The quest for companions to post-common envelope binaries: III. A reexamination of HW Virginis

We report new mid-eclipse times of the short-period sdB/dM binary HW Vir, which differ substantially from the times predicted by a previous model. The proposed orbits of the two planets in that model are found to be unstable. We present a new secularly stable solution, which involves two companions orbiting HW VIr with periods of 12.7 yr and 55 +/-15 yr. For orbits coplanar with the binary, the inner companion is a giant planet with mass M_3 sin i_3 = 14 M_Jup and the outer one a brown dwarf or low-mass star with a mass of M_4 sin i_4 = 30-120 M_Jup. Using the mercury6 code, we find that such a system would be stable over more than 10^7 yr, in spite of the sizeable interaction. Our model fits the observed eclipse-time variations by the light-travel time effect alone, without invoking any additional process, thereby providing support for the planetary hypothesis of the eclipse-time variations in close binaries. The signature of non-Keplerian orbits may be visible in the data.

Two planets orbiting the recently formed post-common envelope binary NN Serpentis

Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by N-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in a 2:1 mean motion resonance, with orbital periods P_c=15.5 yrs and P_d=7.7 yrs, masses M_c sin i_c = 6.9 M_Jup and M_d sin i_d = 2.2 M_Jup, and eccentricities e_c=0 and e_d=0.20. A secondary chi**2 minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with ~2 M_Sun and the present M dwarf secondary at an orbital separation of ~1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10**6 yrs.

Detecting circumstellar disks around gravitational microlenses

We investigate the chance of detecting proto-planetary or debris disks in stars that induce microlensing events (lenses). The modification of the light curves shapes due to occultation and extinction by the disks as well as the additional gravitational deflection caused by the additional mass is considered. The magnification of gravitational microlensing events is calculated using the ray shooting method. The occultation is taken into account by neglecting or weighting the images on the lens plane according to a transmission map of the corresponding disk for a point source point lens (PSPL) model. The estimated frequency of events is obtained by taking the possible inclinations and optical depths of the disk into account. We conclude that gravitational microlensing can be used, in principle, as a tool for detecting debris disks beyond 1 kpc, but estimate that each year of the order of 1 debris disk is expected for lens stars of F, G, or K spectral type and of the order of 10 debris disks might have shown signatures in existing datasets.