Mass Transfer, Transiting Stream and Magnetopause in Close-in Exoplanetary Systems with Applications to WASP-12

TL;DR

This paper investigates mass transfer via Roche lobe overflow and magnetospheric interactions in close-in exoplanets, explaining observed transit asymmetries and early ingress phenomena, with specific application to the WASP-12 system.

Contribution

It introduces a model of accretion streams and magnetopause interactions that explain transit asymmetries and early ingress in close-in exoplanets, especially WASP-12.

Findings

Accretion streams can produce significant optical depth and early ingress.

Asymmetric magnetopause may cause transit asymmetries.

Transiting cylinders can be comparable in size to the planet.

Abstract

We study mass transfer by Roche lobe overflow in close-in exoplanetary systems. The planet's atmospheric gas passes through the inner Lagrangian point and flows along a narrow stream, accelerating to 100-$200\kms$ velocity before forming an accretion disk. We show that the cylinder-shaped accretion stream can have an area (projected in the plane of the sky) comparable to that of the planet and a significant optical depth to spectral line absorption. Such a "transiting cylinder" may produce an earlier ingress of the planet transit, as suggested by recent HST observations of the WASP-12 system. The asymmetric disk produced by the accretion stream may also lead to time-dependent obscuration of the star light and apparent earlier ingress. We also consider the interaction of the stellar wind with the planetary magnetosphere. Since the wind speed is subsonic/sub-Alfvenic and comparable to the orbital velocity of the planet, the head of the magnetopause lies eastward relative to the substellar line (the line joining the planet and the star). The gas around the magnetopause may, if sufficiently compressed, give rise to asymmetric ingress/egress during the planet transit, although more works are needed to evaluate this possibility.