278 research outputs found
FUSE observations of HD 5980: The wind structure of the eruptor
HD 5980 is a unique system containing one massive star (star A) that is
apparently entering the luminous blue variable phase, and an eclipsing
companion (star B) that may have already evolved beyond this phase to become a
Wolf-Rayet star. In this paper we present the results from FUSE observations
obtained in 1999, 2000, and 2002 and one far-UV observation obtained by
ORFEUS/BEFS in 1993 shortly before the first eruption of HD 5980. The eight
phase-resolved spectra obtained by FUSE in 2002 are analyzed in the context of
a wind-eclipse model. This analysis shows that the wind of the eruptor obeyed a
very fast velocity law in 2002, which is consistent with the line-driving
mechanism. Large amplitude line-profile variations on the orbital period are
shown to be due to the eclipse of star B by the wind of star A, although the
eclipse due to gas flowing in the direction of star B is absent. This can only
be explained if the wind of star A is not spherically symmetric, or if the
eclipsed line radiation is "filled-in" by emission originating from somewhere
else in the system, e.g., in the wind-wind collision region. Except for a
slightly lower wind speed, the ORFEUS/BEFS spectrum is very similar to the
spectrum obtained by FUSE at the same orbital phase: there is no indication of
the impending eruption. However, the trend for decreasing wind velocity
suggests the occurrence of the "bi-stability" mechanism, which in turn implies
that the restructuring of the circumbinary environment caused by the transition
from "fast, rarefied wind" to "slow, dense wind" was observed as the eruptive
event. The underlying mechanism responsible for the long-term decrease in wind
velocity that precipitated this change remains an open issue.Comment: 19 pages, 13 figure
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&
A Detailed Study of 2S 0114+650 with the RXTE
We present the results of a detailed study of the high mass X-ray binary 2S
0114+650 made with the pointed instruments onboard the Rossi X-ray Timing
Explorer. The spectral and temporal behaviour of this source was examined over
the pulse, orbital, and super-orbital timescales, covering 2 cycles of
the 30.7 d super-orbital modulation. Marginal evidence for variability of the
power law photon index over the pulse period was identified, similar to that
observed from other X-ray pulsars. If this variability is real it can be
attributed to a varying viewing geometry of the accretion region with the spin
of the neutron star. Variability of the neutral hydrogen column density over
the orbital period was observed, which we attribute to the line of sight motion
of the neutron star through the dense circumstellar environment. A reduction in
the power law photon index was observed during the orbital maximum, which we
speculate is due to absorption effects as the neutron star passes behind a
heavily absorbing region near the base of the supergiant companion's wind. No
significant variability of the column density was observed over the
super-orbital period, indicating that variable obscuration by a precessing warp
in an accretion disc is not the mechanism behind the super-orbital modulation.
In contrast, a significant increase in the power law photon index was observed
during the super-orbital minimum. We conclude that the observed super-orbital
modulation is tied to variability in the mass accretion rate due to some as yet
unidentified mechanism.Comment: 22 pages, 27 figures, accepted for publication in MNRAS after
moderate revisio
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
Emergent properties of electrically coupled smooth muscle cells
Asynchronous and synchronous calcium oscillations occur in a variety of cells. A well-established pathway for intercellular communication is provided by gap junctions which connect adjacent cells and can mediate electrical and chemical coupling. Several experimental studies report that cells presenting only a transient increase when freshly dispersed may oscillate when they are coupled. Such observations suggest that the role of gap junctions is not only to coordinate calcium oscillations of adjacent cells. Gap junctions may also be important to generate oscillations. Here we illustrate the emergent properties of electrically coupled smooth muscle cells using a model that we recently proposed. A bifurcation analysis in the case of two cells reveals that synchronous and asynchronous calcium oscillations can be induced by electrical coupling. In a larger population of smooth muscle cells, electrical coupling may result in the creation of groups of cells presenting synchronous calcium oscillations. The elements of one group may be distant from each other. Moreover, our results highlight a general mechanism by which gap junctional electrical coupling can give rise to out of phase calcium oscillations in smooth muscle cells that are non-oscillating when uncoupled. All these observations remain true in the case of non-identical cells, except that the solution corresponding to synchronous calcium oscillations disappears and that the formation of groups is sensitive to the degree of heterogeneity. © 2005 Society for Mathematical Biology. Published by Elsevier Ltd. All rights reserved
Bolometric luminosity variations in the Luminous Blue Variable AFGL2298
We characterise the variability in the physical properties of the luminous
blue variable AFGL2298 between 1989-2008. In conjunction with published data
from 1989-2001, we have undertaken a long term (2001-2008) near-IR
spectroscopic and photometric observational campaign for this star and utilise
a non-LTE model atmosphere code to interpret these data. We find AFGL2298 to
have been highly variable during the two decades covered by the observational
datasets. Photometric variations of >1.6 mag have been observed in the JHK
wavebands; however, these are not accompanied by correlated changes in near-IR
colour. Non-LTE model atmosphere analysis of 4 epochs of K band spectroscopy
obtained between 2001-7 suggests that the photometric changes were driven by
expansion and contraction of the stellar photosphere accompanied by
comparatively small changes in the stellar temperature. Unclumped mass loss
rates throughout this period were modest and directly comparable to those of
other highly luminous LBVs. However, the bolometric luminosity of AFGL2298
appears to have varied by at least a factor of ~2 between 1989-2008, with it
being one of the most luminous stars in the Galaxy during maximum. Comparison
to other LBVs that have undergone non bolometric luminosity conserving
`eruptions' shows such events to be heterogeneous, with AFGL2298 the least
extreme example. These results - and the diverse nature of both the quiescent
LBVs and associated ejecta - may offer support to the suggestion that more than
one physical mechanism is responsible for such behaviour. [ABRIDGED]Comment: 12 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
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
A Supernova Remnant Collision with a Stellar Wind
Numerical simulations of the interaction between supernova ejecta and a
stellar wind are presented. We follow the temporal evolution of the shock
fronts that are formed through such an interaction and determine the
velocities, temperatures and densities. We model the X-ray emission from the
SNR-stellar wind collision region and we compare it with recent results from
X-ray observations carried out with the Chandra satellite of the SMC supernova
remnant SNR 0057-7226 which could be interacting with the wind of the
Wolf-Rayet system HD 5980. The simulations predict the presence of shell-like
regions of enhanced X-ray emission which are consistent with the presence of
X-ray emitting arcs in the Chandra image. Also the observed X-ray luminosity is
comparable to the X-ray luminosities we obtain from the simulations for a
supernova with an initial energy in the (1-5)E50 erg range.Comment: 9 files into a tar.gz file (the Latex file of manuscript, 7 jpg
figures, 1 style file: aastex.cls). Accepted by ApJ, Vol. 584, 2003 February
1
Dust Formation and He II 4686 emission in the Dense Shell of the Peculiar Type Ib Supernova 2006jc
We present evidence for the formation of dust grains in an unusual Type Ib SN
based on late-time spectra of SN 2006jc. The progenitor suffered an LBV-like
outburst just 2 yr earlier, and we propose that the dust formation is a
consequence of the SN blast wave overtaking that LBV-like shell. The key
evidence for dust formation is (a) the appearance of a red/near-IR continuum
source fit by 1600 K graphite grains, and (b) fading of the redshifted sides of
He I emission lines, yielding progressively more asymmetric blueshifted lines
as dust obscures receding material. This provides the strongest case yet for
dust formation in any SN Ib/c. Both developments occurred between 51 and 75 d
after peak, while other SNe observed to form dust did so after a few hundred
days. Geometric considerations indicate that dust formed in the dense swept-up
shell between the forward and reverse shocks, and not in the freely expanding
SN ejecta. Rapid cooling leading to dust formation may have been aided by
extremely high shell densities, as indicated by He I line ratios. The brief
epoch of dust formation is accompanied by He II 4686 emission and enhanced
X-ray emission. These clues suggest that the unusual dust formation in this
object was not due to properties of the SN itself, but instead -- like most
peculiarities of SN 2006jc -- was a consequence of the dense environment
created by an LBV-like eruption 2 yr before the SN.Comment: ApJ, accepted. added some discussion and 2 figures, better title,
conclusions same as previous version. 12 pages, 4 color fig
- …