Discovery of a Scattering Disk around the Low-Mass T Tauri Star FN Tauri

We have discovered an optically thick, nearly face-on circumstellar disk around a single M5 classical T Tauri star, FN Tau, using the coronagraphic imager CIAO on the Subaru telescope. This is the least massive T Tauri star whose circumstellar structure has been directly imaged as a scattering disk. The surface brightness in the H band declines as a power law of r(-2.5 +/- 0.1) (110 AU <= r <= 260 AU), suggesting that the disk is flared. The disk morphology appears to be relatively featureless except for an azimuthal asymmetry in the surface brightness, indicating that the disk is not perfectly face-on but is slightly tilted. The disk mass, derived from a simple disk emission model with previous photometry at optical to millimeter wavelengths, is similar to 0.007 M-circle dot, 6% of the mass of the central star. The disk around FN Tau is one of the best targets for disk and planet formation studies around M stars.