Results

Of the nine new well-measured comets in our infrared survey of cometary dust emission, the intermediate-aged long period comets C/Hyakutake 1996B2, P/IRAS 1996, and C/Hale-Bopp 199501 demonstrated high rates of dust emission dominated by small particles, including pronounced silicate features, long-wavelength emissivity falloffs, and large superheats, while the extremely processed short period comets P/Schwassmann-Wachmann 3, P/Encke, and P/Tempel-Tuttle emitted mainly large dust particles at relatively low rates. The dust from Comet Hale-Bopp was slightly anomalous in that the particles seemed especially dense and small [2], although of normal composition compared to other comets. Figure 3 presents the strong correlations between the temperature, albedo, and silicate strength of the emitted dust (brighter dust is hotter and has a larger silicate feature), between the dust production rate and the temperature of the dust (hotter dust is produced more rapidly), and the strength of the silicate feature and the Tisserand invariant (largest silicate feature and smallest dust particles are emitted by the LP comets) for the 9 comets in our survey.

Taken in aggregate, there appears to be a real difference in the particle size distributions for our 9 comet dataset of the dust emitted by the new and short period comets, which emit mainly large, dark dust particles, versus the small, bright dust emitted by the long period comets. The new comets also appear to have lower D/G ratios than the others.

Comparison of our results to the two classes of comets found in the optical polarimetric survey of Levasseur-Regourd [16] shows an excellent match of our large particle dominated emission with their "gassy, low polarization comets" and our small particle dominated emission with their "dusty, high polarization comets". This is consistent with the expectation that small particles have a higher albedo and polarization per unit dust mass [4, 12], Hale-Bopp stands out as having unusually high polarization and albedo, which is consistent with our models showing it had unusually solid, small dust [2]. The strength of the 8-13 /¿m silicate emission feature versus the continuum is known to be inversely proportional to the dust particle size [17]; for the 5 of our comets also observed in the silicate feature at 8 - 13 jxm by Hanner et al. [18], we find good agreement between our assignment of large or small particle dominated emission and their finding of large or small amplitude of the silicate feature.

Concern that our results may be preliminary due to small sample size and selection effects led us to search the literature for other infrared comet observations. (Note that while the 5 new comets were selected on the basis of their expected brightness and known orbits, the original 4 comets of the database, detected in the COBE all sky survey of 1989 - 1990, were selected only by their IR brightness against the background.) In Table 2 we present the results of searching for thermal IR observations of comets in the literature (c.f. [19] for references), ordered by whether small or large particle emission was evident via the superheat/silicate feature strength/long wavelength emissivity falloff criteria, and ordered within these two groups by perihelion distance. For the more than 40 literature comets observed, we again find

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