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ABSTRACT: We present the first fully 3D MHD simulation for magnetic channeling and
confinement of a radiatively driven, massive-star wind. The specific parameters
are chosen to represent the prototypical slowly rotating magnetic O star
\theta^1 Ori C, for which centrifugal and other dynamical effects of rotation
are negligible. The computed global structure in latitude and radius resembles
that found in previous 2D simulations, with unimpeded outflow along open field
lines near the magnetic poles, and a complex equatorial belt of inner wind
trapping by closed loops near the stellar surface, giving way to outflow above
the Alfv\'{e}n radius. In contrast to this previous 2D work, the 3D simulation
described here now also shows how this complex structure fragments in azimuth,
forming distinct clumps of closed loop infall within the Alfv\'{e}n radius,
transitioning in the outer wind to radial spokes of enhanced density with
characteristic azimuthal separation of $15-20 \degr$. Applying these results in
a 3D code for line radiative transfer, we show that emission from the
associated 3D `dynamical magnetosphere' matches well the observed H\alpha
emission seen from \theta^1 Ori C, fitting both its dynamic spectrum over
rotational phase, as well as the observed level of cycle to cycle stochastic
variation. Comparison with previously developed 2D models for Balmer emission
from a dynamical magnetosphere generally confirms that time-averaging over 2D
snapshots can be a good proxy for the spatial averaging over 3D azimuthal wind
structure. Nevertheless, fully 3D simulations will still be needed to model the
emission from magnetospheres with non-dipole field components, such as
suggested by asymmetric features seen in the H\alpha equivalent-width curve of
\theta^1 Ori C.
10/2012;
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ABSTRACT: The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such "dynamical magnetospheres" (DMs) with the more steady-state
"centrifugal magnetospheres" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.
03/2012;
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ABSTRACT: We present a generalised formalism for treating the porosity-associated
reduction in continuum opacity that occurs when individual clumps in a
stochastic medium become optically thick. We consider geometries resulting in
either isotropic or anisotropic effective opacity, and, in addition to an
idealised model in which all clumps have the same local overdensity and scale,
we also treat an ensemble of clumps with optical depths set by Markovian
statistics. This formalism is then applied to the specific case of bound-free
absorption of X-rays in hot star winds, a process not directly affected by
clumping in the optically thin limit. We find that the Markov model gives
surprisingly similar results to those found previously for the single clump
model, suggesting that porous opacity is not very sensitive to details of the
assumed clump distribution function. Further, an anisotropic effective opacity
favours escape of X-rays emitted in the tangential direction (the `venetian
blind' effect), resulting in a 'bump' of higher flux close to line centre as
compared to profiles computed from isotropic porosity models. We demonstrate
how this characteristic line shape may be used to diagnose the clump geometry,
and we confirm previous results that for optically thick clumping to
significantly influence X-ray line profiles, very large porosity lengths,
defined as the mean free path between clumps, are required. Moreover, we
present the first X-ray line profiles computed directly from line-driven
instability simulations using a 3-D patch method, and find that porosity
effects from such models also are very small. This further supports the view
that porosity has, at most, a marginal effect on X-ray line diagnostics in O
stars, and therefore that these diagnostics do indeed provide a good `clumping
insensitive' method for deriving O star mass-loss rates.
11/2011;
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ABSTRACT: We fit every emission line in the high-resolution Chandra grating spectrum of ζ Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of 16 lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, , and place confidence limits on this parameter. These 16 lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear increase in τ* with line wavelength, as expected from the wavelength increase of bound–free absorption opacity. The small overall values of τ*, reflected in the rather modest asymmetry in the line profiles, can moreover all be fitted simultaneously by simply assuming a moderate mass-loss rate of 3.5 ± 0.3 × 10−6 M⊙ yr−1, without any need to invoke porosity effects in the wind. The quoted uncertainty is statistical, but the largest source of uncertainty in the derived mass-loss rate is due to the uncertainty in the elemental abundances of ζ Pup, which affects the continuum opacity of the wind, and which we estimate to be a factor of 2. Even so, the mass-loss rate we find is significantly below the most recent smooth-wind Hα mass-loss rate determinations for ζ Pup, but is in line with newer determinations that account for small-scale wind clumping. If ζ Pup is representative of other massive stars, these results will have important implications for stellar and Galactic evolution.
Monthly Notices of the Royal Astronomical Society 07/2010; 405(4):2391 - 2405. · 4.90 Impact Factor
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ABSTRACT: This session dealt with the circumstellar gas surrounding active OB stars, and the discussion broadly focused on disks, wind morphology and structures, X-ray emission, and mass loss rates.
Proceedings of the International Astronomical Union 06/2010; 6:378 - 379.
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ABSTRACT: A magnetic field and rotational line profile variability (lpv) is found in the He-weak star HR 2949. The field measured from metallic lines varies in a clearly non-sinusoidal way, and shows a phase lag relative to the morphologically similar He i equivalent width variations. The surface abundance patterns are strong and complex, and visible even in the hydrogen lines.
Proceedings of the International Astronomical Union 06/2010; 6:210 - 211.
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ABSTRACT: We fit every emission line in the high-resolution Chandra grating spectrum of zeta Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of sixteen lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, tau_* = kappa*Mdot/4{pi}R_{\ast}v_{\infty}, and place confidence limits on this parameter. These sixteen lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear increase in tau_* with line wavelength, as expected from the wavelength increase of bound-free absorption opacity. The small overall values of tau_*, reflected in the rather modest asymmetry in the line profiles, can moreover all be fit simultaneously by simply assuming a moderate mass-loss rate of 3.5 \pm 0.3 \times 10^{-6} Msun/yr, without any need to invoke porosity effects in the wind. The quoted uncertainty is statistical, but the largest source of uncertainty in the derived mass-loss rate is due to the uncertainty in the elemental abundances of zeta Pup, which affects the continuum opacity of the wind, and which we estimate to be a factor of two. Even so, the mass-loss rate we find is significantly below the most recent smooth-wind H-alpha mass-loss rate determinations for zeta Pup, but is in line with newer determinations that account for small-scale wind clumping. If zeta Pup is representative of other massive stars, these results will have important implications for stellar and galactic evolution. Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society. 17 pages, including 14 figures (7 color)
03/2010;
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ABSTRACT: We report on four Chandra grating observations of the oblique magnetic rotator θ1 Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star's 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray-emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hot plasma. We investigate these diagnostics as a function of viewing angle and analyze them in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on θ1 Ori C. This model fits all the data surprisingly well, predicting the temperature, luminosity, and occultation of the X-ray-emitting plasma with rotation phase.
The Astrophysical Journal 12/2008; 628(2):986. · 6.02 Impact Factor
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ABSTRACT: We examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, W(=Vrot/Vorb) and 'wind magnetic confinement', $\eta_\ast$ defined below. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD scaling $\dot {J} \sim \dot {M} \Omega R_A^2$. The key distinction is that for a dipole field Alfv\`en radius $R_A$ is significantly smaller than for the monopole field WD used in their analyses. This leads to a slower stellar spin-down for the dipole field with typical spin-down times of order 1 Myr for several known magnetic massive stars. Comment: 2 pages, Proceedings IAU Symposium No. 259, 2009 Cosmic Magnetic Fields: From Planets, to Stars and Galaxies
12/2008;
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ABSTRACT: We examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous two-dimensional numerical magnetohydrodynamics simulation study that examines the interplay among wind, field and rotation as a function of two dimensionless parameters: one characterizing the wind magnetic confinement () and the other the ratio (W≡Vrot/Vorb) of stellar rotation to critical (orbital) speed. We compare and contrast the two-dimensional, time-variable angular momentum loss of this dipole model of a hot-star wind with the classical one-dimensional steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time closely follows the general WD scaling , where is the mass-loss rate, Ω is the stellar angular velocity and RA is a characteristic Alfvén radius. However, a key distinction here is that for a dipole field, this Alfvén radius has a strong-field scaling RA/R*≈η1/4*, instead of the scaling for a monopole field. This leads to a slower stellar spin-down time that in the dipole case scales as , where is the characteristic mass loss time and k is the dimensionless factor for stellar moment of inertia. The full numerical scaling relation that we cite gives typical spin-down times of the order of 1 Myr for several known magnetic massive stars.
Monthly Notices of the Royal Astronomical Society 12/2008; 392(3):1022 - 1033. · 4.90 Impact Factor
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ABSTRACT: We examine the angular momentum loss and associated rotational spindown for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, one characterizing the wind magnetic confinement ($\eta_{\ast} \equiv B_{eq}^{2} R_{\ast}^{2}/{\dot M} v_{\infty}$), and the other the ratio ($W \equiv V_{rot}/V_{orb}$) of stellar rotation to critical (orbital) speed. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss ${\dot J}$ averaged over both solid angle and time follows closely the general WD scaling ${\dot J} = (2/3) {\dot M} \Omega R_{A}^{2}$, where ${\dot M}$ is the mass loss rate, $\Omega$ is the stellar angular velocity, and $R_{A}$ is a characteristic Alfv\'{e}n radius. However, a key distinction here is that for a dipole field, this Alfv\'{e}n radius has a strong-field scaling $R_{A}/R_{\ast} \approx \eta_{\ast}^{1/4}$, instead of the scaling $R_{A}/R_{\ast} \sim \sqrt{\eta_{\ast}}$ for a monopole field. This leads to a slower stellar spindown time that in the dipole case scales as $\tau_{spin} = \tau_{mass} 1.5k/\sqrt{\eta_{\ast}}$, where $\tau_{mass} \equiv M/{\dot M}$ is the characteristic mass loss time, and $k$ is the dimensionless factor for stellar moment of inertia. The full numerical scaling relation we cite gives typical spindown times of order 1 Myr for several known magnetic massive stars. Comment: 13 pages, 7 figures, accepted for publication in MNRAS. MNRAS in press
10/2008;
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ABSTRACT: By quantitatively fitting simple emission line profile models that include both atomic opacity and porosity to the Chandra X-ray spectrum of $\zeta$ Pup, we are able to explore the trade-offs between reduced mass-loss rates and wind porosity. We find that reducing the mass-loss rate of $\zeta$ Pup by roughly a factor of four, to 1.5 \times 10^{-6} M_sun/yr, enables simple non-porous wind models to provide good fits to the data. If, on the other hand, we take the literature mass-loss rate of 6 \times 10^{-6} M_sun/yr, then to produce X-ray line profiles that fit the data, extreme porosity lengths -- of $h_{\infty} \approx 3$ Rstar -- are required. Moreover, these porous models do not provide better fits to the data than the non-porous, low optical depth models. Additionally, such huge porosity lengths do not seem realistic in light of 2-D numerical simulations of the wind instability.
01/2008;
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ABSTRACT: We present a semi-analytic model for shaping the nebula around eta Carinae that accounts for the simultaneous production of bipolar lobes and an equatorial disk through a rotating surface explosion. Material is launched normal to the surface of an oblate rotating star with an initial kick velocity that scales approximately with the local escape speed. Thereafter, ejecta follow ballistic orbital trajectories, feeling only a central force corresponding to a radiatively reduced gravity. Our model is conceptually similar to the wind-compressed disk model of Bjorkman & Cassinelli, but we modify it to an explosion instead of a steady line-driven wind, we include a rotationally-distorted star, and we treat the dynamics somewhat differently. Continuum-driving avoids the disk inhibition that normally operates in line-driven winds. Our model provides a simple method by which rotating hot stars can simultaneously produce intrinsically bipolar and equatorial mass ejections, without an aspherical environment or magnetic fields. Although motivated by eta Carinae, the model may have generic application to other LBVs, B[e] stars, or SN1987A's nebula. When near-Eddington radiative driving is less influential, our model generalizes to produce bipolar morphologies without disks, as seen in many PNe. Comment: ApJ accepted, 9 pages
05/2007;
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ABSTRACT: We report on four Chandra grating observations of the oblique magnetic
rotator theta^1 Ori C (O5.5 V) covering a wide range of viewing angles with
respect to the star's 1060 G dipole magnetic field. We employ line-width and
centroid analyses to study the dynamics of the X-ray emitting plasma in the
circumstellar environment, as well as line-ratio diagnostics to constrain the
spatial location, and global spectral modeling to constrain the temperature
distribution and abundances of the very hot plasma. We investigate these
diagnostics as a function of viewing angle and analyze them in conjunction with
new MHD simulations of the magnetically channeled wind shock mechanism on
theta^1 Ori C. This model fits all the data surprisingly well, predicting the
temperature, luminosity, and occultation of the X-ray emitting plasma with
rotation phase.
04/2005;
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ABSTRACT: We discuss three different observational diagnostics related to disks around hot stars: absorption line determinations of rotational velocities of Be stars; polarization diagnostics of circumstellar disks; and X-ray line diagnostics of one specific magnetized hot star, $\theta^1$ Ori C. Some common themes that emerge from these studies include (a) the benefits of having a specific physical model as a framework for interpreting diagnostic data; (b) the importance of combining several different types of observational diagnostics of the same objects; and (c) that while there is often the need to reinterpret traditional diagnostics in light of new theoretical advances, there are many new and powerful diagnostics that are, or will soon be, available for the study of disks around hot stars.
11/2004;
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ABSTRACT: We flt X-ray line proflle models, including the efiects of large-scale wind porosity, to the high-resolution Chandra spectrum of the O4 supergiant ‡ Pup. We probe the importance of porosity in two speciflc ways: by comparing the flt quality of porous and non-porous models for individual line proflles, and also by studying the trend in proflle shape for many proflles as a function of wavelength. Individual lines can be flt by both non-porous models and porous models that assume spherical clumps, although for the higher signal-to-noise lines, the non-porous models are always preferred. As the mass-loss rate and the porosity length are nearly degenerate parameters, we explore the trade-ofis between the parameters and flnd that the porosity lengths required to accommodate the traditional mass-loss rate of ‡ Pup, 8 £ 10¡6 Mfl yr¡1, are very high, with h1 > R⁄. Porous models that assume oblate, or ∞attened, clumps, produce proflles with a difierent overall shape, and one that does not provide good flts to the data. We also flnd that there is in fact a signiflcant trend in optical depth as a function of wavelength over the range 6 to 22 "A. This trend is consistent with the expected atomic opacity, but is inconsistent with a highly porous medium, in which the optical depth is governed by the geometrical cross-section of the clumps. From the flts to these lines under the assumption that porosity does not afiect the opacity, we derive a mass-loss rate of 3:0£10¡6 Mfl yr¡1, which represents a factor of » 3 reduction of the traditional mass-loss rate derived assuming no wind clumping, and is consistent with more recent determinations that include small-scale clumping. Note to coauthors: I wonder if we should add a sentence about the CNO abundances and how the sub-solar net abundances make the mass-loss rate determination about a factor of two higher than we'd flnd if we assumed a solar C + N + O.
