ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Organization I-Organization O O O O O B-Fellowship I-Fellowship I-Fellowship I-Fellowship O O B-Observatory I-Observatory I-Observatory O B-Location I-Location O O O O O O O O O O O O
tokens: Acknowledgements I am very grateful to the anonymous referee for his/her helpful comments and suggestions that have led to a significant improvement in the presentation of this manuscript. I am also grateful to Amherst College for its support through a John Woodruff Simpson Lectureship, and to Purple Mountain Observatory in Nanjing, China, for its hospitality while part of this work was being carried out.