Chemical fingerprints of hot Jupiter planet formation

Abstract

Context. The current paradigm to explain the presence of Jupiter-like planets with small orbital periods (P < 10 days; hot Jupiters), which involves their formation beyond the snow line following inward migration, has been challenged by recent works that explore the possibility of in situ formation. Aims. We aim to test whether stars harbouring hot Jupiters and stars with more distant gas-giant planets show any chemical peculiarity that could be related to different formation processes. Methods. Our methodology is based on the analysis of high-resolution échelle spectra. Stellar parameters and abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn for a sample of 88 planet hosts are derived. The sample is divided into stars hosting hot (a < 0.1 au) and cool (a > 0.1 au) Jupiter-like planets. The metallicity and abundance trends of the two sub-samples are compared and set in the context of current models of planet formation and migration. Results. Our results show that stars with hot Jupiters have higher metallicities than stars with cool distant gas-giant planets in the metallicity range +0.00/+0.20 dex. The data also shows a tendency of stars with cool Jupiters to show larger abundances of α elements. No abundance differences between stars with cool and hot Jupiters are found when considering iron peak, volatile elements or the C/O, and Mg/Si ratios. The corresponding p-values from the statistical tests comparing the cumulative distributions of cool and hot planet hosts are 0.20, < 0.01, 0.81, and 0.16 for metallicity, α, iron-peak, and volatile elements, respectively. We confirm previous works suggesting that more distant planets show higher planetary masses as well as larger eccentricities. We note differences in age and spectral type between the hot and cool planet host samples that might affect the abundance comparison. Conclusions. The differences in the distribution of planetary mass, period, eccentricity, and stellar host metallicity suggest a different formation mechanism for hot and cool Jupiters. The slightly larger α abundances found in stars harbouring cool Jupiters might compensate their lower metallicities allowing the formation of gas-giant planets.

Small Planets in the Galactic Context: Host Star Kinematics, Iron, and Alpha-element Enhancement

“…However, we saw no evidence for the positive trend we had detected with V tot for small planets. These results are also consistent with previous works (e.g., Maldonado et al 2018;Brewer et al 2018), suggesting that stars with low iron content (no matter their α content) do not seem to form or maintain close-in giant planets. More distant giant planets (not included in our survey) do seem to be more common around stars of low-iron and high α content, as the latest RV surveys for metal-poor stars suggest (Maldonado et al 2018;Barbato et al 2019).…”

Exocomets: A spectroscopic survey

“…Solar abundances were used For cooler stars (later than F2), a different approach was used to calculate parameters. In those cases we have followed the procedure described in detail by Maldonado & Villaver (2017) and Maldonado et al (2018), which is based on the iron ionisation and excitation equilibrium, and match of the curve of growth conditions. Radial velocities (v rad ) were estimated by measuring the shift between the synthetic spectrum, which is computed using a database of rest wavelengths, and the observed spectrum, corrected for barycentric velocity.…”

Investigating the planet–metallicity correlation for hot Jupiters

“…In-situ formation has, however, been recently reconsidered to be a possibility (e.g Boley et al 2016). Maldonado et al (2018) makes the assumption that hot and cool Jupiters would have similar chemical properties if hot Jupiters were formed at large distances from their star and then migrate inwards, but they find that hot and cool Jupiters have different properties, and that they are two distinct populations. Perhaps they have different formation methods, or perhaps hot Jupiter migration is a metallicity dependent process.…”

“…Perhaps they have different formation methods, or perhaps hot Jupiter migration is a metallicity dependent process. Maldonado et al (2018) argues that the latter is unlikely, as it would not be expected that migration would change the abundance of the host star.…”