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 .
G191 ) with an 8˚ x 8˚ field-of-view . In summary , we have three actors in the scene : a wide binary system with two very similar components , and a hot-Jupiter planet orbiting around the primary ( star A ).
Stars born at different times and locations in our Galaxy commonly present a different initial chemical composition due to the Galactic Chemical Evolution ( GCE ) effect , which leads to different chemical enrichment histories . On the other hand , it is generally assumed that individual components of wide binaries ( and most multiple systems ) have the same age and initial chemical composition , 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 system makes it possible to achieve the highest possible precision in differential chemical studies when compared to classical ( i . e ., non-differential ) methods . Such precision is a requisite in order to detect even slight differences between both stars . That HAT-P-4 is not only a binary with physical similarities between its components , but also one that 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 challenging 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 system . The stellar spectra were obtained under the Fast Turnaround observing mode ( program 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 similarity between both stars and applied a lineby-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 unexpected 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 differences found in similar binary systems ( see , e . g ., Desidera et al ., 2006 ). Second , star A shows a clear enhancement in its photo-
4 GeminiFocus October 2017