Astrometric view of companions in the inner dust cavities of protoplanetary discs

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Context . Protoplanetary discs with inner dust cavities (often referred to as ‘transition discs’) are potential signposts of planet formation. However, few companions have been identified within these cavities, and the role of companions in shaping them remains unclear. Aims . We used Gaia astrometry to search for planetary and stellar companions in a sample of 98 transition discs, assessing the occurrence rate of such companions and their potential influence on cavity formation. Methods . For the 98 young stellar objects (YSOs) with inner dust cavities, we computed Gaia proper motion anomalies, which together with the renormalised unit weight error (RUWE), identify companions with mass ratios q ≳0.01 at ∼0.1−30 au. We assessed the impact of disc gravity, accretion, disc-scattered light, dippers, starspots, jets, and outflows on the measured proper motion anomalies, concluding that these effects are unlikely to affect our analyses and that astrometric techniques such as the one of this work can be robustly applied to YSOs. Results . Significant proper motion anomalies are found in 31 transition discs (32% of the sample), indicative of companions. We recovered 85% of the known companions within our sensitivity range. Assuming that the astrometry of each system is dominated by a single companion, we modelled the semi-major axis and mass required to reproduce the observed astrometric signals. Most inferred companions have M >30 M J , placing many within or near the stellar mass regime. Seven sources host companions compatible with a planetary mass ( M <13 M J , HD 100453, J04343128+1722201, J16102955-3922144, MHO6, MP Mus, PDS 70, and Sz 76). For the non-detections, we provide the companion masses and semi-major axes that can be excluded in future searches. About half (53%) of detected companions cannot be reconciled with having carved the observed dust cavities. Conclusions . We have gathered evidence of the presence of companions in a large sample of transition discs. However, we find that the population of transition discs cannot be fully described as a circumbinary population. Transition discs host as many companions within our sensitivity range as do randomly sampled groups of YSOs and main-sequence stars. If dust cavities are shaped by companions, such companions must reside at larger orbital separations than those of the companions detected here, and we predict them to be of planetary mass.

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