KVKI thanks the Director of the Indian Institute of Astrophysics (IIA) for kindly providing him the facilities of IIA while this work was being carried out. He also thanks the Smithsonian Institution, Washington DC, U.S.A. for the award of a Short-Term Visiting Appointment (and also for a short extension of this appointment) to work at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass, U.S.A. where a major part of this work was carried out. He thanks Dr. G.G. Fazio for his kind hospitality and fruitful discussions. He also thanks the Research Programs Office of the Space Telescope Science Institute (STScI), Baltimore, MD, U.S.A. for providing financial assistance to enable him to visit DJM for discussions related to this work. DJM thanks the STScI for salary support during the visit of KVKI and during analysis of this material. We thank Profs. H. Bhatt and M. Parthasarathy (both of IIA) for several useful
discussions and Dr. C. Barnbaum for a careful reading of the manuscript. We also thank Dr. van der Veen, (the referee of this paper) for several useful comments and suggestions which helped to improve the manuscript. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. DJM's classification work was supported by the NASA Astrophysics Data Program under JPL Contracts 766490 and 957272.
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