Searching for Scatterers: High-Contrast Imaging of Young Stars Hosting Wide-Separation Planetary-Mass Companions

TL;DR

This study used high-contrast imaging to search for closer-in companions to systems with wide-separation planetary-mass objects, aiming to test formation theories and rule out scattering as the primary origin.

Contribution

It provides the first direct constraints on the presence of intermediate-mass bodies that could have caused scattering, challenging the scattering formation scenario for wide-separation PMCs.

Findings

No additional >7 M$_{ m Jup}$ bodies found within 15-50 AU.

Orbital motion detected for some companions, constraining their eccentricities.

Less than 54% chance of additional 5-15 M$_{ m Jup}$ planets beyond 40 AU.

Abstract

We have conducted an angular differential imaging survey with NIRC2 at Keck in search of close-in substellar companions to a sample of seven systems with confirmed planetary-mass companions (PMCs) on wide orbits (>50 AU). These wide-separation PMCs pose significant challenges to all three possible formation mechanisms: core accretion plus scattering, disk instability, and turbulent fragmentation. We explore the possibility that these companions formed closer in and were scattered out to their present-day locations by searching for other massive bodies at smaller separations. The typical sensitivity for this survey is \Delta K ~ 12.5 at 1". We identify eight candidate companions, whose masses would reach as low as one Jupiter mass if gravitationally bound. From our multi-epoch astrometry we determine that seven of these are conclusively background objects, while the eighth near DH Tau is ambiguous and requires additional monitoring. We rule out the presence of >7 M$_{\rm Jup}$ bodies in these systems down to 15 - 50 AU that could be responsible for scattering. This result combined with the totality of evidence suggests that dynamical scattering is unlikely to have produced this population of PMCs. We detect orbital motion from the companions ROXs 42B b and ROXs 12 b, and from this determine 95% upper limits on the companions' eccentricities of 0.58 and 0.83 respectively. Finally, we find that the 95% upper limit on the occurrence rate of additional planets with masses between 5 - 15 M$_{\rm Jup}$ outside of 40 AU in systems with PMCs is 54%.