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<jats:p> Analysis of Hubble Space Telescope (HST) images of comet Hale-Bopp (C/1995 O1) suggests that the effective diameter of the nucleus is between 27 to 42 kilometers, which is at least three times larger than that of comet P/Halley. The International Ultraviolet Explorer and HST spectra showed emissions from OH (a tracer of H <jats:sub>2</jats:sub> O) and CS (a tracer of CS <jats:sub>2</jats:sub> ) starting in April 1996, and from the CO Cameron system (which primarily traces CO <jats:sub>2</jats:sub> ) starting in June 1996. The variation of the H <jats:sub>2</jats:sub> O production rate with heliocentric distance was consistent with sublimation of an icy body near its subsolar point. The heliocentric variation in the production rates of CS <jats:sub>2</jats:sub> and dust was different from that of H <jats:sub>2</jats:sub> O, which implies that H <jats:sub>2</jats:sub> O sublimation did not control the CS <jats:sub>2</jats:sub> or dust production during these observations. </jats:p>

<jats:p>Comet Hale-Bopp (C/1995 O1) was observed at wavelengths from 2.4 to 195 micrometers with the Infrared Space Observatory when the comet was about 2.9 astronomical units (AU) from the sun. The main observed volatiles that sublimated from the nucleus ices were water, carbon monoxide, and carbon dioxide in a ratio (by number) of 10:6:2. These species are also the main observed constituents of ices in dense interstellar molecular clouds; this observation strengthens the links between cometary and interstellar material. Several broad emission features observed in the 7- to 45-micrometer region suggest the presence of silicates, particularly magnesium-rich crystalline olivine. These features are similar to those observed in the dust envelopes of Vega-type stars.</jats:p>

<jats:p>Thermal infrared (IR) imaging and spectroscopy of comet Hale-Bopp (C/1995 O1) during June, August, and September 1996 traced the development of the dust coma several months before perihelion. Images revealed nightly variations in the brightness of the inner coma from 1 to 12 June that were correlated with the appearance of a northward-pointing jet. The central IR flux increased by a factor of 8 between 1 June and 30 September, and the September data showed IR jets that corresponded to similar structures that were visible in reflected sunlight at shorter wavelengths. At all epochs, 8- to 13-micrometer spectra of the central coma revealed a strong silicate emission feature, including an 11.2-micrometer feature indicative of crystalline olivine, even when the comet was at a heliocentric distance of 4.1 astronomical units.</jats:p>

<jats:p> The activity of comet Hale-Bopp (C/1995 O1) was monitored monthly by optical imaging and long-slit spectroscopy of its dust and gas distribution over heliocentric distances of 4.6 to 2.9 astronomical units. The observed band intensities of the NH <jats:sub>2</jats:sub> radical and the H <jats:sub>2</jats:sub> O <jats:sup>+</jats:sup> ion cannot be explained by existing models of fluorescence excitation, warranting a reexamination of the corresponding production rates, at least at large heliocentric distances. Comparing the production rate of the CN radical to its proposed parent, HCN, shows no evidence for the need of a major additional source for CN in Hale-Bopp at large heliocentric distances. The dust and CN production rates are consistent with a significant amount of sublimation occurring from icy dust grains surrounding Hale-Bopp. </jats:p>

<jats:p>Narrowband photometric observations of comet Hale-Bopp (C/1995 O1) between 25 July 1995 and 15 February 1997 indicated gas and dust production rates of 20 and 100 times greater, respectively, than observed at the same heliocentric distances for comet P/Halley in 1985. Hale-Bopp produced dust at a rate greater than has been observed for any other comet at any distance since at least 1977. On the basis of the observed production rate of the hydroxyl molecule, the calculated minimum effective diameter of Hale-Bopp's nucleus is 17 kilometers, but the actual diameter of the nucleus is likely to be at least two to three times larger. The chemical composition of Hale-Bopp is consistent with that of other long-period comets originating from the Oort Cloud.</jats:p>

<jats:p> Spectra obtained from ground-based radio telescopes show the progressive release of CO, CH <jats:sub>3</jats:sub> OH, HCN, H <jats:sub>2</jats:sub> O (from OH), H <jats:sub>2</jats:sub> S, CS, H <jats:sub>2</jats:sub> CO, CH <jats:sub>3</jats:sub> CN, and HNC as comet Hale-Bopp (C/1995 O1) approached the sun from 6.9 to 1.4 astronomical units (AU). The more volatile species were relatively more abundant in the coma far from the sun, but there was no direct correlation between overabundance and volatility. Evidence for H <jats:sub>2</jats:sub> O sublimation from icy grains was seen beyond 3.5 AU from the sun. The change from a CO-driven coma to an H <jats:sub>2</jats:sub> O-driven coma occurred at about 3 AU. The gas outflow velocity and temperature increased as Hale-Bopp approached the sun. </jats:p>

<jats:p> Optical spectra of comet Hale-Bopp (C/1995 O1) at a heliocentric distance of 6.45 astronomical units showed emission from cyanogen gas. The spatial distribution of cyanogen was considerably more diffuse and extended compared to the spatial profile of the dust or grains which were sharply peaked near the center. This behavior is consistent with comets at smaller heliocentric distances suggesting the same or a similar formation mechanism. A cyanogen gas production rate of (1.2 ± 0.3) ×10 <jats:sup>26</jats:sup> molecules per second was derived. A model band profile derived from fluorescence equilibrium calculations for the comet's heliocentric velocity and distance agrees with the observed band profile. </jats:p>

<jats:p> Diffusing-wave spectroscopy was used to measure the microscopic dynamics of grains in the interior of a three-dimensional flow of sand. The correlation functions show that minutely separated grains fly from collision to collision with large random velocities. On a time scale 10 <jats:sup>3</jats:sup> to 10 <jats:sup>4</jats:sup> times longer than the average time between collisions, the grains displayed slow, collective rearrangements, which, at the long-time limit, produced diffusive dynamics. </jats:p>

<jats:p>The electrical properties of individual bundles, or “ropes,” of single-walled carbon nanotubes have been measured. Below about 10 kelvin, the low-bias conductance was suppressed for voltages less than a few millivolts. In addition, dramatic peaks were observed in the conductance as a function of a gate voltage that modulated the number of electrons in the rope. These results are interpreted in terms of single-electron charging and resonant tunneling through the quantized energy levels of the nanotubes composing the rope.</jats:p>

<jats:p> The vibrational properties of amorphous SiO <jats:sub>2</jats:sub> were studied within first-principles density functional theory. The calculated spectrum is in good agreement with neutron data, showing, in particular, a double peak in the high-frequency region. This doublet results from different local modes of the tetrahedral subunits and cannot be ascribed to a longitudinal-optic-transverse-optic (LO-TO) effect. This solves a long-standing controversy about the origin of the doublet in neutron spectra. A LO-TO splitting is recovered only when the long-wavelength limit is probed, as in optical experiments. These findings should be a general feature of tetrahedral AX <jats:sub>2</jats:sub> amorphous networks. </jats:p>