Introduction

Studies of the physical properties of cometary dust are important for understanding the phenomena of comets as a class, and for understanding the formation and evolution of comets and the solar system. The quantities we can derive from infrared observations of the dust include the dust composition, mass, particle size distribution, and emission history. Given a large enough statistical population of observations, we can use the results of the measurements to understand the role of comets in the present day solar system, including, e.g., the contribution of cometary dust to the interplanetary dust cloud, and the evolution and fate of highly aged comets. Since comets are among the most primitive bodies in the inner solar system, despite their evolution, we also want to use the results to understand the formation of the solar system from the proto-solar nebula, e.g., the total and relative abundance of rock forming material, the formation and evolution of the accreting icy planetesimals (comets), and chemical variations in the proto- solar nebula. We hope to learn more about the aging process of comets, e.g.: the effect of weathering and mantling on the cometary surface and the effect of collisions on the cometary population.

Combined thermal infrared and optical observations are sensitive to the 0.1 - 100 um range of dust particle sizes [1, 2, 3]. The presence of a strong 8 - 13 ¡xm silicate emission feature is also an indicator of relatively small dust grains (< 10 pim), as is a spectral color temperature elevated more than 10% above the local equilibrium temperature, or a negative deviation from greybody behavior at 30 - 100 fim [4], A total albedo for scattering, defined as the ratio of the scattered luminosity to the total luminosity observed at a given phase angle, is -6% for large cometary particles (e.g., comets Austin and Encke) and >12% for small particles (e.g. comets Levy, Hyakutake, and Hale-Bopp). Extensions of the dust tail morphology along the projected orbital velocity direction (i.e., "trails") are due to large, heavy particle emission, while morphologies with only anti-solar tails are indicators of small particle emission [3], Trends in long term light curves yield independent estimates of the particle size distribution and emission rate [1, 2],

There is growing evidence, from the Giotto encounter with the P/Halley nucleus [5], the IRAS observations of C/IRAS-Araki-Alcock 1983H1 [6], the COBE/DIRBE large angular scale observations of 4 comets [1], and the sub-mm observations of comet C/Hyakutake 1999 B2 and C/Hale-Bopp 1995 01 [7, 8], that the majority (by mass) of grains emitted by comets are visibly dark and large, requiring the use of mid- to far-IR observations to probe the nature of cometary dust. This conclusion is very different from the current paradigm created mainly from large optical surveys of periodic comets [9, 10], which concludes that the majority of cometary dust mass is emitted by the optically bright comets.

2. OBSERVATIONS

Our study of the statistical properties of cometary dust emission began with the large-scale photometric imaging of 4 comets detected in the COBE/DIRBE all-sky survey of 1989 -1990. These comets were selected only by our ability to detect them against the background in the COBE all sky survey, down to V ~ 12. As summarized [1], C/OLR and C/Austin were found to emit only large, dark dust grains not detected in the visible, while C/Levy was more Halley-like in its emission of many small dust grains. That the comet 73P/SW-3 was detected at all by COBE was quite surprising, as it was optically fainter than 4 other short period comets that were not detected; it was argued [1] that this was due to the enhanced release of large, dark particles due to the incipient breakup of the comet during its next perihelion passage.

The present paper adds 5 more comets to our database of comets well-characterized using multi-wavelength photometry in the thermal infrared (Table 1). Included are the extremely bright long-period comets of 1996 and 1997, C/Hyakutake 1996 B2 [11] and C/Hale-Bopp 1995 01 [2, 12]; and the close-approaching periodic comets P/IRAS 1996, P/Encke 1997 [3], and P/Tempel-Tuttle 1998. Not included are comets like P/Wild2 1997, C/Utsunomiya 1997 Tl, and C/LINEAR 1998 U5, which were observed by our group but only detected in broadband N [13], The reader is referred to the individual observational papers listed for each comet for the details of the observations and the photometric calibration.

3. MODELS

Results from the comet observations of the COBE mission [14, 1], have shown that simultaneous modeling of the shape and scattering/thermal emission spectrum of a comet's dust tail can constrain the silicatexarbonaceous composition ratio and particle size distribution of the dust. Our methods of spectral, dynamical, and temporal modeling of the thermal emission from the cometary dust have been extensively summarized in [1], where we demonstrate the ability to derive absolute dust production rates, dust-to-gas ratios, and rough compositional information for 4 comets using 1 - 300 fim multiwavelength broad-band infrared photometric imaging obtained by the COBE/DIRBE instrument in 1989 - 1990. [2] demonstrates our ability to use the same modeling techniques for comet C/Hale-Bopp from ground based 1-20 ¡xm photometry.

We present here the aggregate results of modeling our IR observations of 9 different comets (Table 1, Figure 2). Our broad-band multi-wavelength photometry observations can be explained by dust with similar silicatexarbonaceous composition ratios of 2 to 3 : 1, but two very different dust particle size distributions (PSD's), the [15] and the nr0-63 (or 1/(5; [1]) distributions. While both these PSD's are mass dominated by the largest (radius >100 um)

Figure 1. Example of small and large particle dominated dust emission infrared spectral observations, a) Spectral energy distribution of comet P/IRAS 1996 as taken by ISO from 3100 fim, at the NASA/IRTF from 1-3 fim, and at the Lowell 42" on October 13, 1996 [3], Squares - ISO observations with 20% error bars. Dotted Curve - best-fit greybody to the infrared data, 20% hotter than the local equilibrium temperature, due to the presence of small, hot grains. From 8 to 13 /im, there is a strong emission feature due to silicates in small dust grains. At 60 and 100 pim, there is a strong fall off from the blackbody curve due to decreasing emission efficiency from small grains at large wavelengths. Dashed Curve - solar spectrum normalized to ISO observation at 1.25 fim. There is good agreement between the observed scattered light spectrum and a blackbody of 5700 K, the average temperature of the Sun's emission, b) Spectral energy distribution of comet P/Encke 1997 as taken by ISO from 3-100 Um on July 16, 1997, and at 5-20 fim at the ESO 3.6m on July 18, 1997 [3], Triangles - ISO observations with 20% error bars. Solid curve - best fit greybody to the emission. The best fit greybody temperature is very close to the local equilibrium temperature, a strong indication that only large grains are being observed. There is no evidence of a silicate emission feature or long wavelength emissivity falloff.

Table 1

Emitted dust properties for the 9 comets in the IR survey.

Emitted dust properties for the 9 comets in the IR survey.

Table 1

Comet

Time (UT Day)

(AU)

(%)

QDust (kg/sec)

Qgas (mol/sec)

Dust/Ga s

C/OLR (New)*

1989 Dec 20

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