Disk Truncation Radius Research Articles

Magnetic Accretion and Photopolarimetric Variability in Classical T Tauri Stars

We employ a Monte Carlo radiation transfer code to investigate the multiwavelength photopolarimetric variability arising from a spotted T Tauri star surrounded by a dusty circumstellar disk. Our aim is to assess the ability of the magnetic accretion model to explain the observed photopolarimetric variability of classical T Tauri stars and to identify potentially useful observational diagnostics of T Tauri star/disk/spot parameters. We model a range of spot sizes, spot latitudes, inner disk truncation radii, and system inclination angles, as well as multiple disk and spot geometries. We find that the amplitude, morphology, and wavelength-dependence of the photopolarimetric variability predicted by our models are generally consistent with existing observations; a flared disk geometry is required to reproduce the largest observed polarization levels and variations. Our models can further explain stochastic polarimetric variability if unsteady accretion is invoked, in which case irregular (but correlated) photometric variability is predicted, in agreement with observations. We find that variability in percent polarization is by itself an unreliable diagnostic, because certain system geometries do not produce any variability in linear polarization (contrary to the commonly held notion that hot spots will necessarily produce periodic polarimetric variability). Observations of variability in polarization position angle, however, could provide useful constraints on system inclination. The observation of wavelength-dependent polarization position angles, attributed by some to interstellar effects, is naturally explained by our models. Certain system geometries yield peculiar photometric light curve morphologies, the observation of which could also serve to constrain system inclination. We do not find useful diagnostics of disk truncation radius, nor do we find significant differences when we model the spots as rings. We also investigate the reliability of modeling spot parameters via analytic fits to multiband photometric variations. We find that commonly used analytic models consistently recover input model parameters but that inferred spot temperatures are more sensitive to uncertainties in the photometric data than previous modeling would suggest.

A Laboratory for Magnetized Accretion Disk Model: Ultraviolet and X‐Ray Emission from Cataclysmic Variable GK Persei

We analyze the ultraviolet spectrum of the cataclysmic variable GK Per at maximum light. The flat ultraviolet spectrum in this system requires a truncated inner accretion disk and an unusually flat radial temperature profile. This requirement is not satisfied by any nonmagnetic steady or nonsteady disk model. We consider a magnetized accretion disk model to explain the ultraviolet spectrum. The available data on the white dwarf spin and possible quasi-periodic oscillations constrain the magnetic field, B*, and the disk accretion rate, , to lie along a well-defined spin-equilibrium condition (/1017 g s−1) ~ 100(B*/107 G)2. Our self-consistent treatment of the magnetic torque on the disk flattens the disk temperature distribution outside the disk truncation radius. This modified temperature distribution is too steep to explain the UV spectrum for reasonable field strengths. X-ray heating is a plausible alternative to magnetic heating in GK Per. We estimate that the disk intercepts ~5% of the accretion energy in outburst, which results in an extra disk luminosity of ~5-10 L☉. Model spectra of optically thick disks are too blue to match observations. The UV spectrum of an optically thick disk with an optically thin, X-ray heated corona resembles the observed spectrum. The X-ray luminosity observed during the outburst indicates < 1018 g s-1, which is a factor of 10 lower than that required to explain the ultraviolet luminosity. Radiation drag on material flowing inward along the accretion column lowers the shock temperature and reduces the X-ray luminosity. Most of the accretion energy is then radiated at extreme ultraviolet wavelengths.