Mg-rich silicate crystals in comet Hale-Bopp: ISM relics or solar nebula condensates?

We compare the HIFOGS 10-mu m spectra of Comet C/1995 (O1) (Hale-Bopp) to t he mid-LR spectra of interplanetary dust particles (IDPs), The four epochs of Hale-Bopp silicate features, at 2.8 AU (pre-perihelion), 1.2 AU (pre-per ihelion), 0.93 AU (10 days after perihelion), and 1.7 AU (post-perihelion) have different spectral shapes. At smaller heliocentric distances, r(h) les s than or equal to 1.7 AU, there appears to be enhanced mid-IR emission (i. e., 9.3- and 10.5-mu m peaks) from Mg-rich pyroxene crystals. A two-tempera ture model of warm amorphous and crystalline olivines, warm amorphous pyrox enes, and similar to 165 K cooler pyroxene crystals is successful with labo ratory silicate minerals (D. H. Wooden et al. 1999, Astrophys. J. 517, 1034 -1058). A two-temperature model using LDPs as cometary dust particle analog s also fits Comet Hale-Bopp's silicate features, with the pyroxene IDP simi lar to 50 K cooler than the olivine and layer-lattice silicate IDPs. The wa rm layer-lattice silicate IDP contributes a broad 20-mu m feature which is not in the Infrared Space Observatory Short Wavelength Spectrometer (ISO SW S) spectrum of Hale-Bopp at 2.8 AU, making it a questionable cometary compo nent. On the other hand, members of the "cluster IDP" subclass of pyroxene IDPs fit Hale-Bopp's 10-mu m spectra better than other pyroxene IDPs. Clust er IDPs are highly fragile and contain significant deuterium enrichments, i ndicating their presolar origin. By spectroscopic analogy, Comet Hale-Bopp' s Mg-rich pyroxene crystals may also be relic interstellar grains. The IDP and mineral models for Comet Hale-Bopp indicate the importance of m ultiple grain components at different temperatures to explaining the tempor al evolution of the observed infrared features. Relative abundances of mine rals depend on their relative temperatures. Cooler grains may be more abund ant than warmer grains and yet may remain hidden from detection in the 10-m u m silicate feature because of their cooler temperatures. The deduced larg er abundance of cooler, pyroxene-dominated grains in Comet Hale-Bopp affect s the interpretation of the origin of cometary dust, and dust evolution dur ing protoplanetesimal accumulation and disk dissipation. (C) 2000 Academic Press.