268 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
International Ultraviolet Explorer Observations of Wolf-Rayet Binaries: Wind Structures
Spectra of six WN + OB Wolf-Rayet systems obtained with the IUE are analyzed for phase-dependent variations. Periodic variability at emission-line frequencies is detected in V444 Cyg, HD 90657, HD 211853, HD 186943 and HD 94546 on low dispersion SWP images. No changes in the low dispersion spectra of HD 193077 are apparent. We find the variations in the UV to be similar in nature to those observed in optical spectra of various WR sources. That is, there is a strengthening of absorption components in P Cygni-type features at orbital phases in which the O-star is behind the WR wind. With the aid of a computer code which models this type of variations, and through a comparison with HD 193077, the dominant mechanism producing the variations is shown to be selective atmospheric eclipses of the O-star by the WR wind. Based on this interpretation, a straightforward technique is applied to the line of N IV 1718, by which an optical depth distribution in the WN winds of the form tau varies as r(-1) is derived for 16 r 66 solar radii. Phase-dependent variations in the width of the C IV 1550 absorption component in V444 Cyg, HD 90657 and HD 211853 are interpretated as wind-wind collision effects
First detection of phase-dependent colliding wind X-ray emission outside the Milky Way
After having reported the detection of X-rays emitted by the peculiar system
HD5980, we assess here the origin of this high-energy emission from additional
X-ray observations obtained with XMM-Newton. This research provides the first
detection of apparently periodic X-ray emission from hot gas produced by the
collision of winds in an evolved massive binary outside the Milky Way. It also
provides the first X-ray monitoring of a Luminous Blue Variable only years
after its eruption and shows that the dominant source of the X-rays is not
associated with the ejecta.Comment: 13 pages, 3 figures and 1 table, accepted for publication in ApJ
(letters
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&
Decoding of the light changes in eclipsing Wolf-Rayet binaries I. A non-classical approach to the solution of light curves
We present a technique to determine the orbital and physical parameters of
eclipsing eccentric Wolf-Rayet + O-star binaries, where one eclipse is produced
by the absorption of the O-star light by the stellar wind of the W-R star. Our
method is based on the use of the empirical moments of the light curve that are
integral transforms evaluated from the observed light curves. The optical depth
along the line of sight and the limb darkening of the W-R star are modelled by
simple mathematical functions, and we derive analytical expressions for the
moments of the light curve as a function of the orbital parameters and the key
parameters of the transparency and limb-darkening functions. These analytical
expressions are then inverted in order to derive the values of the orbital
inclination, the stellar radii, the fractional luminosities, and the parameters
of the wind transparency and limb-darkening laws. The method is applied to the
SMC W-R eclipsing binary HD 5980, a remarkable object that underwent an
LBV-like event in August 1994. The analysis refers to the pre-outburst
observational data. A synthetic light curve based on the elements derived for
the system allows a quality assessment of the results obtained.Comment: Accepted for publication in Astronomy & 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
Investigating the Chemically Homogeneous Evolution Channel and its Role in the Formation of the Enigmatic Binary Black Hole Progenitor Candidate HD 5980
Chemically homogeneous evolution (CHE) is a promising channel for forming
massive binary black holes. The enigmatic, massive Wolf-Rayet (WR) binary HD
5980 A&B has been proposed to have formed through this channel. We investigate
this claim by comparing its observed parameters with CHE models. Using MESA, we
simulate grids of close massive binaries then use a Bayesian approach to
compare them with the stars' observed orbital period, masses, luminosities, and
hydrogen surface abundances. The most probable models, given the observational
data, have initial periods ~3 days, widening to the present-day ~20 day orbit
as a result of mass loss -- correspondingly, they have very high initial
stellar masses (150 M). We explore variations in stellar
wind-mass loss and internal mixing efficiency, and find that models assuming
enhanced mass-loss are greatly favored to explain HD 5980, while enhanced
mixing is only slightly favoured over our fiducial assumptions. Our most
probable models slightly underpredict the hydrogen surface abundances.
Regardless of its prior history, this system is a likely binary black hole
progenitor. We model its further evolution under our fiducial and enhanced wind
assumptions, finding that both stars produce black holes with masses ~19-37
M. The projected final orbit is too wide to merge within a Hubble time
through gravitational waves alone. However, the system is thought to be part of
a 2+2 hierarchical multiple. We speculate that secular effects with the
(possible) third and fourth companions may drive the system to promptly become
a gravitational-wave source.Comment: 23 pages, 9 figures, 2 tables, 2 appendices. Accepted for publication
in Ap
Effects of bearing clearance on the chatter stability of milling process
In the present study, the influences of the bearing clearance, which is a common fault for machines, to the chatter stability of milling process are examined by using numerical simulation method. The results reveal that the presence of bearing clearance could make the milling process easier to enter the status of chatter instability and can shift the chatter frequency. In addition, the spectra analysis to vibration signals obtained under the instable milling processes show that the presence of bearing clearance could introduce more frequency components to the vibration responses but, however, under both the stable and instable milling processes, the generated frequency components will not violate the ideal spectra structures of the vibration responses of the milling process, which are usually characterized by the tooth passing frequency and its associated higher harmonics for the stable milling process and by the complex coupling of the tooth passing frequency and the chatter frequency for the instable milling process. This implies that, even under the case with bearing clearance fault, the stability of the milling process can still be determined by viewing the frequency spectra of the vibration responses. Moreover, the phenomena of the chatter frequency shift and the generation of more components provide potential ways to detect the bearing clearance in machines. (C) 2010 Elsevier Ltd. All rights reserved
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