We analyze interferometric measurements of the luminous blue variable Eta Carinae with the goal of constraining the rotational velocity of the primary star and probing the influence of the companion. ...Using two-dimensional radiative transfer models of latitude-dependent stellar winds, we find that prolate-wind models with a ratio of the rotational velocity (v{sub rot}) to the critical velocity (v{sub crit}) of W = 0.77-0.92, inclination angle of i = 60{sup 0}-90{sup 0}, and position angle (P.A.) =108{sup 0}-142{sup 0} reproduce simultaneously K-band continuum visibilities from VLTI/VINCI and closure phase measurements from VLTI/AMBER. Interestingly, oblate models with W = 0.73-0.90 and i = 80{sup 0}-90{sup 0} produce similar fits to the interferometric data, but require P.A. =210{sup 0}-230{sup 0}. Therefore, both prolate and oblate models suggest that the rotation axis of the primary star is not aligned with the Homunculus polar axis. We also compute radiative transfer models of the primary star allowing for the presence of a cavity and dense wind-wind interaction region created by the companion star. We find that the wind-wind interaction has a significant effect on the K-band image mainly via free-free emission from the compressed walls and, for reasonable model parameters, can reproduce the VLTI/VINCI visibilities taken at {phi}{sub vb03} = 0.92-0.93. We conclude that the density structure of the primary wind can be sufficiently disturbed by the companion, thus mimicking the effects of fast rotation in the interferometric observables. Therefore, fast rotation may not be the only explanation for the interferometric observations. Intense temporal monitoring and three-dimensional modeling are needed to resolve these issues.
The very massive star system η Carinae exhibits regular 5.54 yr (2024 d) period disruptive events in wavebands ranging from the radio to X-ray. There is a growing consensus that these events likely ...stem from periastron passage of an (as yet) unseen companion in a highly eccentric (e∼ 0.9) orbit. This Letter presents 3D smoothed particle hydrodynamics (SPH) simulations of the orbital variation of the binary wind–wind collision, and applies these to modelling the X-ray light curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global 3D model of the phase variation of the density of the interacting winds, the simulations allow computation of the associated variation in X-ray absorption, presumed here to originate from near the apex of the wind–wind interaction cone. We find that the observed RXTE light curve can be readily fitted if the observer's line of sight is within this cone along the general direction of apastron. Specifically, the data are well fitted by an assumed inclination i= 45° for the orbit's polar axis, which is thus consistent with orbital angular momentum being along the inferred polar axis of the Homunculus nebula. The fits also constrain the position angle φ that an orbital-plane projection makes with the apastron side of the semimajor axis, strongly excluding positions φ < 9° along or to the retrograde side of the axis, with the best-fitting position given by φ= 27°. Overall the results demonstrate the utility of a fully 3D dynamical model for constraining the geometric and physical properties of this complex colliding wind binary system.
We investigate the degree to which the nearly symmetric form of X-ray emission lines seen in Chandra spectra of early-type supergiant stars could be explained by the possibly porous nature of their ...spatially structured stellar winds. Such porosity could effectively reduce the bound-free absorption of X-rays emitted by embedded wind shocks, and thus allow a more similar transmission of redshifted and blueshifted emission from the back and front hemispheres, respectively. To obtain the localized self-shielding that is central to this porosity effect, it is necessary that the individual clumps be optically thick. In a medium consisting of clumps of size l and volume filling factor f, we argue that the general modification in effective opacity should scale approximately as sub(eff)- / super(1+t)dc), where, for a given atomic opacity and mean density r, the clump optical thickness scales as t sub(c) = rl/f. For a simple wind structure parameterization in which the "porosity length" h =l/f increases with local radius r as h = h r, we find that a substantial reduction in wind absorption requires a quite large porosity scale factor, h >1, implying large porosity lengths h >r. The associated wind structure must thus have either a relatively large scale l<r, or a small volume filling factor f-l/r <<1, or some combination of these. We argue that the relatively small-scale, moderate compressions generated by intrinsic instabilities in line driving are unlikely to give such large porosity lengths. This raises questions about whether porosity effects could play a significant role in explaining nearly symmetric X-ray line profiles, leaving the prospect of instead having to invoke a substantial (approximately a factor of 5) downward revision in the assumed mass-loss rates.
We present a practical, efficient, semianalytic formalism for computing steady state X-ray emission from radiative shocks between colliding stellar winds in relatively close (orbital period up to ...order tens of days) massive- star, binary systems. Our simplified approach idealizes the individual wind flows as smooth and steady, ignoring the intrinsic instabilities and associated structure thought to occur in such flows. By also suppressing thin-shell instabilities for wind-collision radiative shocks, our steady state approach avoids the extensive structure and mixing that has thus far precluded reliable computation of X-ray emission spectra from time-dependent hydrodynamical simulations of close-binary, wind-collision systems; but in ignoring the unknown physical level of such mixing, the luminosity and hardness of X-ray spectra derived here represent upper limits to what is possible for a given set of wind and binary parameters. A key feature of our approach is the separation of calculations for the small-scale shock-emission from the ram-pressure-balance model for determining the large-scale, geometric form of the wind-wind interaction front. Integrating the localized shock emission over the full interaction surface and using a warm-absorber opacity to take account of attenuation by both the smooth wind and the compressed, cooled material in the interaction front, the method can predict spectra for a distant observer at any arbitrary orbital inclination and phase. We illustrate results for a sample selection of wind, stellar, and binary parameters, providing both full X-ray light curves and detailed spectra at selected orbital phases. The derived spectra typically have a broad characteristic form, and by synthetic processing with the standard XSPEC package, we demonstrate that they simply cannot be satisfactorily fitted with the usual attenuated single- or two-temperature thermal-emission models. We conclude with a summary of the advantages and limitations of our approach and outline its potential application for interpreting detailed X-ray observations from close, massive-star binary systems.
We have obtained 18 new high-resolution spectropolarimetric observations of the B2Vp star σ Ori E with both the Narval and ESPaDOnS spectropolarimeters. The aim of these observations is to test, with ...modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki, applied to the specific case of σ Ori E by Townsend, Owocki & Groote. This model includes a substantially offset dipole magnetic field configuration, and approximately reproduces previous observational variations in longitudinal field strength, photometric brightness and Hα emission. We analyse new spectroscopy, including H i, He i, C ii, Si iii and Fe iii lines, confirming the diversity of variability in photospheric lines, as well as the double S-wave variation of circumstellar hydrogen. Using the multiline analysis method of least-squares deconvolution (LSD), new, more precise longitudinal magnetic field measurements reveal a substantial variance between the shapes of the observed and RRM model time-varying field. The phase-resolved Stokes V profiles of He i 5876 and 6678 Å lines are fitted poorly by synthetic profiles computed from the magnetic topology assumed by Townsend et al.. These results challenge the offset dipole field configuration assumed in the application of the RRM model to σ Ori E, and indicate that future models of its magnetic field should also include complex, higher order components.
We present an efficient technique to study the 1D evolution of instability-generated structure in winds of hot stars out to very large distances (∼1000 stellar radii). This technique makes use of our ...previous finding that external forces play little rôle in the outer evolution of structure. Rather than evolving the entire wind, as is traditionally done, the technique focuses on a representative portion of the structure and follows it as it moves out with the flow. This requires the problem to be formulated in a moving reference frame. The lack of Galilean invariance of the spherical equations of hydrodynamics is circumvented by recasting them in a pseudo-planar form. By applying the technique to a number of problems we show that it is fast and accurate, and has considerable conceptual advantages. It is particularly useful to test the dependence of solutions on the Galilean frame in which they were obtained. As an illustration, we show that, in a one-dimensional approximation, the wind can remain structured out to distances of more than 1300 stellar radii from the central star.
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