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

By A. ud-Doula, S. Owocki. 2002

468 citations recorded by the source corpus.

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/Ṁv∞), 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1a58225a555c5d2d455574af558d3c4025aa8da9

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Citations in the corpus

• The surprising magnetic topology of τ Sco: fossil remnant or dynamo output? (2006, unknown): “In the now-standard picture, initially proposed by Ba-bel & Montmerle (1997) and further investigated by Donati et al. (2001, 2002), ud-Doula & Owocki (2002), Townsend & Owocki (2005) and Gagn´e et al. (2005b,a), the magnetic field is assumed to be dipolar; the dense wind coming from each magnetic…”
• Rotating massive main-sequence stars: II. Simulating a population of LMC early B-type stars as a test of rotational mixing (2011, background): “…et al. 2007; Puls et al. 2008), intermittent discrete absorption components in UV absorption lines (Prinja & Howarth 1988; Kaper et al. 1997), micro- and macro-turbulence (Cantiello et al. 2009; Aerts et al. 2009), and non-thermal X-ray emission (Babel & Montmerle 1997; ud-Doula & Owocki 2002).”
• Chandra HETGS Multiphase Spectroscopy of the Young Magnetic O Star θ1 Orionis C (2005, methodology): “One of the key results of ud-Doula & Owocki (2002) is that the overall degree to which the wind is influenced by the magnetic field depends largely on a single parameter, the magnetic confinement parameter, η⋆, defined by equation 1.”
• THE MASS-DEPENDENCE OF ANGULAR MOMENTUM EVOLUTION IN SUN-LIKE STARS (2014, background): “The exponent factor m is determined primarily by the magnetic field geometry and wind acceleration profile (Réville et al. 2014) and likely falls in the range m =0.20–0.25 (Washimi & Shibata 1993; Matt & Pudritz 2008; Ud-Doula et al. 2009; Pinto et al. 2011; Matt et al. 2012).”
• An investigation of the close environment of β Cephei with the VEGA/CHARA interferometer (2010, background): “…work based on dynamical modeling of the wind-magnetic-field interaction, however, indicates that the thick ring (in the X-ray domain) predicted by Babel & Montmerle (1997) would be unlikely to form around a star such as β Cep (Gagné et al. 2005; ud-Doula & Owocki 2002; Townsend & Owocki 2005).”
• Magnetic Fields of Nondegenerate Stars (2009, methodology): “MHD numerical simulations have been carried out for various cases, confirmi g and extending the predictions of the earlier models (ud-Doula & Owocki 2002; Owocki & ud-Doula 2004; Townsend et al. 2005; ud-Doula et al. 2006; Townsend et al. 2007; Ud-Doula et al. 2008).”
• Mass Loss: Its Effect on the Evolution and Fate of High-Mass Stars (2014, background): “…discrete absorption components (Cranmer & Owocki 1996, Fullerton et al. 1997, Howarth et al. 1995, Massa et al. 1995), or axisymmetric winds due to rapid rotation (Owocki et al. 1996), and a wide array of magnetically induced inhomogeneities (Townsend et al. 2005, ud-Doula & Owocki 2002).”
• Grids of stellar models with rotation - II. WR populations and supernovae/GRB progenitors at Z = 0.014 (2012, background): “Conservation of angular momentum will exert a torque and thus slow down the star (Ud-Doula et al. 2009)10.”
• The MiMeS survey of Magnetism in Massive Stars : magnetic analysis of the O-type stars (2016, background): “…and evolution, and the significance of magnetic fields in determining their wind structure (e.g. Shore & Brown 1990; Babel & Montmerle 1997; ud-Doula & Owocki 2002; Townsend & Owocki 2005), rotation (e.g. ud-Doula et al. 2009; Mikulášek et al. 2008; Townsend 2010), and evolution (e.g.…”
• THE ROTATION RATES OF MASSIVE STARS: THE ROLE OF BINARY INTERACTION THROUGH TIDES, MASS TRANSFER, AND MERGERS (2012, methodology): “Our models do not account for effects of magnetic fields on angular momentum loss by stellar winds (e.g., Ud-Doula et al. 2009).”
• Massive pulsating stars observed by BRITE-Constellation - I. The triple system β Centauri (Agena) (2016, background): “Nevertheless, it is reasonable to suspect that its magnetic nature has contributed to its slow (relative to the primary) rotation (e.g. ud-Doula et al. 2009).”
• A rigidly rotating magnetosphere model for circumstellar emission from magnetic OB stars (2004, methodology): “This is what ud-Doula & Owocki (2002) found in their MHD simulations of wind outflow from a nonrotating star, in the case of a moderately strong magnetic field (their η∗ = 10).”