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