Browsing by Author "Owocki, Stanley P."
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Item 3D MHD models of the centrifugal magnetosphere from a massive star with an oblique dipole field Get access Arrow(Monthly Notices of the Royal Astronomical Society, 2023-02-01) ud-Doula, Asif; Owocki, Stanley P.; Russell, Christopher; Gagné, Marc; Daley-Yates, SimonWe present results from new self-consistent 3D magnetohydrodynamics (MHD) simulations of the magnetospheres from massive stars with a dipole magnetic axis that has a non-zero obliquity angle (β) to the star’s rotation axis. As an initial direct application, we compare the global structure of co-rotating discs for nearly aligned (β = 5°) versus half-oblique (β = 45°) models, both with moderately rapid rotation (∼0.5 critical). We find that accumulation surfaces broadly resemble the forms predicted by the analytical rigidly rotating magnetosphere model, but the mass buildup to near the critical level for centrifugal breakout against magnetic confinement distorts the field from the imposed initial dipole. This leads to an associated warping of the accumulation surface towards the rotational equator, with the highest density concentrated in wings centred on the intersection between the magnetic and rotational equators. These MHD models can be used to synthesize rotational modulation of photometric absorption and H α emission for a direct comparison with observations.Item An ‘analytic dynamical magnetosphere’ formalism for X-ray and optical emission from slowly rotating magnetic massive stars(Oxford University Press on behalf of the Royal Astronomical Society., 2016-08-01) Owocki, Stanley P.; ud-Doula, Asif; Sundqvist, Jon O.; Petit, Veronique; Cohen, David H.; Townsend, Richard H. D.; Stanley P. Owocki, Asif ud-Doula, Jon O. Sundqvist, Veronique Petit, David H. Cohen, and Richard H. D. Townsend; Owocki, Stanley P.Slowly rotating magnetic massive stars develop ‘dynamical magnetospheres’ (DMs), characterized by trapping of stellar wind outflow in closed magnetic loops, shock heating from collision of the upflow from opposite loop footpoints, and subsequent gravitational infall of radiatively cooled material. In 2D and 3D magnetohydrodynamic (MHD) simulations, the interplay among these three components is spatially complex and temporally variable, making it difficult to derive observational signatures and discern their overall scaling trends. Within a simplified, steady-state analysis based on overall conservation principles, we present here an ‘analytic dynamical magnetosphere’ (ADM) model that provides explicit formulae for density, temperature, and flow speed in each of these three components – wind outflow, hot post-shock gas, and cooled inflow – as a function of colatitude and radius within the closed (presumed dipole) field lines of the magnetosphere. We compare these scalings with time-averaged results from MHD simulations, and provide initial examples of application of this ADM model for deriving two key observational diagnostics, namely hydrogen H α emission line profiles from the cooled infall, and X-ray emission from the hot post-shock gas. We conclude with a discussion of key issues and advantages in applying this ADM formalism towards derivation of a broader set of observational diagnostics and scaling trends for massive stars with such dynamical magnetospheres.Item The spectral temperature of optically thick outflows with application to light echo spectra from η Carinae’s giant eruption(Oxford University Press on behalf of the Royal Astronomical Society, 2016-07-07) Owocki, Stanley P.; Shaviv, Nir J.; Stanley P. Owocki and Nir J. Shaviv; Owocki, Stanley P.The detection by Rest et al. of light echoes from η Carinae has provided important new observational constraints on the nature of its 1840s era giant eruption. Spectra of the echoes suggest a relatively cool spectral temperature of about 5500 K, lower than the lower limit of about 7000 K suggested in the optically thick wind-outflow analysis of Davidson. This has led to a debate about the viability of this steady wind model relative to alternative, explosive scenarios. Here we present an updated analysis of the wind-outflow model using newer low- temperature opacity tabulations and accounting for the stronger mass-loss implied by the > 10 M mass now inferred for the Homunculus. A major conclusion is that, because of the sharp drop in opacity due to recombination loss of free electrons for T < 6500 K, a low temperature of about 5000 K is compatible with, and indeed expected from, a wind with the extreme mass-loss inferred for the eruption. Within a spherical grey model in radiative equilibrium, we derive spectral energy distributions for various assumptions for the opacity variation of the wind, providing a basis for comparisons with observed light echo spectra. The scaling results here are also potentially relevant for other highly optically thick outflows, including those from classical novae, giant eruptions of luminous blue variables and supernovae Type IIn precursors. A broader issue therefore remains whether the complex, variable features observed from such eruptions are better understood in terms of a steady or explosive paradigm, or perhaps a balance of these idealizations.