Implications of the spectroscopic abundances in α Centauri A and B

Regardless of their close proximity, abundance measurements for both stars in α Centauri by different groups have led to varying results. We have chosen to combine the abundance ratios from five similar data sets in order to reduce systematic effects that may have caused inconsistencies. With these collated relative abundance measurements, we find that the α Cen system and the Sun were likely formed from the same material, despite the [Fe/H] enrichment observed in the α Cen binaries: 0.28 and 0.31 dex, respectively. Both α Centauri A and B exhibit relative abundance ratios that are generally solar, with the mean at 0.002 and 0.03 dex, respectively. The refractory elements (condensation temperature ≳ 900 K) in each have a mean of −0.02 and 0.01 dex and a 1σ uncertainty of 0.09 and 0.11 dex, respectively. Given the trends seen when analysing the refractory abundances [X/Fe] with condensation temperature, we find it possible that α Centauri A may host a yet undiscovered planet

On the Habitable Zones of Circumbinary Planetary Systems

The effect of the stellar flux on exoplanetary systems is becoming an increasingly important property as more planets are discovered in the habitable zone (HZ). The Kepler mission has recently uncovered circumbinary planets with relatively complex HZs due to the combined flux from the binary host stars. Here, we derive HZ boundaries for circumbinary systems and show their dependence on the stellar masses, separation, and time while accounting for binary orbital motion and the orbit of the planet. We include stability regimes for planetary orbits in binary systems with respect to the HZ. These methods are applied to several of the known circumbinary planetary systems such as Kepler-16, 34, 35, and 47. We also quantitatively show the circumstances under which single-star approximations break down for HZ calculations

Habitability of Exomoons at the Hill or Tidal Locking Radius

Moons orbiting extrasolar planets are the next class of object to be observed and characterized for possible habitability. Like the host-planets to their host-star, exomoons have a limiting radius at which they may be gravitationally bound, or the Hill radius. In addition, they also have a distance at which they will become tidally locked and therefore in synchronous rotation with the planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets μ Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on a moon at its furthest, stable distance from the planet achieves its largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in orbit to the planet. We have also analyzed the effect of planetary eccentricity on the flux on the moon, examining planets that traverse the habitable zone either fully or partially during their orbit. Looking solely at the stellar contributions, we find that moons around planets that are totally within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. This means that a planet does not need to spend its entire orbit within the habitable zone in order for the exomoon to be habitable. Because the applied systems comprise giant planets around bright stars, we believe that the transit detection method is most likely to yield an exomoon discovery