868 research outputs found
Acceleration-Induced Nonlocality: Uniqueness of the Kernel
We consider the problem of uniqueness of the kernel in the nonlocal theory of
accelerated observers. In a recent work, we showed that the convolution kernel
is ruled out as it can lead to divergences for nonuniform accelerated motion.
Here we determine the general form of bounded continuous kernels and use
observational data regarding spin-rotation coupling to argue that the kinetic
kernel given by is the only physically acceptable
solution.Comment: LaTeX file, 2 figures, 14 page
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
A Constraint on brown dwarf formation via ejection: radial variation of the stellar and substellar mass function of the young open cluster IC2391
Using the Wide Field Imager (WFI) at the ESO 2.2m telescope at La Silla and
the CPAPIR camera at the CTIO 1.5m telescope at Cerro Tololo, we have performed
an extensive, multiband photometric survey of the open cluster IC2391 (D~146pc,
age~50Myr, solar metallicity). Here we present the results from our photometric
survey and from a spectroscopic follow-up of the central part of the survey.Comment: 4 pages, 3 figures, to appear in the proceedings of the Cool Stars 15
conferenc
Magnetic hardening in rapidly quenched Fe-Pr and Fe-Nd alloys
We report studies of high-field magnetization and thermomagnetic effects in rapidly quenched and heat treated alloys based on Fe-Pr and Fe-Nd. Coercivities up to ~40 kOe and large energy products result from the precipitation of a finely dispersed crystalline phase. Studies of varying the alloy composition and heat treatment are reported. Journal of Applied Physics is copyrighted by The American Institute of Physics
Discovery of superthermal hydroxyl (OH) in the HH211 outflow
We present a 5-37 micron infrared spectrum obtained with the Spitzer Space
Telescope toward the southeastern lobe of the young protostellar outflow HH211.
The spectrum shows an extraordinary sequence of OH emission lines arising in
highly excited rotational levels up to an energy E/k~28200K above the ground
level. This is, to our knowledge, by far the highest rotational excitation of
OH observed outside Earth. The spectrum also contains several pure rotational
transitions of H2O (v=0), H2 (v=0) S(0) to S(7), HD (v=0) R(3) to R(6), and
atomic fine-structure lines of [Fe II], [Si II], [Ne II], [S I], and [Cl I].
The origin of the highly excited OH emission is most likely the
photodissociation of H2O by the UV radiation generated in the terminal outflow
shock of HH211.Comment: ApJ Letters, in pres
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
A multiwavelength study of embedded clusters in W5-east, NGC 7538, S235, S252 and S254-S258
articleWe present Spitzer, near-IR (NIR) and millimetre observations of the massive star-forming regions W5-east, S235, S252, S254-S258 and NGC 7538. Spitzer data is combined with NIR observations to identify and classify the young population while 12CO and 13CO observations are used to examine the parental molecular cloud. We detect in total 3021 young stellar objects (YSOs). Of those, 539 are classified as Class I, and 1186 as Class II sources. YSOs are distributed in groups surrounded by a more scattered population. Class I sources are more hierarchically organized than Class II and associated with the most dense molecular material. We identify in total 41 embedded clusters containing between 52 and 73 per cent of the YSOs. Clusters are in general non-virialized, turbulent and have star formation efficiencies between 5 and 50 per cent. We compare the physical properties of embedded clusters harbouring massive stars (MEC) and low-mass embedded clusters (LEC) and find that both groups follow similar correlations where the MEC are an extrapolation of the LEC. The mean separation between MEC members is smaller compared to the cluster Jeans length than for LEC members. These results are in agreement with a scenario where stars are formed in hierarchically distributed dusty filaments where fragmentation is mainly driven by turbulence for the more massive clusters. We find several young OB-type stars having IR-excess emission which may be due to the presence of an accretion disc.This work is based in part on observations made with the Spitzer
Space Telescope, which is operated by the Jet Propulsion Laboratory,
Caltech, under a contract with NASA. Support for this work
was provided by NASA through a contract issued by JPL/Caltech.
We also thank NOAO for their student thesis support. The Five College
Radio Astronomy Observatory was supported by NSF grant
AST 0540852. CB is supported by an RCUK Fellowship at the University
of Exeter, UK. This work is based in part on the IRAC postBCD
processing software ‘IRACPROC’ developed by Mike Schuster,
Massimo Marengo and Brian Patten at the Smithsonian Astrophysical
Observatory. This research used the facilities of the Canadian
Astronomy Data Centre operated by the National Research Council
of Canada with the support of the Canadian Space Agency. This research
has made use of the NASA/ IPAC Infrared Science Archive,
which is operated by the Jet Propulsion Laboratory, California Institute
of Technology, under contract with the National Aeronautics
and Space Administration. We thank the Spanish MINECO
for funding support from grants CSD2009-00038, AYA2009-07304
and AYA2012-32032
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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