GeminiFocus October 2017 | Page 8

They also display simultaneously significant rotational velocities, activity, and very low lithium content; alas, we see none of these characteristics in the stars of the HAT-P-4 bi- nary system. On the Planet Engulfment Scenario To explain the chemical abundance differ- ence between stars A and B, we propose that at the time of planet formation, star A locked the orbiting refractory material (in the shape of planetesimals, rocky planets, or both), and formed a gas giant planet in its external disk. This was followed by the accretion of most of these refractories descending onto star A – possibly due to the migration of the de- tected giant planet which finally ended as a hot-Jupiter transiting planet. We estimate that some 10 M Earth of rocky refractory mate- rial must have accreted onto star A in order to reproduce the observed T C trends and metallicity. This scenario agrees with the fol- lowing observational facts: • The enhancement of ~0.1 dex in metallic- ity of star A compared to B. These objects do not show a peculiar chemical pattern (λ Bootis, δ Scuti, or blue straggler). In ad- dition, the binary nature of stars A and B discards possible GCE or age effects. • The enhancement of refractory elements in the HAT-P-4 A star compared to its B companion. • The slightly higher mass of A compared to B, which corresponds to a lower convec- tive mass and a lower mixing of the pos- sible accreted material. • The detection of a hot Jupiter and no ad- ditional planets in the A star, which do not discard a possible migration and accre- tion process. In other words, the proposed scenario of planet engulfment fits all the observation- al pieces of the puzzle. Very few previous works claim a similar accretion scenario on a main-sequence star, such as the case of HD 82943 (which was then strongly disputed), or HIP 68468 (Meléndez et al., 2017). That leaves HAT-P-4 as the only main candidate in a remarkable system, studied through the unique combination of Gemini North as collector with the high-resolution spec- tra of GRACES. We also want to stress that this work was carried out thanks to the Fast Turnaround observing mode being offered by Gemini, which is the only observatory that provides this kind of time proposal. We expect to continue this excit