Signatures of rocky planet engulfment in HAT-P-4

Abstract

Aims. We aim to explore the possible chemical signature of planet formation in the binary system HAT-P-4 by studying the trends of abundance vs. condensation temperature T c . The star HAT-P-4 hosts a planet detected by transits, while its stellar companion does not have any detected planet. We also study the lithium content, which might shed light on the problem of Li depletion in exoplanet host stars. Methods. We derived for the first time both stellar parameters and high-precision chemical abundances by applying a line-by-line full differential approach. The stellar parameters were determined by imposing ionization and excitation equilibrium of Fe lines, with an updated version of the FUNDPAR program, together with ATLAS9 model atmospheres and the MOOG code. We derived detailed abundances of different species with equivalent widths and spectral synthesis with the MOOG program. Results. The exoplanet host star HAT-P-4 is found to be ∼0.1 dex more metal rich than its companion, which is one of the highest differences in metallicity observed in similar systems. This could have important implications for chemical tagging studies. We rule out a possible peculiar composition for each star, such as is the case for λ Boötis and δ Scuti, and neither is this binary a blue straggler. The star HAT-P-4 is enhanced in refractory elements relative to volatile when compared to its stellar companion. Notably, the Li abundance in HAT-P-4 is greater than that of its companion by ∼0.3 dex, which is contrary to the model that explains the Li depletion by the presence of planets. We propose a scenario where at the time of planet formation, the star HAT-P-4 locked the inner refractory material in planetesimals and rocky planets, and formed the outer gas giant planet at a greater distance. The refractories were then accreted onto the star, possibly as a result of the migration of the giant planet. This explains the higher metallicity, the higher Li content, and the negative T c trend we detected. A similar scenario was recently proposed for the solar-twin star HIP 68468, which is in some aspects similar to HAT-P-4. We estimate a mass of at least M rock ∼ 10 M ⊕ locked in refractory material in order to reproduce the observed T c trends and metallicity.

Detailed chemical compositions of the wide binary HD 80606/80607: revised stellar properties and constraints on planet formation

“…These three studies favour the scenario proposed by Meléndez et al (2009). Meanwhile, Oh et al (2017) and Saffe et al (2017) reported two cases where the planet host star in the binary system shows an enrichment in elemental abundances at the level of ∼ 0.2 dex for HD 240430/240429 and ∼ 0.1 dex for HAT-P-4 system, respectively. These two studies instead favour the scenario of later accretion of inner planets.…”

The Chemical Signatures of Planetary Engulfment Events in Binary Systems

“…Follow-up observations are required to establish if the two stars also have two different architectures for their planetary systems. This is necessary to understand the origin of similar anomalies observed within other stellar associations (Spina et al 2015(Spina et al , 2018Tucci Maia et al 2014Teske et al 2015Oh et al 2018;Biazzo et al 2015;Saffe et al 2016Saffe et al , 2017, where the stars richer in refractory elements could also be the ones with a more chaotic architecture of their planetary systems.…”

High-precision abundances of elements inKeplerLEGACY stars

“…Differences of the [X/H] − T c slope are also seen between twin stars in binary systems (e.g. Tucci Maia et al 2014;Teske et al 2016a,b;Saffe et al 2017), which is particular interesting, because dustgas segregation before planet formation is likely to affect the abundances of the two components in the same way making it more likely that the observed abundance differences are due to planet-star interactions.…”