Dynamics of extrasolar planets around massive stars and binary stars

 

Since the 1990's hundreds of extrasolar planets have been detected in different kinds of planetary systems ranging from Jupiter-like planets orbiting their stars within the orbit of Mercury and planets around pulsars to possibly free floating planets which do not seem to be gravitationally bound to any host star. The diversity of extrasolar planetary systems requires different methods for the detection of the planets, e.g. radial velocity measurements and transit detections which are the two most successful detection methods.

A method that can be used to detect planets in systems with three or more objects, i.e. multiple planet systems or planets around binary stars, is the eclipse timing variation. It uses the detectable changes in the timing of transits compared to an undisturbed two-body system due to the gravitational acceleration or deceleration of the orbiting objects during close encounters.

So far, several planets around binary star systems have been detected via this method. Among them are planets in post-common envelope binaries where one or both of the stellar components evolved past the Main Sequence to White Dwarfs and during this evolution shared a common gaseous envelope with their stellar companion. The change in the gravitational potential of the binary combined with momentum transfer between planets and expelled gas can have significant effects on the dynamics of these planets.

Exemplary system: NN Serpentis

 

 

Simulations with the FLASH code

 

On the one hand, possible outcomes of the stellar evolution include the expulsion or destruction of planets. On the other hand, a circumbinary disk could be formed and lead to gas accretion onto existing planets or even the formation of new (second generation) planets.

Therefore, we make use of FLASH, an adaptive mesh refinement simulation code for multiple physical demands, to perform full three-dimensional hydrodynamical simulations of planets around close binary stars during and after the common envelope phase. We trace the dynamics of the planetary system and follow the evolution of the expanding envelope.

The following images are snapshots of one simulation. They show the density distribution in the x,y-plane at

(a) t = 0 yr, (b) t = 0.15 yr and (c) t = 0.27 yr. Image (d) shows the density distribution at t = 0.27 yr in the x,z-plane.

 

 

 

 

(a)

(b)

dens 0000

 

dens xy early

(c)

(d)

dens xy

dens xz