Mg-Rich Silicate Crystals in Comet Hale-Bopp: ISM Relics or Solar Nebula Condensates?

We compare the HIFOGS 10-μm spectra of Comet C/1995 (O1) (Hale-Bopp) to the mid-IR spectra of interplanetary dust particles (IDPs). The four epochs of Hale-Bopp silicate features, at 2.8 AU (pre-perihelion), 1.2 AU (pre-perihelion), 0.93 AU (10 days after perihelion), and 1.7 AU (post-perihelion) have different spectral shapes. At smaller heliocentric distances, r_h≤1.7 AU, there appears to be enhanced mid-IR emission (i.e., 9.3- and 10.5-μm peaks) from Mg-rich pyroxene crystals. A two-temperature model of warm amorphous and crystalline olivines, warm amorphous pyroxenes, and ~165 K cooler pyroxene crystals is successful with laboratory silicate minerals (D. H. Wooden et al. 1999, Astrophys. J.517, 1034-1058). A two-temperature model using IDPs as cometary dust particle analogs also fits Comet Hale-Bopp's silicate features, with the pyroxene IDP ~50 K cooler than the olivine and layer-lattice silicate IDPs. The warm layer-lattice silicate IDP contributes a broad 20-μm feature which is not in the Infrared Space Observatory Short Wavelength Spectrometer (ISO SWS) spectrum of Hale-Bopp at 2.8 AU, making it a questionable cometary component. On the other hand, members of the ''cluster IDP" subclass of pyroxene IDPs fit Hale-Bopp's 10-μm spectra better than other pyroxene IDPs. Cluster IDPs are highly fragile and contain significant deuterium enrichments, indicating 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 multiple grain components at different temperatures to explaining the temporal evolution of the observed infrared features. Relative abundances of minerals depend on their relative temperatures. Cooler grains may be more abundant than warmer grains and yet may remain hidden from detection in the 10-μm silicate feature because of their cooler temperatures. The deduced larger abundance of cooler, pyroxene-dominated grains in Comet Hale-Bopp affects the interpretation of the origin of cometary dust, and dust evolution during protoplanetesimal accumulation and disk dissipation.