210 research outputs found
The X-ray binary 2S0114+650=LSI+65 010:A slow pulsar or tidally-induced pulsations?
The X-ray source 2S0114+650=LSI+65 010 is a binary system containing a B-type
primary and a low mass companion believed to be a neutron star. The system has
three reported periodicities: the orbital period, P{orb}~11.6 d, X-ray flaring
with P{flare}~2.7 hr, and a "superorbital" X-ray periodicity P{super}~30.7 d.
The objective of this paper is to show that the puzzling periodicities in the
system may be explained in the context of scenarios in which tidal interactions
drive oscillations in the B-supergiant star. We calculate the solution of the
equations of motion for one layer of small surface elements distributed along
the equator of the star, as they respond to the forces due to gas pressure,
centrifugal, coriolis, viscous forces, and the gravitational forces of both
stars. This calculation provides variability timescales that can be compared
with the observations. In addition, we use observational data obtained at the
Observatorio Astron\'omico Nacional en San Pedro M\'artir (OAN/SPM) between
1993-2004 to determine which of the periodicities may be present in the optical
region. We suggest that the tidal oscillations lead to a structured stellar
wind which, when fed to the neutron star, produces the X-ray modulations. The
connection between the stellar oscillations and the modulation of the mass
ejection may lie in the shear energy dissipation generated by the tangential
motions that are produced by the tidal interaction, particularly in the tidal
bulge region. The tidal oscillation scenario weakens the case for 2S0114+650
containing a magnetar descendent.Comment: 12 pages, 14 figure
Eccentric binaries: Tidal flows and periastron events
A number of binary systems present evidence of enhanced activity around
periastron passage, suggesting a connection between tidal interactions and
these periastron effects. The aim of this investigation is to study the
time-dependent response of a star's surface as it is perturbed by a binary
companion. We derive expressions for the rate of dissipation, , of the
kinetic energy by the viscous flows driven by tidal interactions on the surface
layer. The method is tested by comparing the results from a grid of model
calculations with the analytical predictions of Hut (1981) and the
synchronization timescales of Zahn (1977, 2008). Our results for the orbital
cycle averaged energy dissipation on orbital separation are consistent with
those of Hut for model binaries with orbital separations at periastron >8
stellar radii. The model also reproduces the predicted pseudo-synchronization
angular velocity for moderate eccentricities and the same scaling of
synchronization timescales for circular orbits with separation as given by
Zahn. The computations gives the distribution of over the stellar
surface, and show that it is generally concentrated at the equatorial latitude,
with maxima generally located around four clearly defined longitudes,
corresponding to the fastest azimuthal velocity perturbations. Maximum
amplitudes occur around periastron passage or slightly thereafter for
supersynchronously rotating stars. In very eccentric binaries, the distribution
of over the surface changes significantly as a function of orbital
phase, with small spatial structures appearing after periastron. An exploratory
calculation for the highly eccentric binary system delta Sco suggests that the
sudden and large amplitude variations in surface properties around periastron
may contribute toward the activity observed around this orbital phase.Comment: Accepted for publication in A&
An experimental investigation of chatter effects on tool life
Tool wear is one of the most important considerations in machining operations as it affects surface quality and integrity, productivity and cost. The most commonly used model for tool life analysis is the one proposed by F.W. Taylor about a century ago. Although the extended form of this equation includes the effects of important cutting conditions on tool wear, tool life studies are mostly performed under stable cutting conditions where the effect of chatter vibrations are not considered. This paper presents an empirical attempt to understand tool life under vibratory cutting conditions. Tool wear data are collected in turning and milling on different work materials under stable and chatter conditions. The effects of cutting conditions as well as severity of chatter on tool life are analyzed. The results indicate significant reduction in tool life due to chatter as expected. They also show that the severity of chatter, and thus the vibration amplitude, strongly reduces the life of cutting tools. These results can be useful in evaluating the real cost of chatter by including the reduced tool life. They can also be useful in justifying the cost of chatter suppression and more rigid machining systems
The development of an instrument for measuring strength of elbow flexion in elementary school children
This study was carried out in an attempt to develop and evaluate an instrument which would satisfactorily measure die strength of elbow flexion of boys and girls in grades three, four, five and six. Elbow flexion was chosen as an important element of total strength and as a factor in many of the activities in which children participate. The bar-scale apparatus used in this study consisted of either a Chatillon spring scale or a regular bathroom scale and a portable chinning bar set in a doorway. The test using this apparatus was administered to 112 children and evaluated for reliability, validity and objectivity. Since strength varies widely in children of different ages and between the sexes, the evaluation was done separately by grade and sex. A cable-tension test of elbow flexion strength was used as the criterion for the validity coefficients
A Changing Wind Collision
We report on the first detection of a global change in the X-ray emitting properties of a wind–wind collision, thanks to XMM-Newton observations of the massive Small Magellenic Cloud (SMC) system HD 5980. While its light curve had remained unchanged between 2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and slightly hardened. The new observations also extend the observational coverage over the entire orbit, pinpointing the light-curve shape. It has not varied much despite the large overall brightening, and a tight correlation of fluxes with orbital separation is found without any hysteresis effect. Moreover, the absence of eclipses and of absorption effects related to orientation suggests a large size for the X-ray emitting region. Simple analytical models of the wind–wind collision, considering the varying wind properties of the eruptive component in HD 5980, are able to reproduce the recent hardening and the flux-separation relationship, at least qualitatively, but they predict a hardening at apastron and little change in mean flux, contrary to observations. The brightness change could then possibly be related to a recently theorized phenomenon linked to the varying strength of thin-shell instabilities in shocked wind regions
Generative models of the human connectome
The human connectome represents a network map of the brain's wiring diagram
and the pattern into which its connections are organized is thought to play an
important role in cognitive function. The generative rules that shape the
topology of the human connectome remain incompletely understood. Earlier work
in model organisms has suggested that wiring rules based on geometric
relationships (distance) can account for many but likely not all topological
features. Here we systematically explore a family of generative models of the
human connectome that yield synthetic networks designed according to different
wiring rules combining geometric and a broad range of topological factors. We
find that a combination of geometric constraints with a homophilic attachment
mechanism can create synthetic networks that closely match many topological
characteristics of individual human connectomes, including features that were
not included in the optimization of the generative model itself. We use these
models to investigate a lifespan dataset and show that, with age, the model
parameters undergo progressive changes, suggesting a rebalancing of the
generative factors underlying the connectome across the lifespan.Comment: 38 pages, 5 figures + 19 supplemental figures, 1 tabl
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
Two New LBV Candidates in the M33 Galaxy
We present two new luminous blue variable (LBV) candidate stars discovered in
the M33 galaxy. We identified these stars (Valeev et al. 2010) as massive star
candidates at the final stages of evolution, presumably with a notable
interstellar extinction. The candidates were selected from the Massey et al.
(2006) catalog based on the following criteria: emission in Halpha, V<18.5 and
0.35<(B-V)<1.2. The spectra of both stars reveal a broad and strong Halpha
emission with extended wings (770 and 1000 km/s). Based on the spectra we
estimated the main parameters of the stars. Object N45901 has a bolometric
luminosity log(L/Lsun)=6.0-6.2 with the value of interstellar extinction
Av=2.3+-0.1. The temperature of the star's photosphere is estimated as
Tstar~13000-15000K its probable mass on the Zero Age Main Sequence is
M~60-80Msun. The infrared excess in N45901 corresponds to the emission of warm
dust with the temperature Twarm~1000K, and amounts to 0.1% of the bolometric
luminosity. A comparison of stellar magnitude estimates from different catalogs
points to the probable variability of the object N45901. Bolometric luminosity
of the second object, N125093, is log(L/Lsun)=6.3-6.6, the value of
interstellar extinction is Av=2.75+-0.15. We estimate its photosphere's
temperature as Tstar~13000-16000K, the initial mass as M~90-120Msun. The
infrared excess in N125093 amounts to 5-6% of the bolometric luminosity. Its
spectral energy distribution reveals two thermal components with the
temperatures Twarm~1000K and Tcold~480K. The [CaII] lines (7291A and 7323A),
observed in LBV-like stars VarA and N93351 in M33, are also present in the
spectrum of N125093. These lines indicate relatively recent gas eruptions and
dust activity linked with them. High bolometric luminosity of these stars and
broad Halpha emissions allow classifying the studied objects as LBV candidates.Comment: 14 pages, 4 figure
Investigating the X-ray emission from the massive WR+O binary WR 22 using 3D hydrodynamical models
We examine the dependence of the wind-wind collision and subsequent X-ray
emission from the massive WR+O star binary WR~22 on the acceleration of the
stellar winds, radiative cooling, and orbital motion. Simulations were
performed with instantaneously accelerated and radiatively driven stellar
winds. Radiative transfer calculations were performed on the simulation output
to generate synthetic X-ray data, which are used to conduct a detailed
comparison against observations. When instantaneously accelerated stellar winds
are adopted in the simulation, a stable wind-wind collision region (WCR) is
established at all orbital phases. In contrast, when the stellar winds are
radiatively driven, and thus the acceleration regions of the winds are
accounted for, the WCR is far more unstable. As the stars approach periastron,
the ram pressure of the WR's wind overwhelms the O star's and, following a
significant disruption of the shocks by non-linear thin-shell instabilities
(NTSIs), the WCR collapses onto the O star. X-ray calculations reveal that when
a stable WCR exists the models over-predict the observed X-ray flux by more
than two orders of magnitude. The collapse of the WCR onto the O star
substantially reduces the discrepancy in the keV flux to a factor of
at . However, the observed spectrum is not well matched
by the models. We conclude that the agreement between the models and
observations could be improved by increasing the ratio of the mass-loss rates
in favour of the WR star to the extent that a normal wind ram pressure balance
does not occur at any orbital phase, potentially leading to a sustained
collapse of the WCR onto the O star. Radiative braking may then play a
significant r\^{o}le for the WCR dynamics and resulting X-ray emission.Comment: 16 pages, 11 figures, 2 tables. Accepted for publication in A&
Eta Carinae across the 2003.5 minimum: Spectroscopic Evidence for Massive Binary Interactions
We have analyzed high spatial, moderate spectral resolution observations of
Eta Carinae obtained with the STIS from 1998.0 to 2004.3. The spectra show
prominent P-Cygni lines in H I, Fe II and He I which are complicated by blends
and contamination by nebular emission and absorption along the line-of-sight
toward the observer. All lines show phase and species dependent variations in
emission and absorption. For most of the cycle the He I emission is blueshifted
relative to the H I and Fe II P-Cygni emission lines, which are approximately
centered at system velocity. The blueshifted He I absorption varies in
intensity and velocity throughout the 2024 day period. We construct radial
velocity curves for the absorption component of the He I and H I lines. The He
I absorption shows significant radial velocity variations throughout the cycle,
with a rapid change of over 200 km/s near the 2003.5 event. The H I velocity
curve is similar to that of the He I absorption, though offset in phase and
reduced in amplitude. We interpret the complex line profile variations in He I,
H I and Fe II to be a consequence of the dynamic interaction of the dense wind
of Eta Car A with the less dense, faster wind plus the radiation field of a hot
companion star, Eta Car B. During most of the orbit, Eta Car B and the He+
recombination zone are on the near side of Eta Car A, producing blueshifted He
I emission. He I absorption is formed in the part of the He+ zone that
intersects the line-of-sight toward Eta Car. We use the variations seen in He I
and the other P-Cygni lines to constrain the geometry of the orbit and the
character of Eta Car B.Comment: 16 pages, 18 figure
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