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
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