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W.O. Love, D.G. Furton (Rhode Island College), A.N. Witt, T.L. Smith (University of Toledo)
It has been proposed by Witt, Gordon, and Furton (1998, ApJ, 501, L111) that extended red emission (ERE), the broad, red dust luminescence band which is observed in a wide range of astrophysical environments, is due to luminescence of silicon nanoparticles (SNPs). Furthermore, it seems to be the case that the ERE band is both more efficient and bluer in environments where the energy density is lower (e.g. the diffuse interstellar medium) than where the energy density is higher (e.g. reflection and planetary nebulae). Recent laboratory results indicate that luminescence of SNPs is dependent upon particle size. Specifically, smaller grains have higher luminescence efficiencies and bluer emission spectra than larger ones. In interstellar environments, photodissociation prevents grains below a certain critical size from existing; a very small grain absorbing a single ultraviolet photon will be heated to a temperature in excess of 1000 K, which can lead to photodissociation. This thermal spiking is also believed to be the mechanism which gives rise to excess near infrared continuum emission. Following Guhathakurta and Draine (1989, ApJ, 345, 230) we calculate the temperature fluctuations and lower size limits of SNPs as a function of radiation field energy density. Our calculations differ from those of Guhathakurta and Draine in that we employ experimentally measured values of the optical constants of SNPs and experimentally determined dissociation energy as a function of cluster size. This obviates the need to make fiducial corrections to physical parameters determined from bulk solids. These calculations show that in environments where the energy density of the ambient radiation field is higher the lower size limit of the grains is larger and the wavelength of peak ERE is lower. This work is supported by NASA grant NAG5-4338 to the University of Toledo with a subcontract to Rhode Island College.
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