G191) with an 8˚ x 8˚ field-of-view. In sum-
mary, we have three actors in the scene: a
wide binary system with two very similar
components, and a hot-Jupiter planet orbit-
ing around the primary (star A).
Figure 1.
Image of the HAT-P-4
binary system. The two
brightest stars in the
field are HAT-P-4 A and
its B companion (lower
left). Their Hipparcos V
magnitudes are 11.12
and 11.38, respectively,
and they lie 91.8
arcseconds apart.
Stars born at different times and locations
in our Galaxy commonly present a different
initial chemical composition due to the Ga-
lactic Chemical Evolution (GCE) effect, which
leads to different chemical enrichment his-
tories. On the other hand, it is generally as-
sumed that individual components of wide
binaries (and most multiple systems) have
the same age and initial chemical composi-
tion, and formed coevally from a common
molecular cloud.
This latter case is a strong advantage for
comparative chemical studies, where GCE
effects are greatly diminished or ruled-out;
in addition, the notable physical similarity
between both components of a binary sys-
tem makes it possible to achieve the high-
est possible precision in differential chemi-
cal studies when compared to classical (i.e.,
non-differential) methods. Such precision is
a requisite in order to detect even slight dif-
ferences between both stars. That HAT-P-4
is not only a binary with physical similarities
between its components, but also one that
8
GeminiFocus
harbors a planetary companion, makes it an
ideal case study on the possible chemical
signature of the planet formation process in
a binary star system. So far, this kind of chal-
lenging analysis has been performed in only
a very few systems.
A high-precision chemical abundance study
requires both high signal-to-noise (S/N) and
high-resolution spectra. This fact, together
with the relative brightness of both stars,
made the combination of Gemini North with
the Gemini Remote Access to CFHT ESPa-
DOnS Spectrograph (GRACES) an excellent
choice for the observation of this binary sys-
tem. The stellar spectra were obtained under
the Fast Turnaround observing mode (pro-
gram ID: GN-2016A-FT-25; with the author as
the Principal Investigator). We acquired the
observations using the 1-fiber (object-only)
observing mode, which provides a maximum
resolving power of ~67,500 between 4,000
and 10,000 Ångstroms (Å). The exposure
times were 2 x 16 minutes and 2 x 18 minutes
for the stars A and B, respectively, obtaining
a final S/N ~ 400 measured at ~6,000 Å in the
combined spectra of each target.
A Surprising Chemical Difference
Between Sibling Stars
We took advantage of the physical similar-
ity between both stars and applied a line-
by-line full differential technique in order
to determine fundamental parameters and
detailed chemical abundances. To do so, we
used the FUNDPAR program (Saffe, 2011)
together with ATLAS9 model atmospheres.
The results showed mainly three unexpect-
ed differences in the chemical pattern of
both stars. First, the exoplanet host A star
is ~0.1 dex more metal-rich than its stellar
companion. This difference is remarkable
and much higher than most metallicity dif-
ferences found in similar binary systems
(see, e.g., Desidera et al., 2006). Second, star
A shows a clear enhancement in its photo-
January 2018 / 2017 Year in Review