We model the interior of Europa using a method by which we set three layers of Europa and the composition of each layer in advance. Comparing the calculated mass and radius related to each model with the data derived by the observations, we propose three possible internal structure models for Europa. All models are assumed to be differentiated into a metallic core, a (Mg,Fe)2SiO4 silicate mantle and an outer water shell. The only difference amongst these models are the composition of the core. The core of Model I is composed of Fe, while Model II composed of FeS, and the two models are based on the inferences provided by Europa's external gravitational field. Model III has a core which consists of Fe-FeS alloy, similar to that of Earth's outer core. Model I has a Fe core with a radius of 431 km, a (Mg,Fe)2SiO4 mantle with 999 km and a water shell with 132 km. Model II has a 777 km FeS core, a 619 km (Mg,Fe)2SiO4 mantle and a 167 km water shell. Model III has a 571 km Fe0.8(FeS)0.2 core, a 832 km (Mg,Fe)2SiO4 mantle and a 166 km water shell. In all three models, the density range and the radius of each layer is consistent with those deduced by Anderson et al.
Embedded clusters are ideal laboratories for understanding the early phase of the dynamical evolution of clusters as well as massive star formation. An interesting observational phenomenon is that some of the embedded clusters show mass segregation, i.e., the most massive stars are preferentially found near the cluster center. We develop a new approach to describe mass segregation. Using this approach and the Two Micron All Sky Survey Point Source Catalog （2MASS PSC）, we analyze 18 embedded clusters in the Galaxy. We find that 11 of them are mass-segregated and that the others are not mass-segregated. No inversely mass-segregated cluster is found.
HD 6840 is a double-lined visual binary with an orbital period of N7.5 years. By fitting the speckle interferometric measurements made by the 6 m BTA telescope and 3.5 m WIYN telescope, Balega et al. gave a preliminary astrometric orbital solution of the system in 2006. Recently, Griffin derived a precise spectroscopic orbital solution from radial velocities observed with OPH and Cambridge Coravel. However, due to the low precision of the determined orbital inclination, the derived component masses are not satisfying. By adding the newly collected astrometric data in the Fourth Catalog of Interferometric Measurements of Binary Stars, we give a three-dimensional orbit solution with high precision and derive the preliminary physical parameters of HD 6840 via a simultaneous fit including both astrometric and radial velocity measurements.
We perform numerical simulations to investigate tidal evolution of two single-planet systems, that is, WASP-50 and GJ 1214 and a two-planet system CoRoT-7. The results of orbital evolution show that tidal decay and circularization may play a significant role in shaping their final orbits, which is related to the initial orbital data in the simulations. For GJ 1214 system, different cases of initial eccentricity are also considered as only an upper limit of its eccentricity （0.27） is shown, and the outcome suggests a possible maximum initial eccentricity （0.4） in the adopted dynamical model. Moreover, additional runs with alternative values of dissipation factor Q＇I are carried out to explore tidal evolution for GJ 1214b, and these results further indicate that the real Q＇1 of GJ 1214b may be much larger than its typical value, which may reasonably suggest that GJ 1214b bears a present-day larger eccentricity, undergoing tidal circularization at a slow rate. For the CoRoT-7 system, tidal forces make two planets migrating towards their host star as well as producing tidal circularization, and in this process tidal effects and mutual gravitational interactions are coupled with each other. Various scenarios of the initial eccentricity of the outer planet have also been done to investigate final planetary configuration. Tidal decay arising from stellar tides may still work for each system as the eccentricity decreases to zero, and this is in association with the remaining lifetime of each planet used to predict its future.
The Damocloids are a group of unusual asteroids that recently added a new member： 2010 EJ104. The dynamical evolution of the Damocloids may reveal a connection from the Main Belt to the Kuiper Belt and beyond the scattered disk. According to our simulations, two regions may be considered as possible origins of the Damocloids： the scattered disk, or a part of the Oort cloud, which will be perturbed to a transient region located between 700 AU and 1000 AU. Based on their potential origin, the Damocloids can be classified into two types, depending on their semi-major axes, and about 65.5% of the Damocloids are classified into type I which mainly originate from the Oort cloud. Whether the Damocloids are inactive nuclei of the Halley Family of Comets may depend on their origin. K
Su WangHai-Bin ZhaoJiang-Hui JiSheng JinYan XiaHao LuMin WangJin-Sheng Yao
We show that there exists a new class of symmetric periodic solutions of the spatial elliptic restricted three-body problem. In such a solution, the infinitesimal body is confined to the vicinity of a primary and moves on a nearly circular orbit. This orbit is almost perpendicular to the orbital plane of the primaries, where the line of symmetry of the orbit lies. The existence is shown by applying a corollary of Arenstorf's fixed point theorem to a periodicity equation system of the problem. And this existence doesn't require any restriction on the mass ratio of the primaries, nor on the eccentricity of their relative elliptic orbit. Potential relevance of this new class of periodic solutions to real celestial body systems and the follow-up studies in this respect are also discussed.
XU XingBo1,2 & FU YanNing1 1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
