Instability of mass transfer in a planet-star system

Instability of mass transfer in a planet-star system

Authors:

Jia et al

Abstract:

We show that the angular momentum exchange mechanism governing the evolution of mass transferring binary stars does not apply to Roche-lobe filling planets, because most of the angular momentum of the mass transferring stream is absorbed by the host star. Apart from a correction for the difference in specific angular momentum of the stream and the centre of mass of the planet, the orbit does not expand much on Roche-lobe overflow. We explore the conditions for dynamically unstable Roche-lobe overflow as a function of planet mass and mass and radius (age) of host star and equation of state of planet. For a Sun-like host, gas giant planets in a range of mass and entropy can undergo dynamical mass transfer. Examples of the evolution of the mass transfer process are given. Dynamic mass transfer of rocky planets depend somewhat sensitively on equation of state used. Silicate planets in the range 1 M⊕ < Mp < 10 M⊕ typically go through a phase of dynamical mass transfer before settling to slow overflow when their mass drops to less than 1 M⊕.

Could a hot Jupiter Bleed out its Atmosphere to Necome a hot Mini Neptune?

Tidally-driven Roche-Lobe Overflow of Hot Jupiters with MESA

Authors:

Valsecchi et al

Abstract:

Many exoplanets have now been detected in orbits with ultra-short periods, very close to the Roche limit. Building upon our previous work, we study the possibility that mass loss through Roche lobe overflow (RLO) may affect the evolution of these planets, and could possibly transform a hot Jupiter into a lower-mass planet (hot Neptune or super-Earth). We focus here on systems in which the mass loss occurs slowly ("stable mass transfer" in the language of binary star evolution) and we compute their evolution in detail with the binary evolution code MESA. We include the effects of tides, RLO, irradiation and photo-evaporation of the planet, as well as the stellar wind and magnetic braking. Our calculations all start with a hot Jupiter close to its Roche limit, in orbit around a sun-like star. The initial orbital decay and onset of RLO are driven by tidal dissipation in the star. We confirm that such a system can indeed evolve to produce lower-mass planets in orbits of a few days. The RLO phase eventually ends and, depending on the details of the mass transfer and on the planetary core mass, the orbital period can remain around a few days for several Gyr. The remnant planets have a rocky core and some amount of envelope material, which is slowly removed via photo-evaporation at nearly constant orbital period; these have properties resembling many of the observed super-Earths and sub-Neptunes. For these remnant planets we also predict an anti-correlation between mass and orbital period; very low-mass planets (Mpl≲5M⊕) in ultra-short periods (Porb less than 1d) cannot be produced through this type of evolution.