Anisotropic Ionizing Illumination from an M-type Pre-main-sequence Star, DM Tau

The powerful, high-energy magnetic activities of young stars play important roles in the magnetohydrodynamics in the innermost parts of the protoplanetary disks. In addition, the associated UV and X-ray emission dictates the photochemistry; moreover, the corona activities can affect the atmosphere of a newborn extrasolar planet. How the UV and X-ray photons are generated and how they illuminate the disks are not well understood. Here we report the analyses of the optical and infrared (OIR) photometric monitoring observations and the high angular resolution centimeter-band images of the low-mass (M1-type) pre-main-sequence star DM Tau. We found that the OIR photometric light curves present periodic variations, which suggests that the host young star is rotating in the same direction as the natal disk and is hosting at least one giant cold spot. In addition, we resolved that the ionized gas in the DM Tau disk is localized and its spatial distribution is varying with time. All the present observations can be coherently interpreted, if the giant cold spot is the dominant anisotropic UV and/or X-ray source that illuminates the ambient cone-like region. These results indicate that a detailed theoretical model of the high-energy protostellar emission is essential in understanding the space weather around extrasolar planets and the origin of life.