38,171 research outputs found
Runaway and walkaway stars from the ONC with Gaia DR2
Theory predicts that we should find fast, ejected (runaway) stars of all
masses around dense, young star-forming regions. -body simulations show that
the number and distribution of these ejected stars could be used to constrain
the initial spatial and kinematic substructure of the regions. We search for
runaway and slower walkaway stars within 100 pc of the Orion Nebula Cluster
(ONC) using DR2 astrometry and photometry. We compare our findings to
predictions for the number and velocity distributions of runaway stars from
simulations that we run for 4 Myr with initial conditions tailored to the ONC.
In DR2, we find 31 runaway and 54 walkaway candidates based on proper
motion, but not all of these are viable candidates in three dimensions. About
40 per cent are missing radial velocities, but we can trace back 9 3D-runaways
and 24 3D-walkaways to the ONC, all of which are low/intermediate-mass (<8
M). Our simulations show that the number of runaways within 100 pc
decreases the older a region is (as they quickly travel beyond this boundary),
whereas the number of walkaways increases up to 3 Myr. We find fewer walkaways
in DR2 than the maximum suggested from our simulations, which may be due
to observational incompleteness. However, the number of DR2 runaways
agrees with the number from our simulations during an age of 1.3-2.4 Myr,
allowing us to confirm existing age estimates for the ONC (and potentially
other star-forming regions) using runaway stars.Comment: 19 pages, 7 figures, accepted for publication in MNRA
Photoionization models of the Eskimo nebula: evidence for a binary central star?
The ionizing star of the planetary nebula NGC 2392 is too cool to explain the
high excitation of the nebular shell, and an additional ionizing source is
necessary. We use photoionization modeling to estimate the temperature and
luminosity of the putative companion. Our results show it is likely to be a
very hot (Teff ~ 250kK), dense white dwarf. If the stars form a close binary,
they may merge within a Hubble time, possibly producing a Type Ia supernova.Comment: 2 pages, 1 figure, presented at the IAU Symposium 282 "From
Interacting Binaries to Exoplanets: Essential Modeling Tools", Tatransk\'a
Lomnica, Slovakia, 201
Wolf-Rayet Central Stars of Planetary Nebulae: Their Evolution and Properties
Over the past decade, the number of planetary nebula central stars (CSPN)
known to exhibit the Wolf-Rayet (WR) phenomenon has grown substantially. Many
of these discoveries have resulted from the Macquarie/AAO/Strasbourg Ha (MASH)
PN Survey. While WR CSPN constitute a relatively rare stellar type (<10% of
CS), there are indications that the proportion of PN harbouring them may
increase as spectroscopy of more central stars is carried out. In addition,
with new and better distances from the Ha surface brightness-radius
relationship of Frew (2008), we can attempt a dynamical age sequence which may
provide insight into the evolution of these stars.Comment: 4 pages, 2 figures. Presented at the Asymmetric Planetary Nebulae V
Conference in Bowness-on-Windermere, UK, June 201
Spatial differences between stars and brown dwarfs: a dynamical origin?
We use -body simulations to compare the evolution of spatial distributions
of stars and brown dwarfs in young star-forming regions. We use three different
diagnostics; the ratio of stars to brown dwarfs as a function of distance from
the region's centre, , the local surface density of
stars compared to brown dwarfs, , and we compare the global
spatial distributions using the method. From a suite of
twenty initially statistically identical simulations, 6/20 attain
, indicating that dynamical interactions could be responsible for
observed differences in the spatial distributions of stars and brown dwarfs in
star-forming regions. However, many simulations also display apparently
contradictory results - for example, in some cases the brown dwarfs have much
lower local densities than stars (), but their global
spatial distributions are indistinguishable () and the
relative proportion of stars and brown dwarfs remains constant across the
region (). Our results suggest that extreme caution
should be exercised when interpreting any observed difference in the spatial
distribution of stars and brown dwarfs, and that a much larger observational
sample of regions/clusters (with complete mass functions) is necessary to
investigate whether or not brown dwarfs form through similar mechanisms to
stars.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Acceleration of the universe, vacuum metamorphosis, and the large-time asymptotic form of the heat kernel
We investigate the possibility that the late acceleration observed in the
rate of expansion of the universe is due to vacuum quantum effects arising in
curved spacetime. The theoretical basis of the vacuum cold dark matter (VCDM),
or vacuum metamorphosis, cosmological model of Parker and Raval is revisited
and improved. We show, by means of a manifestly nonperturbative approach, how
the infrared behavior of the propagator (related to the large-time asymptotic
form of the heat kernel) of a free scalar field in curved spacetime causes the
vacuum expectation value of its energy-momentum tensor to exhibit a resonance
effect when the scalar curvature R of the spacetime reaches a particular value
related to the mass of the field. we show that the back reaction caused by this
resonance drives the universe through a transition to an accelerating expansion
phase, very much in the same way as originally proposed by Parker and Raval.
Our analysis includes higher derivatives that were neglected in the earlier
analysis, and takes into account the possible runaway solutions that can follow
from these higher-derivative terms. We find that the runaway solutions do not
occur if the universe was described by the usual classical FRW solution prior
to the growth of vacuum energy-density and negative pressure (i.e., vacuum
metamorphosis) that causes the transition to an accelerating expansion of the
universe in this theory.Comment: 33 pages, 3 figures. Submitted to Physical Review D15 (Dec 23, 2003).
v2: 1 reference added. No other change
Phase diagram and dependence of the critical temperature T_c on the pressure for Tl_{0.5}Pb_{0.5}Sr_2Ca_{1-x}Y_xCu_2)_7
Using a mean-field BCS-like approach on the bidimensional extended Hubbard
Hamiltonian we calculate the superconducting transition temperature Tc as a
function of the hole content nh, for the d-wave and extended-s wave gap
symmetries. To describe the pressure effect on Tc we assume it induces a change
in the magnitude V of the attractive superconductor potential. This assumption
yields an explanation for the intrinsic term, and together with the well known
change in nh, we set the critical temperature as Tc=Tc(nh(P),V(P)). With this
we obtain a general expansion of Tc in terms of the pressure P and the hole
content nh. We apply this expansion to the
Tl_{0.5}Pb_{0.5}Sr_2Ca_{1-x}Y_xCu_2)_7 system
Magnetic reconnection in flux-tubes undergoing spinning footpoint motions
Aims. Photospheric motions acting on the coronal magnetic field have the potential to build up huge amounts of magnetic energy. The energy may be released through magnetic reconnection, and so a detailed understanding of the 3D process is crucial if its implications for coronal heating are to be fully addressed. Methods. A 3D MHD experiment is described in which misaligned magnetic flux tubes are subjected to simple spinning boundary motions. Results. The resulting shear between adjacent flux systems generates a twisted central separator current sheet that extends vertically throughout the domain. Current density is amplified to a sufficient extent that reconnection begins, and occurs everywhere along the separator current sheet, while the separatrix current sheets that exist in the early stages of the experiment are found to be unimportant in the systems dynamical evolution. In 2D cross-sections, the reconnection process exhibits many similarities to the regime of flux pile-up reconnection
A Newtonian Model for the Quantum Gravitational Back-Reaction on Inflation
Quantum gravitational back-reaction offers a simultaneous explanation for why
the cosmological constant is so small and a natural model of inflation in which
scalars play no role. In this talk I review previous work and present a simple
model of the mechanism in which the induced stress tensor behaves like negative
vacuum energy with a density proportional to . The model also highlights the essential role of
causality in back-reaction.Comment: 4 pages, LaTeX 2.6, no figures. Talk given at the International
Meeting on Quantum Gravity and Spectral Geometry, Naples, Italy, July 2-7,
200
Short and user-friendly: The development and validation of the Mini-DBQ
The Driver Behavior Questionnaire (DBQ) is used to measure aberrant driver behavior by asking drivers how often they engage in various aberrant driver behaviors. Since the development of the original DBQ several modified versions have been developed. The difference between the various versions is that new items are added or existing items modified or excluded. However, despite the differences, all versions are relatively long and therefore time-consuming and tiring to answer, which might limit the usability of the instrument. The main purpose of the present study was to develop a mini DBQ version by reducing the 27-item original DBQ to the shortest possible DBQ version. A second aim was to explore the feasibility of a second-order structure within the data, which means that violations, errors and lapses factors load on a higher-order aberrant driver behavior factor. The presence of a second-order structure further indicates the validity of the DBQ and its theoretical structure. Confirmatory factor analysis (CFA) was used to test the fit (i.e., how well the models explain the data) of the original DBQ versus the fit of the shortest possible DBQ as well as the presence of a second-order structure for the DBQ. The results indicated a nine-item Mini-DBQ In addition, a second-order structure was established in the data. These findings indicate that the Mini-DBQ is a valid and useful short measure of aberrant driver behavior
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