J.N. Cuzzi

Closed-form expressions for particle relative velocities induced by turbulence (Research Note)

In this note we present complete, closed-form expressions for random relative velocities between colliding particles of arbitrary size in nebula turbulence. These results are exact for very small particles (those with stopping times much shorter than the large eddy overturn time) and are also surprisingly accurate in complete generality (that is, also apply for particles with stopping times comparable to, or much longer than, the large eddy overturn time). We note that some previous studies may have adopted previous simple expressions, which we find to be in error regarding the size dependence in the large particle regime.

Compositions of Saturn's rings A, B, and C from high resolution near-infrared spectroscopic observations

We used the NASA IRTF spectrograph SpeX to obtain near-infrared spectra (0.9-5.4 µm) of Saturns rings, achieving spectral resolution λ/∆λ of about 2000. The spatial resolution (about 1 arcsec) is sufficient to distinguish the three main ring components (A, B and C rings) from one another. These new observations of Saturns rings are the first to combine an extended spectral range with high spectral resolution and good spatial resolution. We combined these data with recent photometric observations acquired by HST in the 0.3-1.0 µm range. The spectra of the A band B rings are dominated by strong features due to crystalline water ice. The shape and the depth of these absorptions differ for each ring, which indicates different water ice grain sizes and abundances. No spectral evidence for volatile ices other than water ice has been detected. Both the lower albedo and the less blue slope in the near-infrared reflectance of the C ring indicate a concentration of dark material different from that in the A and B rings. The broader triangular Fresnel reflection peak at 3.1 µm may support the presence of some amount of amorphous ice. The C ring spectrum exhibits bands centered at 1.73 and 3.4 µm which agree in position quite well with the C-H bands. Although the detection is probable, it requires confirmation. With a radiative transfer model, we constrain the grain sizes and the relative abundances of water ice, a dark colorless component (amorphous carbon) to adjust the albedo and a second contaminant to reproduce the reddening in the UV-visible range represented here by organic tholins. The dark component of the C ring spectrum is included as an intra-mixture only. The cosmogenic implications of the inferred compositions are discussed.

Co-accretion of chondrules and dust in the solar nebula

We present a mechanism for chondrules to stick together by means of compaction of a porous dust rim they sweep up as they move through the dusty nebula gas. It is shown that dust aggregates formed out of micron-size grains stick to chondrules, forming a porous dust rim. When chondrules collide, this dust can be compacted by means of rolling motions within the porous dust layer. This mechanism dissipates the collisional energy, compacting the rim and allowing chondrules to stick. The structure of the obtained chondrule-dust agglomerates (referred to as compounds) then consists of three phases: chondrules, porous dust, and dust that has been compacted by collisions. Subsequently, these compounds accrete their own dust and collide with other compounds. The evolution of the compound size distribution and the relative importance of the phases is calculated by a Monte Carlo code. Growth ends, and a simulation is terminated when all the dust in the compounds has been compacted. Numerous runs are performed, reflecting the uncertainty in the physical conditions at the chondrule formation time. It is found that compounds can grow by 1-2 orders of magnitudes in radius, up to dm sizes when turbulence levels are low. However, relative velocities associated with radial drift form a barrier for further growth. Earlier findings that the dust sweep-up by chondrules is proportional to their sizes are confirmed. We contrast two scenarios regarding how this dust evolved further toward the densely packed rims seen in chondrites.

On the Persistence of Small Regions of Vorticity in the Protoplanetary Nebula

The fate of small regions of vorticity in a barotropic model of the protoplanetary nebula is investigated over thousands of years using a finite difference model. It is found that the coherence time for a small island of vorticity depends on its size, strength, orientation, and radial location in the nebula. Anticyclonic vorticity retains its coherence for longer times than cyclonic vorticity due to favorable interactions with the Keplerian shear flow. Rossby waves are generated as a result of mean vorticity gradients across the disk. The two-dimensional nebula evolves from discrete vortices into an axisymmetric flow consisting of small-amplitude vortex sheets at the radial locations of the initial vorticity. These vortex sheets induce an additional small, potential flow velocity superimposed on the Keplerian rotation curve.

A high-sensitivity search for extraterrestrial intelligence at γ18 cm

Abstract We have conducted a targeted high-sensitivity search for narrow-band signals near γ18 cm using the 91-m radiotelescope of the National Radio Astronomy Observatory. The search included 201 nearby solar-type stars and achieved a frequency resolution of 5.5 Hz over a 1.4-MHz bandwidth. This high spectral Mark I VLBI recording terminal in conjunction with the CDC 7600 computational facility at the NASA-Ames Research Center. This is the first high-resolution search for narrow-band signals in this wavelength regime. To date it is the most sensitive search per unit observing time of any search strategy which does not postulate a unique magic frequency. Our data show no evidence for narrow-band signals due to extraterrestrial intelligence at a 12-σ upper limit on signal strength of 1.1 × 10 −23 W m −2 .