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تسجيل مستخدم جديدModelling the photometric variability of magnetic massive stars with the Analytical Dynamical Magnetosphere model
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In this paper, we investigate the photometric variability of magnetic O-type stars. Such stars possess oblique, predominantly dipolar magnetic fields that confine their winds roughly axisymmetrically about the magnetic equator, thus forming a magnetosphere. We interpret their photometric variability as phase-dependent magnetospheric occultations. For massive star winds dominated by electron scattering opacity in the optical and NIR, we can compute synthetic light curves from simply knowing the magnetospheres mass density distribution. We exploit the newly-developed Analytical Dynamical Magnetosphere model (ADM) in order to obtain the predicted circumstellar density structures of magnetic O-type stars. The simplicity in our light curve synthesis model allows us to readily conduct a parameter space study. For validation purposes, we first apply our algorithm to HD 191612, the prototypical Of?p star. Next, we attempt to model the photometric variability of the Of?p-type stars identified in the Magellanic Clouds using OGLE photometry. We evaluate the compatibility of the ADM predictions with the observed photometric variations, and discuss the magnetic field properties that are implied by our modelling.
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Massive star winds are important contributors to the energy, momentum and chemical enrichment of the interstellar medium. Strong, organized and predominantly dipolar magnetic fields have been firmly detected in a small subset of massive O-type stars.
Magnetic massive stars are known to exhibit phase-locked variability of numerous observable quantities that is hypothesized to arise due to the presence of an obliquely rotating magnetosphere formed via the magnetic confinement of their strong outflowing winds. Analyzing the observed modulations of magnetic O-type stars is thus a key step towards the better understanding of the physical processes that occur within their magnetospheres. The dynamical processes that lead to the formation of a magnetosphere are formally solved utilizing complex MHD simulations. Recently, an Analytic Dynamical Magnetosphere (ADM) model has been developed that can quickly be employed to compute the time-averaged density, temperature and velocity gradients within a dynamical magnetosphere. Here, we exploit the ADM model to compute photometric and polarimetric observables of magnetic Of?p stars, to test geometric models inferred from magnetometry. We showcase important results on the prototypical Of?p-type star HD 191612, that lead to a better characterization of massive star wind and magnetic properties.
High-precision photometric observations have revealed ubiquitous stochastic low-frequency photometric variability in early type stars. It has been suggested that this variability arises due to either subsurface convection or internal gravity waves la
unched by the convective core. Here we show that relevant properties of convection in subsurface convective layers correlate very well with the timescale and amplitude of stochastic low-frequency photometric variability, as well as with the amplitude of macroturbulence. We suggest that low-frequency, stochastic photometric variability and surface turbulence in massive stars are caused by the the presence of subsurface convection. We show that an explanation for the observed surface photometric variability and macroturbulence relying on convective core driven internal gravity waves encounters a number of difficulties and seems unlikely to be able to explain the observed trends.
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 gravit
ational 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-alpha 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 toward derivation of a broader set of observational diagnostics and scaling trends for massive stars with such dynamical magnetospheres.
Galactic stars belonging to the Of?p category are all strongly magnetic objects exhibiting rotationally modulated spectral and photometric changes on timescales of weeks to years. Five candidate Of?p stars in the Magellanic Clouds have been discovere
d, notably in the context of ongoing surveys of their massive star populations. Here we describe an investigation of their photometric behaviour, revealing significant variability in all studied objects on timescales of one week to more than four years, including clearly periodic variations for three of them. Their spectral characteristics along with these photometric changes provide further support for the hypothesis that these are strongly magnetized O stars, analogous to the Of?p stars in the Galaxy.
The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observ
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