The current paper studies funding and collaboration effect on citation impact of publications of top-3 Chinese and US universities. Funding support focuses on the two agencies supporting basic... Show moreThe current paper studies funding and collaboration effect on citation impact of publications of top-3 Chinese and US universities. Funding support focuses on the two agencies supporting basic research of the two countries, namely, the National Natural Science Foundation of China (NSFC), and the National Science Foundation (NSF) of the USA. The NSFC is significantly more important to Chinese universities than the NSF to US counterparts due to absolute high share of the former in publications. Both the NSFC and NSF grants raise citation impact. International collaboration raises citation impact of Chinese publications but plays a negative role to US publications. Cross-institutional collaboration is helpful to increasing publication productivity but may decrease citation impact. Increasing the number of authors in collaboration may increase citation impact, but negative effect happens if the number of institutions is increased. There is no significant effect if the number of countries is increased. Show less
Cai, X.; Portegies Zwart, S.F.; Elteren, A.K. van 2018
Due to the high stellar densities in young clusters, planetary systems formed in these environments are likely to have experienced perturbations from encounters with other stars. We carry out... Show moreDue to the high stellar densities in young clusters, planetary systems formed in these environments are likely to have experienced perturbations from encounters with other stars. We carry out direct N-body simulations of multiplanet systems in star clusters to study the combined effects of stellar encounters and internal planetary dynamics. These planetary systems eventually become part of the Galactic field population as the parental cluster dissolves, which is where most presently known exoplanets are observed. We show that perturbations induced by stellar encounters lead to distinct signatures in the field planetary systems, most prominently, the excited orbital inclinations and eccentricities. Planetary systems that form within the cluster's half-mass radius are more prone to such perturbations. The orbital elements are most strongly excited in the outermost orbit, but the effect propagates to the entire planetary system through secular evolution. Planet ejections may occur long after a stellar encounter. The surviving planets in these reduced systems tend to have, on average, higher inclinations and larger eccentricities compared to systems that were perturbed less strongly. As soon as the parental star cluster dissolves, external perturbations stop affecting the escaped planetary systems, and further evolution proceeds on a relaxation time-scale. The outer regions of these ejected planetary systems tend to relax so slowly that their state carries the memory of their last strong encounter in the star cluster. Regardless of the stellar density, we observe a robust anticorrelation between multiplicity and mean inclination/eccentricity. We speculate that the ‘Kepler dichotomy’ observed in field planetary systems is a natural consequence of their early evolution in the parental cluster. Show less
Qian, X.; Cai, X.; Portegies Zwart, S.F.; Zhu, M. 2017
Scientific discovery via numerical simulations is important in modern astrophysics. This relatively new branch of astrophysics has become possible due to the development of reliable numerical... Show moreScientific discovery via numerical simulations is important in modern astrophysics. This relatively new branch of astrophysics has become possible due to the development of reliable numerical algorithms and the high performance of modern computing technologies. These enable the analysis of large collections of observational data and the acquisition of new data via simulations at unprecedented accuracy and resolution. Ideally, simulations run until they reach some pre-determined termination condition, but often other factors cause extensive numerical approaches to break down at an earlier stage. In those cases, processes tend to be interrupted due to unexpected events in the software or the hardware. In those cases, the scientist handles the interrupt manually, which is time-consuming and prone to errors. We present the Simulation Monitor (SiMon) to automatize the farming of large and extensive simulation processes. Our method is light-weight, it fully automates the entire workflow management, operates concurrently across multiple platforms and can be installed in user space. Inspired by the process of crop farming, we perceive each simulation as a crop in the field and running simulation becomes analogous to growing crops. With the development of SiMon we relax the technical aspects of simulation management. The initial package was developed for extensive parameter searchers in numerical simulations, but it turns out to work equally well for automating the computational processing and reduction of observational data reduction. Show less
Cai, X.; Kouwenhoven, M.B.N.; Portegies Zwart, S.F.; Spurzem, R. 2017
Most stars form in star clusters and stellar associations. However, only about ˜1 per cent of the presently known exoplanets are found in these environments. To understand the roles of star cluster... Show moreMost stars form in star clusters and stellar associations. However, only about ˜1 per cent of the presently known exoplanets are found in these environments. To understand the roles of star cluster environments in shaping the dynamical evolution of planetary systems, we carry out direct N-body simulations of four planetary system models in three different star cluster environments with respectively N = 2k, 8k and 32k stars. In each cluster, an ensemble of initially identical planetary systems are assigned to solar-type stars with ˜1 M⊙ and evolved for 50 Myr. We found that following the depletion of protoplanetary discs, external perturbations and planet-planet interactions are two driving mechanisms responsible for the destabilization of planetary systems. The planet survival rate varies from ˜95 per cent in the N = 2k cluster to ˜60 per cent in the N = 32k cluster, which suggests that most planetary systems can indeed survive in low-mass clusters, except in the central regions. We also find that planet ejections through stellar encounters are cumulative processes, as only ˜3 per cent of encounters are strong enough to excite the eccentricity by Δe ≥ 0.5. Short-period planets can be perturbed through orbit crossings with long-period planets. When taking into account planet-planet interactions, the planet ejection rate nearly doubles, and therefore multiplicity contributes to the vulnerability of planetary systems. In each ensemble, ˜0.2 per cent of planetary orbits become retrograde due to random directions of stellar encounters. Our results predict that young low-mass star clusters are promising sites for next-generation planet surveys, yet low planet detection rates are expected in dense globular clusters such as 47 Tuc. Nevertheless, planets in denser stellar environments are likely to have shorter orbital periods, which enhance their detectability. Show less
Cai, X.; Gieles, M.; Heggie, D.C.; Varri, A.L. 2016
We study the evolution of star clusters on circular and eccentric orbits using direct N-body simulations. We model clusters with initially N = 8k and 16k single stars of the same mass, orbiting... Show moreWe study the evolution of star clusters on circular and eccentric orbits using direct N-body simulations. We model clusters with initially N = 8k and 16k single stars of the same mass, orbiting around a point-mass galaxy. For each orbital eccentricity that we consider, we find the apogalactic radius at which the cluster has the same lifetime as the cluster with the same N on a circular orbit. We show that then, the evolution of bound particle number and half-mass radius is approximately independent of eccentricity. Secondly, when we scale our results to orbits with the same semimajor axis, we find that the lifetimes are, to first order, independent of eccentricity. When the results of Baumgardt and Makino for a singular isothermal halo are scaled in the same way, the lifetime is again independent of eccentricity to first order, suggesting that this result is independent of the galactic mass profile. From both sets of simulations, we empirically derive the higher order dependence of the lifetime on eccentricity. Our results serve as benchmark for theoretical studies of the escape rate from clusters on eccentric orbits. Finally, our results can be useful for generative models for cold streams and cluster evolution models that are confined to spherical symmetry and/or time-independent tides, such as Fokker-Planck models, Monte Carlo models, and (fast) semi-analytic models. Show less
Kouwenhoven, M.B.N.; Shu, Q.; Cai, X.; Spurzem, R. 2016
Astrophysical research in recent decades has made significant progress thanks to the availability of various N-body simulation techniques. With the rapid development of high-performance computing... Show moreAstrophysical research in recent decades has made significant progress thanks to the availability of various N-body simulation techniques. With the rapid development of high-performance computing technologies, modern simulations have been able to use the computing power of massively parallel clusters with more than 105 GPU cores. While unprecedented accuracy and dynamical scales have been achieved, the enormous amount of data being generated continuously poses great challenges for the subsequent procedures of data analysis and archiving. In this paper, we propose an adaptive storage scheme for simulation data, inspired by the block time step (BTS) integration scheme found in a number of direct N-body integrators available nowadays, as an urgent response to these challenges. The proposed scheme, namely, the BTS storage scheme, works by minimizing the data redundancy by assigning individual output frequencies to the data as required by the researcher. As demonstrated by benchmarks, the proposed scheme is applicable to a wide variety of simulations. Despite the main focus of developing a solution for direct N-body simulation data, the methodology is transferable for grid-based or tree-based simulations where hierarchical time stepping is used. Show less
