Asif ud-Doula

Dynamical Simulations of Magnetically Channeled Line-driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow

We present numerical magnetohydrodynamic (MHD) simulations of the effect of stellar dipole magnetic fields on line-driven wind outflows from hot, luminous stars. Unlike previous fixed-field analyses, the simulations here take full account of the dynamical competition between field and flow and thus apply to a full range of magnetic field strength and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless wind magnetic confinement parameter η* (= BR/v∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement, η* ≤ 1, the field is fully opened by the wind outflow, but nonetheless, for confinements as small as η* = 1/10 it can have a significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement, η* > 1, the magnetic field remains closed over a limited range of latitude and height about the equatorial surface, but eventually is opened into a nearly radial configuration at large radii. Within closed loops, the flow is channeled toward loop tops into shock collisions that are strong enough to produce hard X-rays, with the stagnated material then pulled by gravity back onto the star in quite complex and variable inflow patterns. Within open field flow, the equatorial channeling leads to oblique shocks that are again strong enough to produce X-rays and also lead to a thin, dense, slowly outflowing disk at the magnetic equator. The polar flow is characterized by a faster-than-radial expansion that is more gradual than anticipated in previous one-dimensional flow tube analyses and leads to a much more modest increase in terminal speed (less than 30%), consistent with observational constraints. Overall, the results here provide a dynamical groundwork for interpreting many types of observations—e.g., UV line profile variability, redshifted absorption or emission features, enhanced density-squared emission, and X-ray emission—that might be associated with perturbation of hot-star winds by surface magnetic fields.

Simulating Radiating and Magnetized Flows in Multiple Dimensions with ZEUS-MP

This paper describes ZEUS-MP, a multi-physics, massively parallel, message-passing implementation of the ZEUS code. ZEUS-MP differs significantly from the thoroughly documented ZEUS-2D code, the completely undocumented (in peer-reviewed literature) ZEUS-3D code, and a marginally documented “version 1” of ZEUS-MP first distributed publicly in 1999. ZEUS-MP offers an MHD algorithm which is better suited for multidimensional flows than the ZEUS-2D module by virtue of modifications to the Method of Characteristics scheme first suggested by Hawley & Stone (1995). This MHD module is shown to compare quite favorably to the TVD scheme described by Ryu et al. (1998). ZEUS-MP is the first publicly-available ZEUS code to allow the advection of multiple chemical (or nuclear) species. Radiation hydrodynamic simulations are enabled via an implicit flux-limited radiation diffusion (FLD) module. The hydrodynamic, MHD, and FLD modules may be used, singly or in concert, in one, two, or three space dimensions. Additionally, so-called “1.5-D” and “2.5-D” grids, in which the “half-D” denotes a symmetry axis along which a constant but non-zero value of velocity or magnetic field is evolved, are supported. Self gravity may be included either through the assumption of a GM/r potential or a solution of Poisson’s equation using one of three linear solver packages (conjugategradient, multigrid, and FFT) provided for that purpose. Point-mass potentials are also supported. Because ZEUS-MP is designed for large simulations on parallel computing platforms, considerable attention is paid to the parallel performance characteristics of each module in the code. Strong-scaling tests involving pure hydrodynamics (with and without self-gravity), MHD, and RHD are performed in which large problems (256 3 zones) are distributed among as many as 1024 processors of an IBM SP3. Parallel efficiency is a strong function of the amount of communication required between processors in a given algorithm, but all modules are shown to scale well on up to 1024 processors for the chosen fixed problem size. Subject headings: hydrodynamics – methods:numerical – methods:parallel – MHD – radiative transfer

Chandra HETGS Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C

We report on four Chandra grating observations of the oblique magnetic rotator � 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 hotplasma. We investigate these diagnostics as a function of viewing angle andanalyzetheminconjunctionwithnewMHDsimulationsofthemagneticallychanneledwindshockmechanism on � 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. Subject headingg stars: early-type — stars: individual (HD 37022) — stars: magnetic fields — stars: mass loss — stars: rotation — stars: winds, outflows — X-rays: stars Online material: color figure

Dynamical simulations of magnetically channelled line‐driven stellar winds – III. Angular momentum loss and rotational spin‐down

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 Alfven radius. However, a key distinction here is that for a dipole field, this Alfven 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.

A magnetic confinement versus rotation classification of massive-star magnetospheres

Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement (which sets the Alfv´ en radius RA) and stellar rotation (which sets the Kepler co-rotation radius RK) provides a useful organization of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with Teff 16 kK. Using these parameters, we locate the stars in the magnetic confinement–rotation diagram, a log–log plot of RK versus RA. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with RA > RK ,v ersusdynamical magnetospheres (DM), with RK > RA. We show how key observational diagnostics, like the presence and characteristics of Hα emission, depend on a star’s position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with Hα emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall time-scale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown time-scales. Finally, we discuss how other observational diagnostics, e.g. variability of UV wind lines or X-ray emission, relate to the inferred magnetic properties of these stars, and summarize prospects for future developments in our understanding of massive-star magnetospheres.

AN INTRODUCTION TO THE CHANDRA CARINA COMPLEX PROJECT

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatorys Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carinas regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandras unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carinas massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this special issue and that present detailed catalogs, methods, and science results.

The MiMeS survey of magnetism in massive stars: introduction and overview

The Magnetism in Massive Stars (MiMeS) survey represents a highprecision systematic search for magnetic fields in hot, massive OB stars. To date, MiMeS Large Programs (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO3.6) and associated PI programs (FORS@VLT) have yielded nearly 1200 circular spectropolarimetric observations of over 350 OB stars. Within this sample, 20 stars are detected as magnetic. Follow-up observations of new detections reveals (i) a large diversity of magnetic properties, (ii) ubiquitous evidence for magnetic wind confinement in optical spectra of all magnetic O stars, and (iii) the presence of strong, organized magnetic fields in all known Galactic Of?p stars, and iv) a complete absence of magnetic fields in classical Be stars.

Backbone Dipoles Generate Positive Potentials in all Proteins: Origins and Implications of the Effect

Asymmetry in packing the peptide amide dipole results in larger positive than negative regions in proteins of all folding motifs. The average side chain potential in 305 proteins is 109 +/- 30 mV (2. 5 +/- 0.7 kcal/mol/e). Because the backbone has zero net charge, the non-zero potential is unexpected. The larger oxygen at the negative and smaller proton at the positive end of the amide dipole yield positive potentials because: 1) at allowed phi and psi angles residues come off the backbone into the positive end of their own amide dipole, avoiding the large oxygen; and 2) amide dipoles with their carbonyl oxygen surface exposed and amine proton buried make the protein interior more positive. Twice as many amides have their oxygens exposed than their amine protons. The distribution of acidic and basic residues shows the importance of the bias toward positive backbone potentials. Thirty percent of the Asp, Glu, Lys, and Arg are buried. Sixty percent of buried residues are acids, only 40% bases. The positive backbone potential stabilizes ionization of 20% of the acids by >3 pH units (-4.1 kcal/mol). Only 6.5% of the bases are equivalently stabilized by negative regions. The backbone stabilizes bound anions such as phosphates and rarely stabilizes bound cations.

New findings on the prototypical Of?p stars

Aims. In recent years several in-depth investigations of the three prototypical Of?p stars were undertaken. These multiwavelength studies revealed the peculiar properties of these objects (in the X-rays as well as in the optical): magnetic fields, periodic line profile variations, recurrent photometric changes. However, many questions remain unsolved. Methods. To clarify some of the properties of the Of?p stars, we have continued their monitoring. A new xmm-Newton observation and two new optical datasets were obtained. Results. Additional information about the prototypical Of?p trio has been found. HD 108 has now reached its quiescent, minimumemission state for the first time in 50–60 yr. The echelle spectra of HD 148937 confirm the presence of the 7d variations in the Balmer lines and reveal similar periodic variations (though of lower amplitudes) in the He i λ 5876 and He ii λ 4686 lines, underlining its similarities with the other two prototypical Of?p stars. The new xmm-Newton observation of HD 191612 was taken at the same phase in the line modulation cycle, but at a different orbital phase from previous data. It clearly shows that the X-ray emission of HD 191612 is modulated by the 538d period and not by the orbital period of 1542d – it is thus not of colliding-wind origin. The phenomenon responsible for the optical changes appears also at work in the high-energy domain. There are problems however: our MHD simulations of the wind magnetic confinement predict both a harder X-ray flux of a much larger strength than what is observed (the modelled differential emission measure peaks at 30–40 MK, whereas the observed one peaks at 2 MK) and narrow lines (hot gas moving with velocities of 100–200 km s −1 , whereas the observed full width at half maximum is ∼2000 km s −1 ).

CENTRIFUGAL BREAKOUT OF MAGNETICALLY CONFINED LINE-DRIVEN STELLAR WINDS

We present two-dimensional MHD simulations of the radiatively driven outflow from a rotating hot star with a dipole magnetic field aligned with the stars rotation axis. We focus primarily on a model with moderately rapid rotation (half the critical value) and also a large magnetic confinement parameter, η* ≡ BR/V∞ = 600. The magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk extending from near the Kepler corotation radius outward. Even with fine-tuning at lower magnetic confinement, none of the MHD models produce a stable Keplerian disk. Instead, material below the Kepler radius falls back onto the stellar surface, while the strong centrifugal force on material beyond the corotation escape radius stretches the magnetic loops outward, leading to the episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperatures of nearly 108 K, high enough to emit hard (several keV) X-rays. Such centrifugal mass ejection represents a novel mechanism for driving magnetic reconnection and seems a very promising basis for modeling X-ray flares recently observed in rotating magnetic Bp stars like σ Ori E.