There is extensive evidence that the line-driven stellar winds of OB-type stars are not the stead... more There is extensive evidence that the line-driven stellar winds of OB-type stars are not the steady, smooth outflows envisioned in classical models, but instead exhibit extensive structure and variability on a range of temporal and spatial scales. This thesis examines the possible role of stellar magnetic fields in forming large-scale wind structure based on numerical magnetohydrodynamic (MHD) simulations of the
Proceedings of The International Astronomical Union, 2003
We present numerical magnetohydrodynamic simulations of the effect of stellar dipole magnetic fie... more We present numerical magnetohydrodynamic 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
We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with ... more We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with a dipole magnetic field aligned to the star's rotation axis. For moderate to strong fields, much wind outflow is initially along closed magnetic loops that nearly corotate as a solid body with the underlying star, thus providing a torque that results in an effective angular momentum
We present numerical magnetohydrodynamic simulations of the effect of weak magnetic field on idea... more We present numerical magnetohydrodynamic simulations of the effect of weak magnetic field on idealized Standing Accretion Shocks (SAS) that arise in classical core-collapse supernovae, wherein an expanding shock front stalls at a radius of order 200 km and remains quite stationary for a relatively long period of time (300 ms or more). In the models we present here, specific angular momentum is fixed at the outer boundary where outer core material is free-falling onto the stalled accretion shock. To ensure that the initial seed magnetic field has a poloidal component, a necessary condition for the possible growth of magneto-rotational instability (MRI), we use a weak dipole magnetic field. Our fully dynamical simulations of this interaction of rotation and the magnetic field in SAS in the context of core-collapse supernovae, show a substantial exponential growth of the magnetic field energy that can exceed 8 orders of magnitude, and which dominates the linear growth process of ``fiel...
High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the f... more High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce X-rays on very young stars. For zeta Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced
ABSTRACT All main sequence stars lose mass via stellar winds. The winds of cool stars like the su... more ABSTRACT All main sequence stars lose mass via stellar winds. The winds of cool stars like the sun are driven by gas pressure gradient. However, the winds of hot massive stars which tend to be luminous are driven by emitted by the star radiation pressure. Mass loss from such winds are significantly higher. In this article, I describe the nature of such radiatively driven winds and show how they interact with rotation and magnetic fields leading to stellar spindown and large-scale disk-like structures. In particular, I show that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, "wind magnetic confinement parameter", η ∗ ({=} B_{eq}2 R_{ast}2/{dot{M}} v_{infty}), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.
Proceedings of the International Astronomical Union, 2014
ABSTRACT Early-type stars are well-known to be sources of soft X-rays. However, this high-energy ... more ABSTRACT Early-type stars are well-known to be sources of soft X-rays. However, this high-energy emission can be supplemented by bright and hard X-rays when magnetically confined winds are present. In an attempt to clarify the systematics of the observed X-ray properties of this phenomenon, a large series of Chandra and XMM observations was analyzed, over 100 exposures of 60% of the known magnetic massive stars listed recently by Petit et al. (2013). It is found that the X-ray luminosity is strongly correlated with mass-loss rate, in agreement with predictions of magnetically confined wind models, though the predictions of higher temperature are not always verified. We also investigated the behaviour of other X-ray properties (absorption, variability), yielding additional constraints on models. This work not only advances our knowledge of the X-ray emission of massive stars, but also suggests new observational and theoretical avenues to further explore magnetically confined winds.
There is extensive evidence that the line-driven stellar winds of OB-type stars are not the stead... more There is extensive evidence that the line-driven stellar winds of OB-type stars are not the steady, smooth outflows envisioned in classical models, but instead exhibit extensive structure and variability on a range of temporal and spatial scales. This thesis examines the possible role of stellar magnetic fields in forming large-scale wind structure based on numerical magnetohydrodynamic (MHD) simulations of the
Proceedings of The International Astronomical Union, 2003
We present numerical magnetohydrodynamic simulations of the effect of stellar dipole magnetic fie... more We present numerical magnetohydrodynamic 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
We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with ... more We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with a dipole magnetic field aligned to the star's rotation axis. For moderate to strong fields, much wind outflow is initially along closed magnetic loops that nearly corotate as a solid body with the underlying star, thus providing a torque that results in an effective angular momentum
We present numerical magnetohydrodynamic simulations of the effect of weak magnetic field on idea... more We present numerical magnetohydrodynamic simulations of the effect of weak magnetic field on idealized Standing Accretion Shocks (SAS) that arise in classical core-collapse supernovae, wherein an expanding shock front stalls at a radius of order 200 km and remains quite stationary for a relatively long period of time (300 ms or more). In the models we present here, specific angular momentum is fixed at the outer boundary where outer core material is free-falling onto the stalled accretion shock. To ensure that the initial seed magnetic field has a poloidal component, a necessary condition for the possible growth of magneto-rotational instability (MRI), we use a weak dipole magnetic field. Our fully dynamical simulations of this interaction of rotation and the magnetic field in SAS in the context of core-collapse supernovae, show a substantial exponential growth of the magnetic field energy that can exceed 8 orders of magnitude, and which dominates the linear growth process of ``fiel...
High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the f... more High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce X-rays on very young stars. For zeta Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced
ABSTRACT All main sequence stars lose mass via stellar winds. The winds of cool stars like the su... more ABSTRACT All main sequence stars lose mass via stellar winds. The winds of cool stars like the sun are driven by gas pressure gradient. However, the winds of hot massive stars which tend to be luminous are driven by emitted by the star radiation pressure. Mass loss from such winds are significantly higher. In this article, I describe the nature of such radiatively driven winds and show how they interact with rotation and magnetic fields leading to stellar spindown and large-scale disk-like structures. In particular, I show that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, "wind magnetic confinement parameter", η ∗ ({=} B_{eq}2 R_{ast}2/{dot{M}} v_{infty}), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.
Proceedings of the International Astronomical Union, 2014
ABSTRACT Early-type stars are well-known to be sources of soft X-rays. However, this high-energy ... more ABSTRACT Early-type stars are well-known to be sources of soft X-rays. However, this high-energy emission can be supplemented by bright and hard X-rays when magnetically confined winds are present. In an attempt to clarify the systematics of the observed X-ray properties of this phenomenon, a large series of Chandra and XMM observations was analyzed, over 100 exposures of 60% of the known magnetic massive stars listed recently by Petit et al. (2013). It is found that the X-ray luminosity is strongly correlated with mass-loss rate, in agreement with predictions of magnetically confined wind models, though the predictions of higher temperature are not always verified. We also investigated the behaviour of other X-ray properties (absorption, variability), yielding additional constraints on models. This work not only advances our knowledge of the X-ray emission of massive stars, but also suggests new observational and theoretical avenues to further explore magnetically confined winds.
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Papers by Asif Ud-doula