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tokens: We thank Wenfei Yu for helpful discussions. This work was supported by the National Basic Research Program of China (973 Program) under grants 2014CB845800 and 2015CB857004 , the National Natural Science Foundation of China under grants 11573023 , 11573009 , 11473022 , 11403074 , 11333004 , 11222328 , 11133005 and U1331101 , the Knowledge Innovation Program of the Chinese Academy of Sciences , the CAS Open Research Program of Key Laboratory for the Structure and Evolution of Celestial Objects under grant OP201503 , and the Fundamental Research Funds for the Central Universities under grants 20720140532 and 20720160024 .