The likelihood of hosting undetected binaries and planets in transition discs

Protoplanetary discs with cavities, also known as ‘’transition discs'', constitute ~10% of the brightest sources in the sky and have been previously suggested to host an undetected stellar or planetary companion, responsible for carving the cavity. In this talk, I will present a novel approach to quantify the likelihood that a cavity-opening (sub)stellar companion — such as a binary star or a massive planet — remains undetected because it is too “close” to the central star (small projected separation) or faint to be resolved. To do this, we generate a sample of stellar/planetary companions, randomly oriented in the plane of the sky, assuming a distribution of their eccentricity, mass ratio, and accounting for the distribution of orbital phases (companions spend more time at apocentre than at pericentre). On this sample, we calculate the probability that a companion mass with a given semi-major axis appears with a certain projected separation in the plane of the sky. We apply a conservative disc truncation model on the generated sample to predict the cavity size each companion can carve, and compare it to the projected separation, obtaining the distribution of d/a_cav, i.e., the distribution of the ratio between companion projected separations and cavity size. Coupling this information with observational detection limits, we can quantify the likelihood that a companion with a mass below a certain threshold (set by observational upper limits), responsible for opening the observed cavity, remains undetected. Applying this method to known transition discs, we conclude that the presence of an undetected stellar binary companion can be safely ruled out in most systems, whereas undetected planets cannot yet be excluded.