2), CCS (2_1,1_0), and HC5N (9,8) emission toward 46 of these cores. An
atlas of the 2MASS extinction maps is also presented. In total, we detect 63%
of the cores in NH3 (1,1) 22% in NH3 (2,2), 28% in CCS, and 9% in HC5N
emission. We find the cores are associated with dense gas (~10^4 cm-3) with 9.5
< T_k < 17 K. Compared to C18O, we find the NH3 linewidths are systematically
narrower, implying that the NH3 is tracing the dense component of the gas and
that these cores are relatively quiescent. We find no correlation between core
linewidth and size. The derived properties of the Pipe cores are similar to
cores within other low-mass star-forming regions: the only differences are that
the Pipe cores have weaker NH3 emision and most show no current star formation
as evidenced by the lack of embedded infrared sources. Such weak NH3 emission
could arise due to low column densities and abundances or reduced excitation
due to relatively low core volume densities. Either alternative implies that
the cores are relatively young. Thus, the Pipe cores represent an excellent
sample of dense cores in which to study the initial conditions for star
formation and the earliest stages of core formation and evolution.Comment: 35 pages, 10 figures (excluding the appendix). For the complete
appendix contact [email protected]. Accepted for publication in ApJ
The nature of the dense core population in the pipe nebula: core and cloud kinematics from C18O observations
We present molecular-line observations of 94 dark cloud cores identified in
the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio
Observatory 12m telescope, we obtained spectra of these cores in the J=1-0
transition of C18O. We use the measured core parameters, i.e., antenna
temperature, linewidth, radial velocity, radius and mass, to explore the
internal kinematics of these cores as well as their radial motions through the
larger molecular cloud. We find that the vast majority of the dark extinction
cores are true cloud cores rather than the superposition of unrelated
filaments. While we identify no significant correlations between the core's
internal gas motions and the cores' other physical parameters, we identify
spatially correlated radial velocity variations that outline two main kinematic
components of the cloud. The largest is a 15pc long filament that is
surprisingly narrow both in spatial dimensions and in radial velocity.
Beginning in the Stem of the Pipe, this filament displays uniformly small C18O
linewidths (dv~0.4kms-1) as well as core to core motions only slightly in
excess of the gas sound speed. The second component outlines what appears to be
part of a large (2pc; 1000 solar mass) ring-like structure. Cores associated
with this component display both larger linewidths and core to core motions
than in the main cloud. The Pipe Molecular Ring may represent a primordial
structure related to the formation of this cloud.Comment: Accepted to ApJ. 14 pages, 11 figures. Complete table at end of
documen
The mass function of dense molecular cores and the origin of the IMF
Context: Stars form in the cold dense cores of interstellar molecular clouds
and the detailed knowledge of the spectrum of masses of such cores is clearly a
key for the understanding of the origin of the IMF. To date, observations have
presented somewhat contradictory evidence relating to this issue. Aims: In this
paper we propose to derive the mass function of a complete sample of dense
molecular cores in a single cloud employing a robust method that uses uses
extinction of background starlight to measure core masses and enables the
reliable extension of such measurements to lower masses than previously
possible. Methods: We use a map of near-infrared extinction in the nearby Pipe
dark cloud to identify the population of dense cores in the cloud and measure
their masses. Results: We identify 159 dense cores and construct the mass
function for this population. We present the first robust evidence for a
departure from a single power-law form in the mass function of a population of
cores and find that this mass function is surprisingly similar in shape to the
stellar IMF but scaled to a higher mass by a factor of about 3. This suggests
that the distribution of stellar birth masses (IMF) is the direct product of
the dense core mass function and a uniform star formation efficiency of
30%+/-10%, and that the stellar IMF may already be fixed during or before the
earliest stages of core evolution. These results are consistent with previous
dust continuum studies which suggested that the IMF directly originates from
the core mass function. The typical density of ~10^4/cm^3 measured for the
dense cores in this cloud suggests that the mass scale that characterizes the
dense core mass function may be the result of a simple process of thermal
(Jeans) fragmentation.Comment: A&A accepte
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