GeminiFocus October 2017 | Page 5

Carlos Saffe Rocky Planet Engulfment Explains Stellar Odd Couple To date, astronomers have found more than 3,600 planets orbiting around stars in the solar neighborhood. Nevertheless, current observational techniques challenge those searching for possible planets engulfed by their host star. By using Gemini North + GRACES high-resolution spectra in the Fast Turnaround mode, we have found a notable difference in the chemical pattern between the stars of the HAT-P-4 binary system, which could be attributed to the ingestion of at least ~10 M Earth of rocky material onto the primary star. The story begins with the star HAT-P-4, which hosts a giant planet detected by the Hungar- ian Automated Telescope Network (HATNet) transit survey (Kovács et al., 2007). This plane- tary companion, designated HAT-P-4b, has a mass of 0.68 M JUP (between the mass of Jupiter and Saturn), and orbits the star at a distance of only 0.04 astronomical units — about 10 times closer than the distance between Mercury and the Sun. The planet’s estimated densi- ty of ~0.4 grams per cubic centimeter (g cm -3 ) is even lower than that of Saturn (~0.7 g cm -3 ), so we consider it as a low-density hot-Jupiter planet. Searches for additional planets around HAT-P-4 using both transits and radial-velocity techniques have met without success. The Dance Between Two Stars and a Single Planet A few years after the discovery of HAT-P-4b, Mugrauer et al. (2014) showed that HAT-P-4 (hereafter, star A) forms a wide binary system together with TYC 2569-744-1 (hereafter, star B). The two stars are separated by 91.8 arcseconds in the sky and appear nearly equal in brightness (Figure 1). In addition to having common proper motions and similar radial velocities, the discoverers also showed that both stellar components present very similar spectral types, being about G0V and G2V. To date, no planet has been detected around the B component, which was also included in the same HATNet transit survey field (called October 2017 GeminiFocus 3