Hot-Jupiter core mass from Roche lobe overflow

The orbits of many observed hot Jupiters are decaying rapidly due to tidal interaction, eventually reaching the Roche limit.We analytically study the ensuing coupled mass-loss and orbital evolution during the Roche lobe overflow and find two possible scenarios. Planets with light cores Mc ≲ 6M⊕ (assuming a nominal tidal dissipation factor Q ∼ 106 for the host star) are transformed into Neptune-mass gas planets, orbiting at a separation (relative to the stellar radius) a/R∗≈3.5. Planets with heavier coresMc ≳6M⊕ plunge rapidly until they are destroyed at the stellar surface. Remnant gas Neptunes, which are stable to photoevaporation, are absent from the observations despite their unique transit radius (5-10 R⊕). This result suggests that Mc ≳ 6M⊕, providing a useful constraint on the poorly known core mass that may distinguish between different formation theories of gas giants. Alternatively, if one assumes a prior of Mc ≈ 6M⊕ from the core-accretion theory, our results suggest that Q does not lie in the range 106 ≲ Q ≲ 107.