Demographics of transition discs in Ophiuchus and Taurus

Abstract

Transition disc systems are young stars that appear to be on the verge of dispersing their protoplanetary discs. We explore the nature of these systems by comparing the stellar accretion rates M˙ * and disc masses Md of transition discs and normal T Tauri stars in Taurus and Ophiuchus. After controlling for the known dependences of M˙ * and Md on age, M˙ * on stellar mass and Md on the presence of stellar or substellar companions, we find that the normal T Tauri stars show a trend of M˙ * increasing with Md. The transition discs tend to have higher average disc masses than normal T Tauri stars as well as lower accretion rates than normal T Tauri stars of the same disc mass. These results are most consistent with the interpretation that the transition discs have formed objects massive enough to alter the accretion flow, i.e. single or multiple giant planets. Several Ophiuchus T Tauri stars that are not known transition disc systems also have very low accretion rates for their disc masses. We speculate on the possible nature of these sources.