New evidence for planets on S-type orbits in close binary systems

Abstract

We present evidence for two Jovian planets orbiting the evolved giant stars 39 Cygni and HR 2877, based on more than 10 years of high-precision Doppler data taken at the Lick Observatory. Both stars are the primary components of compact binary systems, and thus these systems provide important clues on how planets could form and remain stable in S-type orbit around a star under the strong gravitational influence from a close stellar companion. We investigate large sets of orbital fits for both systems by applying systematic $chi^2$ grid-search techniques coupled with self-consistent dynamical fitting. We also perform long-term dynamical simulations to constrain the permitted orbital configurations. We find that 39 Cygni is accompanied by a low-mass star having nearly circular orbit at $a_B$ > 7.5 AU. The planet orbiting the primary is well separated ($a_b sim$ 1.6 AU) from the secondary and thus the system is generally stable. HR 2877 has a stellar companion of at least $0.6 M_odot$ on a highly eccentric orbit with $e_b$ = 0.73. The binary semimajor axis is $a_B sim$ 13.6 AU, but the pericentre distance is only 3.7 AU leading to strong interactions with the planet, which is at $a_b sim$ 1.1 AU. If the binary and the planet in this system have prograde and aligned coplanar orbits, there are only narrow regions of stable orbital solutions. For this system we also test dynamical models with the planet having a retrograde orbit, and we find that in this case the system is fully stable in a large set of orbital solutions. Only a handful of S-type planetary candidates in compact binary systems are known in the literature, and the 39 Cygni and HR 2877 systems are significant additions to the sample.