84,950 research outputs found
Star Formation in Transient Molecular Clouds
We present the results of a numerical simulation in which star formation
proceeds from an initially unbound molecular cloud core. The turbulent motions,
which dominate the dynamics, dissipate in shocks leaving a quiescent region
which becomes gravitationally bound and collapses to form a small multiple
system. Meanwhile, the bulk of the cloud escapes due to its initial supersonic
velocities. In this simulation, the process naturally results in a star
formation efficiency of 50%. The mass involved in star formation depends on the
gas fraction that dissipates sufficient kinetic energy in shocks. Thus, clouds
with larger turbulent motions will result in lower star formation efficiencies.
This implies that globally unbound, and therefore transient giant molecular
clouds (GMCs), can account for the low efficiency of star formation observed in
our Galaxy without recourse to magnetic fields or feedback processes.
Observations of the dynamic stability in molecular regions suggest that GMCs
may not be self-gravitating, supporting the ideas presented in this letter.Comment: 5 pages, 3 figures, accepted for MNRAS as a lette
Helmet weight simulator
A device for providing acceleration cues to the helmet of a simulator pilot is described. Pulleys are attached to both shoulders of the pilot. A cable is attached to both sides of the helmet and extends through the pulleys to a takeup reel that is controlled by a torque motor. Control signals are applied to a servo system including the torque motor, the takeup reel and a force transducer which supplies the feedback signal. In one embodiment of the invention the force transducer is in the cable and in another it is in the takeup reel
Native and Non-Native Speaker Judgements on the Quality of Synthesized Speech
The difference between native speakers' and non-native speak- ers' naturalness judgements of synthetic speech is investigated. Similar/difference judgements are analysed via a multidimen- sional scaling analysis and compared to Mean opinion scores. It is shown that although the two groups generally behave in a similar manner the variance of non-native speaker judgements is generally higher. While both groups of subject can clearly distinguish natural speech from the best synthetic examples, the groups' responses to different artefacts present in the synthetic speech can vary
Propagating plane harmonic waves through finite length plates of variable thickness using finite element techniques
An analysis is given using finite element techniques which addresses the propagaton of a uniform incident pressure wave through a finite diameter axisymmetric tapered plate immersed in a fluid. The approach utilized in developing a finite element solution to this problem is based upon a technique for axisymmetric fluid structure interaction problems. The problem addressed is that of a 10 inch diameter axisymmetric fixed plate totally immersed in a fluid. The plate increases in thickness from approximately 0.01 inches thick at the center to 0.421 inches thick at a radius of 5 inches. Against each face of the tapered plate a cylindrical fluid volume was represented extending five wavelengths off the plate in the axial direction. The outer boundary of the fluid and plate regions were represented as a rigid encasement cylinder as was nearly the case in the physical problem. The primary objective of the analysis is to determine the form of the transmitted pressure distribution on the downstream side of the plate
Beta lives - some statistical perspectives on the capital asset pricing model
This note summarizes some technical issues relevant to the use of the idea of excess return in empirical modelling. We cover the case where the aim is to construct a measure of expected return on an asset and a model of the CAPM type is used. We review some of the problems and show examples where the basic CAPM may be used to develop other results which relate the expected returns on assets both to the expected return on the market and other factors
RTP control protocol (RTCP) extended report (XR) block for independent reporting of burst/fgp discard metrics
This document defines an RTP Control Protocol (RTCP) Extended Report
(XR) block that allows the reporting of burst/gap discard metrics
independently of the burst/gap loss metrics for use in a range of RTP
applications
Diamond-Like Carbon film from Liquid Gas on Metallic Substrates
Liquid gas was used to produce DLC films on Cu, Al and steel substrates by DC
plasma technique. The absorption in IR reflectance indicates, grown films are
DLC. By deconvolution of room temperature UV-visible spectra of the films grown
at 50 mtorr and 200C, in addition to the spectra lines reported for
CL, PL, PLC and ESR, some new spectra lines were obtained. We also, have seen
exciton absorption line at room temperature.Comment: 6 pages, Postscript, 2 figure
Subglacial floods beneath ice sheets.
Subglacial floods (jökulhlaups) are well documented as occurring beneath present day glaciers and ice caps. In addition, it is known that massive floods have occurred from ice-dammed lakes proximal to the Laurentide ice sheet during the last ice age, and it has been suggested that at least one such flood below the waning ice sheet was responsible for a dramatic cooling event some 8000 years ago. We propose that drainage of lakes from beneath ice sheets will generally occur in a time-periodic fashion, and that such floods can be of severe magnitude. Such hydraulic eruptions are likely to have caused severe climatic disturbances in the past, and may well do so in the future
The star formation efficiency and its relation to variations in the initial mass function
We investigate how the dynamical state of a turbulently supported, 1000 solar
mass, molecular cloud affects the properties of the cluster it forms, focusing
our discussion on the star formation efficiency (SFE) and the initial mass
function (IMF). A variety of initial energy states are examined in this paper,
ranging from clouds with PE = 0.1 KE to clouds with PE = 10 KE, and for both
isothermal and piece-wise polytropic equations of state (similar to that
suggested by Larson). It is found that arbitrary star formation efficiencies
are possible, with strongly unbound clouds yielding very low star formation
efficiencies. We suggest that the low star formation efficiency in the
Maddelena cloud may be a consequence of the relatively unbound state of its
internal structure. It is also found that competitive accretion results in the
observed IMF when the clouds have initial energy states of PE >= KE. We show
that under such conditions the shape of the IMF is independent of time in the
calculations. This demonstrates that the global accretion process can be
terminated at any stage in the cluster's evolution, while still yielding a
distribution of stellar masses that is consistent with the observed IMF. As the
clouds become progressively more unbound, competitive accretion is less
important and the protostellar mass function flattens. These results predict
that molecular clouds should be permeated with a distributed population of
stars that follow a flatter than Salpeter IMF.Comment: 8 pages, 6 figures, accepted by MNRAS for publictaion. Now available
through the 'Online Early' schem
Ionisation-induced star formation II: External irradiation of a turbulent molecular cloud
In this paper, we examine numerically the difference between triggered and
revealed star formation. We present Smoothed Particle Hydrodynamics (SPH)
simulations of the impact on a turbulent 10^4 solar-mass molecular cloud of
irradiation by an external source of ionising photons. In particular, using a
control model, we investigate the triggering of star formation within the
cloud. We find that, although feedback has a dramatic effect on the morphology
of our model cloud, its impact on star formation is relatively minor. We show
that external irradiation has both positive and negative effects, accelerating
the formation of some objects, delaying the formation of others, and inducing
the formation of some that would not otherwise have formed. Overall, the
calculation in which feedback is included forms nearly twice as many objects
over a period of \sim0.5 freefall times (\sim2.4 Myr), resulting in a
star--formation efficiency approximately one third higher (\sim4% as opposed to
\sim3% at this epoch) as in the control run in which feedback is absent.
Unfortunately, there appear to be no observable characteristics which could be
used to differentiate objects whose formation was triggered from those which
were forming anyway and which were simply revealed by the effects of radiation,
although this could be an effect of poor statistics.Comment: 12 pages, 9 figures, accepted by MNRA
- …