On the ultraviolet anomalies of the WASP-12 and HD 189733 systems: Trojan satellites as a plasma source

TL;DR

This paper proposes that Trojan satellites near Lagrange points of hot Jupiters like WASP-12b and HD 189733b can outgas elements due to surface melting, explaining ultraviolet anomalies observed in their light curves.

Contribution

It introduces a novel plasma source hypothesis involving Trojan satellites for explaining UV anomalies in hot Jupiter systems, supported by stability analysis and outgassing rate estimates.

Findings

Trojan outgassing can explain the lack of Mg II and Ca II emission lines in WASP-12.

The proposed mechanism is less effective for HD 189733b due to lower temperatures.

Stable Trojan satellites can persist throughout the system's lifetime.

Abstract

We suggest an additional possible plasma source to explain part of the phenomena observed for the transiting hot Jupiters WASP-12b and HD 189733b in their ultraviolet (UV) light curves. In the proposed scenario, material outgasses from the molten surface of Trojan satellites on tadpole orbits near the Lagrange points L$_4$ and L$_5$. We show that the temperature at the orbital location of WASP-12b is high enough to melt the surface of rocky bodies and to form shallow lava oceans on them. In case of WASP-12b, this leads to the release of elements such as Mg and Ca, which are expected to surround the system. The predicted Mg and Ca outgassing rates from two Io-sized WASP-12b Trojans are $\approx 2.2 \times 10^{27}$ s$^{-1}$ and $\approx 2.2 \times 10^{26}$ s$^{-1}$, respectively. Trojan outgassing can lead to the apparent lack of emission in Mg{\sc ii}\,h\&k and Ca{\sc ii}\,H\&K line cores of WASP-12. For HD 189733b, the mechanism is only marginally possible due to the lower temperature. This may be one of the reasons that couldn't explain the early ingress of HD 189733b observed in the far-UV (FUV) C{\sc ii} doublet due to absence of carbon within elements outgassed by molten lava. We investigate the long-term stability region of WASP-12b and HD 189733b in case of planar and inclined motion of these satellites and show that unlike the classical exomoons orbiting the planet, Io-sized Trojans can be stable for the whole systems life time.