Scattered Light Signatures of Magnetic Accretion in Classical T Tauri Stars

Abstract

The magnetic accretion model as applied to T Tauri stars predicts the formation of hot spots or rings on the stellar surface where accreting disk material impacts the stellar surface at or near the magnetic poles. When the magnetic poles are not aligned with the stellar rotation axis, or the hot spots are nonuniform, brightness variations arise as the hot spots rotate into and out of view. We have investigated the effects on the scattered light images of a T Tauri star-plus-disk system of nonaxisymmetric illumination of the circumstellar disk by hot starspots. We find that changes in the scattered light pattern occur during a stellar rotation period. These changes are observable with current high-resolution imagery and provide a further observational test of the magnetic accretion theory. The observed brightening and dimming of HH 30's scattered light disk may be explained by such a model, although further synoptic observations are required to confirm this.

Moving shadows on the dusty disks of young stars

“…Asymmetry was present in 1998, but was absent in the 1994 and 1995 images. Wood & Whitney (1998) consider the possibility that such asymmetry could be caused by a variable, asymmetric illumination of the CS disk by a rotating T Tauri star with spots on its surface. Cool spots could be the result of a large-scale magnetic field, and hot spots could be caused by accretion activity controlled by the stellar magnetic field.…”

Modelling the photopolarimetric variability of AA Tau

“…For some warped disc models, the azimuthal extent of the warp may mimic scattered light images as a result of hot star spots, but multiwavelength photometry can discriminate models: star spots yield chromatic variability, whereas a disc warp yields achromatic photometric variability. Notice that for edge‐on viewing the time sequence images for warped discs and discs illuminated by a spotted star (Wood & Whitney 1998) are very similar, and multiwavelength photopolarimetry is required to break the degeneracy.…”

“…7 shows a series of close-up scattered light images of the warp occulting the star at a viewing angle of 75 • . These models may be compared with hot star spot models (Wood & Whitney 1998), which show a lighthouse effect of a bright pattern sweeping around show the shadowed area caused by the warp occulting the star moving round the disc. To overcome the large dynamic range between starlight and scattered light in the disc, the images are presented on a one-tenth root (square root for the edge-on disc) stretch.…”

“…For the edge‐on disc around HH 30 IRS, variability has been observed in both Hubble Space Telescope ( HST ) images (Burrows et al 1996; Stapelfeldt et al 1999; Cotera et al 2001) and ground based VRI photometry (Wood et al 2000). The variability in HH 30 IRS has been modelled as a result of hotspots (Wood & Whitney 1998; Wood et al 2000) and a disc warp (Stapelfeldt et al 1999), but no period has so far been determined. Photopolarimetric variability has recently been observed in the cTTS AA Tauri (Bouvier et al 1999, 2003) and has been interpreted as the result of occultation of a star by a warp in the inner regions of a circumstellar disc.…”

“…In this paper, we extend our Monte Carlo photopolarimetry simulations of systems with hot star spots (Wood et al 1996; Wood & Whitney 1998; Stassun & Wood 1999) to include the effects of disc warps. We construct a warped disc model that reproduces the photopolarimetric variability of AA Tau.…”