Thermophysical Characteristics of OSIRIS-REx Target Asteroid (101955) Bennu

In this work, we investigate the thermophysical properties, including thermal inertia, roughness fraction and surface grain size of OSIRIS-REx target asteroid (101955) Bennu by using a thermophysical model with the recently updated 3D radar-derived shape model (\cite[Nolan et al., 2013]{Nolan2013}) and mid-infrared observations (\cite[M$\ddot{u}$ller et al, 2012]{Muller2012}, \cite[Emery et al., 2014]{Emery2014}). We find that the asteroid bears an effective diameter of $510^{+6}_{-40}$ m, a geometric albedo of $0.047^{+0.0083}_{-0.0011}$, a roughness fraction of $0.04^{+0.26}_{-0.04}$, and thermal inertia of $240^{+440}_{-60}\rm~Jm^{-2}s^{-0.5}K^{-1}$ for our best-fit solution. The best-estimate thermal inertia suggests that fine-grained regolith may cover a large portion of Bennu's surface, where a grain size may vary from $1.3$ to $31$~mm. Our outcome suggests that Bennu is suitable for the OSIRIS-REx mission to return samples to Earth.Comment: 3 pages, 1 figures, accepted to IAU Symposium 318: Asteroids: New Observations, New Model

Surface Thermophysical Properties determination of OSIRIS-REx target asteroid (101955) Bennu

In this work, we investigate the thermophysical properties of OSIRIS-REx target asteroid (101955) Bennu (hereafter, Bennu), where thermal inertia plays an important role in understanding the nature of the asteroid's surface, and will definitely provide substantial information for the sampling return mission. Using a thermophysical model incorporating the recently updated 3D radar-derived shape model \citep{Nolan2013} and mid-infrared observations of Spitzer-PUI, Spitzer-IRAC, Herschel/PACS and ESO VLT/VISIR \citep{Muller2012,Emery2014}, we derive the surface thermophysical properties of Bennu. The asteroid has an effective diameter of $510^{+6}_{-40}$ m, a geometry albedo of $0.047^{+0.0083}_{-0.0011}$, a roughness fraction of $0.04^{+0.26}_{-0.04}$, and thermal inertia of $240^{+440}_{-60}\rm~Jm^{-2}s^{-0.5}K^{-1}$ for a best-fit solution at 1$\sigma$ level. The best-estimate thermal inertia indicates that fine-grained regolith may cover a large area of Bennu's surface, with a grain size that may range from $1.3$ to $31$~mm, and our outcome further supports that Bennu would be a suitable target for the OSIRIS-REx mission to return samples from the asteroid to Earth.Comment: 10 pages, 7 figures, 5 tables, accepted to MNRA

Near 3:2 and 2:1 mean motion resonances formation in the systems observed by Kepler

The Kepler mission has released ~4229 transiting planet candidates. There are approximately 222 candidate systems with three planets. Among them, the period ratios of planet pairs near 1.5 and 2.0 reveal that two peaks exist for which the proportions of the candidate systems are ~7.0% and 18.0%, respectively. In this work, we study the formation of mean motion resonance (MMR) systems, particularly for the planetary configurations near 3:2 and 2:1 MMRs, and we concentrate on the interplay between the resonant configuration and the combination of stellar accretion rate, stellar magnetic field, speed of migration and additional planets. We perform more than 1000 runs by assuming a system with a solar-like star and three surrounding planets. From the statistical results, we find that under the formation scenario, the proportions near 1.5 and 2.0 can reach 14.5% and 26.0%, respectively. In addition, $\dot M=0.1\times 10^{-8} ~M_\odot ~{\rm yr^{-1}}$ is propitious toward the formation of 3:2 resonance, whereas $\dot M=2\times 10^{-8} ~M_\odot ~{\rm yr^{-1}}$ contributes to the formation of 2:1 resonance. The speed-reduction factor of type I migration $f_1\geq 0.3$ facilitates 3:2 MMRs, whereas $f_1\geq 0.1$ facilitates 2:1 MMRs. If additional planets are present in orbits within the innermost or beyond the outermost planet in a three-planet system, 3:2:1 MMRs can be formed, but the original systems trapped in 4:2:1 MMRs are not affected by the supposed planets. In summary, we conclude that this formation scenario will provide a likely explanation for Kepler candidates involved in 2:1 and 3:2 MMRs.Comment: 13 pages, 8 figures, accepted for publication in Ap

Using Chebyshev polynomials interpolation to improve the computation efficiency of gravity near an irregular-shaped asteroid

In asteroid rendezvous missions, the dynamical environment near the asteroid's surface should be made clear prior to the mission launch. However, most of the asteroids have irregular shapes, which lower the efficiency of calculating their gravitational field by adopting the traditional polyhedral method. In this work, we propose a method to partition the space near the asteroid adaptively along three spherical coordinates and use Chebyshev polynomials interpolation to represent the gravitational acceleration in each cell. Moreover, we compare four different interpolation schemes to obtain the best precision with the identical initial parameters. An error-adaptive octree division is combined to improve the interpolation precision near the surface. As an example, we take the typical irregular-shaped near-Earth asteroid 4179 Toutatis to show the advantage of this method, as a result, we show that the efficiency can be increased by hundreds to thousands times with our method. In a word, this method can be applicable to other irregular-shaped asteroids and can greatly improve the evaluation efficiency.Comment: 17 pages, 10 figures, 3 tables, accepted to Research in Astronomy and Astrophysic

Near Mean-motion Resonances in the Systems Observed by Kepler: Affected by Mass Accretion and Type I Migration

The Kepler mission has released over 4496 planetary candidates, among which 3483 planets have been confirmed as of April 2017. The statistical results of the planets show that there are two peaks around 1.5 and 2.0 in the distribution of orbital period ratios. The observations indicate that a plenty of planet pairs could have firstly been captured into mean motion resonances (MMRs) in planetary formation. Subsequently, these planets depart from exact resonant locations to be near MMRs configurations. Through type I migration, two low-mass planets have a tendency to be trapped into first-order MMRs (2:1 or 3:2 MMRs), however two scenarios of mass accretion of planets and potential outward migration play an important role in reshaping their final orbital configurations. Under the scenario of mass accretion, the planet pairs can cross 2:1 MMRs and then enter into 3:2 MMRs, especially for the inner pairs. With such formation scenario, the possibility that two planets are locked into 3:2 MMRs can increase if they are formed in a flat disk. Moreover, the outward migration can make planets have a high likelihood to be trapped into 3:2 MMRs. We perform additional runs to investigate the mass relationship for those planets in three-planet systems, and we show that two peaks near 1.5 and 2.0 for the period ratios of two planets can be easily reproduced through our formation scenario. We further show that the systems in chain resonances (e.g., 4:2:1, 3:2:1, 6:3:2 and 9:6:4 MMRs), have been observed in our simulations. This mechanism can be applicable to understand the formation of systems of Kepler-48, Kepler-53, Kepler-100, Kepler-192, Kepler-297, Kepler-399, and Kepler-450.Comment: 12 pages, 6 figures, accepted for publication in A

Formation of S-type planets in close binaries: scattering induced tidal capture of circumbinary planets

Although several S-type and P-type planets in binary systems were discovered in past years, S-type planets have not yet been found in close binaries with an orbital separation not more than 5 au. Recent studies suggest that S-type planets in close binaries may be detected through high-accuracy observations. However, nowadays planet formation theories imply that it is difficult for S-type planets in close binaries systems to form in situ. In this work, we extensively perform numerical simulations to explore scenarios of planet-planet scattering among circumbinary planets and subsequent tidal capture in various binary configurations, to examine whether the mechanism can play a part in producing such kind of planets. Our results show that this mechanism is robust. The maximum capture probability is $\sim 10\%$, which can be comparable to the tidal capture probability of hot Jupiters in single star systems. The capture probability is related to binary configurations, where a smaller eccentricity or a low mass ratio of the binary will lead to a larger probability of capture, and vice versa. Furthermore, we find that S-type planets with retrograde orbits can be naturally produced via capture process. These planets on retrograde orbits can help us distinguish in situ formation and post-capture origin for S-type planet in close binaries systems. The forthcoming missions (PLATO) will provide the opportunity and feasibility to detect such planets. Our work provides several suggestions for selecting target binaries in search for S-type planets in the near future.Comment: 12 pages, 6 figures, accepted for publication in MNRA

The scattering outcomes of Kepler circumbinary planets: planet mass ratio

Recent studies reveal that the free eccentricities of Kepler-34b and Kepler-413b are much larger than their forced eccentricities, implying that the scattering events may take place in their formation. The observed orbital configuration of Kepler-34b cannot be well reproduced in disk-driven migration models, whereas a two-planet scattering scenario can play a significant role of shaping the planetary configuration. These studies indicate that circumbinary planets discovered by Kepler may have experienced scattering process. In this work, we extensively investigate the scattering outcomes of circumbinary planets focusing on the effects of planet mass ratio. We find that the planetary mass ratio and the the initial relative locations of planets act as two important parameters which affect the eccentricity distribution of the surviving planets. As an application of our model, we discuss the observed orbital configurations of Kepler-34b and Kepler-413b. We first adopt the results from the disk-driven models as the initial conditions, then simulate the scattering process occurred in the late evolution stage of circumbinary planets. We show that the present orbital configurations of Kepler-34b and Kepler-413b can be well reproduced when considering two unequal-mass planet ejection model. Our work further suggests that some of the currently discovered circumbinary single-planet systems may be the survivals of original multiple-planet systems. The disk-driven migration and the scattering events occurring in the late stage both play an irreplaceable role in sculpting the final systems.Comment: 18 pages, 9 figures, accepted for publication in A

Revisit of rotational dynamics of Asteroid 4179 Toutatis from Chang'e-2's flyby

This paper presents analysis of the rotational parameters of Toutatis based on the observational results from Chang'e-2's close flyby. The 3-D shape model derived from ground-based radar observation is used to calculate the 3-1-3 Euler angles at the flyby epoch, which are evaluated to be $-20.1^\circ\pm1^\circ$, $27.6^\circ\pm1^\circ$ and $42.2^\circ\pm1^\circ$. The large amplitude of Toutatis' tumbling attitude is demonstrated to be the result of the large deviation of the angular momentum axis and the rotational axis. Two rotational periods are evaluated to be $5.38\pm0.03$ days for rotation about the long axis and $7.40\pm0.03$ days for precession of the long axis about the angular momentum vector based on Fourier analysis. These results provide a further understanding of rotational state of Toutatis.Comment: 4 pages, 3 figures, accepted to IAU Symposium 318: Asteroids: New Observations, New Model

Investigation of Thermal Inertia and Surface Properties for Near-Earth Asteroid (162173) 1999 JU3

In order to obtain the substantial information about the surface physics and thermal property of the target asteroid (162173) 1999 JU3, which will be visited by Hayabusa 2 in a sample return mission, with the Advanced Thermal Physical Model (ATPM) we estimate the possible thermal inertia distribution over its surface, and infer the major material composition of its surface materials. In addition, the effective diameter and geometric albedo are derived to be $D_{\rm eff}=1.13\pm0.03\rm~km$, $p_{\rm v}=0.042\pm0.003$, respectively, and the average thermal inertia is estimated to be about $(300\pm50)\rm~J\cdot m^{-2}\cdot s^{-0.5}\cdot K^{-1}$. According to the derived thermal inertia distribution, we infer that the major area on the surface of the target asteroid may be covered by loose materials, such as rock debris, sands, and so on, but few bare rocks may exist in a very small region. In this sense, the sample return mission of Hayabusa 2 is feasible, when it is performed successfully, it will certainly bring significant scientific information to the research of asteroids.Comment: 15 pages, 7 figures, published in Chinese Astronomy and Astrophysic

Dynamical evolution and stability maps of the Proxima Centauri system

Proxima Centauri was recently discovered to host an Earth-mass planet of Proxima b, and a 215-day signal which is probably a potential planet c. In this work, we investigate the dynamical evolution of the Proxima Centauri system with the full equations of motion and semi-analytical models including relativistic and tidal effects. We adopt the modified Lagrange-Laplace secular equations to study the evolution of eccentricity of Proxima b, and find that the outcomes are consistent with those from the numerical simulations. The simulations show that relativistic effects have an influence on the evolution of eccentricities of planetary orbits, whereas tidal effects primarily affects the eccentricity of Proxima b over long timescale. Moreover, using the MEGNO (the Mean Exponential Growth factor of Nearby Orbits) technique, we place dynamical constraints on orbital parameters that result in stable or quasi-periodic motions for coplanar and non-coplanar configurations. In the coplanar case, we find that the orbit of Proxima b is stable for the semi-major axis ranging from 0.02 au to 0.1 au and the eccentricity being less than 0.4. This is where the best-fitting parameters for Proxima b exactly fall. Additional simulations show that the robust stability of this system would favor an eccentricity of Proxima b less than 0.45 and that of Proxima c below 0.65. In the non-coplanar case, we find that mutual inclinations of two planets must be lower than $50^{\circ}$ in order to provide stability. Finally, we estimate the mass of Proxima c to be $3.13~{M_ \oplus } \le {m_c} \le 70.7~{M_ \oplus }$ when $1.27~{M_ \oplus } \le {m_b} \le 1.6~{M_ \oplus }$, if ${i_{mutual}} \le {50^{\circ}}$ and $\Delta \Omega = 0^{\circ}$.Comment: 11 pages, 11 figures, accepted for publication in MNRA