Disentangling Hot Jupiters formation location from their chemical composition

TL;DR

This study uses a population synthesis model to compare hot-Jupiter formation scenarios, focusing on chemical composition differences, especially C/O ratios, to determine observational signatures distinguishing in-situ formation from migration.

Contribution

It demonstrates that C/O ratios and core masses can help differentiate hot-Jupiter formation pathways, highlighting the impact of chemical assumptions and formation location.

Findings

In-situ hot-Jupiters tend to have higher C/O ratios.

Formation outside the snowline is more probable even with rapid pebble accretion.

C/O ratios vary moderately under solar composition, with larger variations if all carbon and oxygen are volatile.

Abstract

We use a population synthesis model that includes pebbles and gas accretion, planetary migration, and a simplified chemistry scheme to study the formation of hot-Jupiters. Models have been proposed that these planets can either originate beyond the snowline and then move inward via disk migration, or form "in-situ" inside the snowline. The goal of this work is to verify which of these two scenarios is more compatible with pebble accretion, and whether we can distinguish observationally between them via the resulting planetary C/O ratios and core masses. Our results show that, for solar system composition, the C/O ratios will vary but moderately between the two populations, since a significant amount of carbon and oxygen are locked up in refractories. In this case, we find a strong correlation between the carbon and oxygen abundances and core mass. The C/O ratio variations are more pronounced in the case where we assume that all carbon and oxygen are in volatiles. On average, Hot-Jupiters forming "in-situ" inside the snowline will have higher C/O ratios because they accrete less water ice. However, only Hot-Jupiters forming in-situ around stars with C/O=0.8 can have a C/O ratio higher than unity. We finally find that, even with fast pebble accretion, it is significantly easier to form Hot-Jupiters outside of the snowline, even if forming these "in-situ" is not impossible in the limit of the simplifying assumptions made.