Joseph M. Hahn

Orbital Evolution of Planets Embedded in a Planetesimal Disk

The existence of the Oort comet cloud, the Kuiper belt, and plausible inefficiencies in planetary core formation all suggest that there was once a residual planetesimal disk of mass ~10–100 M⊕ in the vicinity of the giant planets following their formation. Since removal of this disk requires an exchange of orbital energy and angular momentum with the planets, significant planetary migration can ensue. The planet migration phenomenon is examined numerically by evolving the orbits of the giant planets while they are embedded in a planetesimal disk having a mass of MD = 10–200 M⊕. We find that Saturn, Uranus, and Neptune evolve radially outward as they scatter the planetesimals, while Jupiters orbit shrinks as it ejects mass. Higher mass disks result in more rapid and extensive planet migration. If orbital expansion and resonance trapping by Neptune are invoked to explain the eccentricities of Pluto and its cohort of Kuiper belt objects at Neptunes 3:2 mean motion resonance, then our simulations suggest that a disk mass of order MD ~ 50 M⊕ is required to expand Neptunes orbit by Δa ~ 7 AU, in order to pump up Plutino eccentricities to e ~ 0.3. Such planet migration implies that the solar system was more compact in the past, with the initial Jupiter-Neptune separation having been smaller by about 30%. We discuss the fate of the remnants of the primordial planetesimal disk. We point out that most of the planetesimal disk beyond Neptunes 2:1 resonance should reside in nearly circular, low-inclination orbits, unless there are (or were) additional, unseen, distant perturbers. The planetesimal disk is also the source of the Oort cloud of comets. Using the results of our simulations together with a simple treatment of Oort cloud dynamics, we estimate that ~12 M⊕ of disk material was initially deposited in the Oort cloud, of which ~4 M⊕ will persist over the age of the solar system. The majority of these comets originated from the Saturn-Neptune region of the solar nebula.

Ground-based Near-Infrared Imaging of the HD 141569 Circumstellar Disk

We present the first ground-based near-infrared image of the circumstellar disk around the post-Herbig Ae/Be star HD 141569A initially detected with the Hubble Space Telescope (HST). Observations were carried out in the near-IR (2.2 μm) at the Palomar 200 inch telescope using the adaptive optics system PALAO. The main large-scale asymmetric features of the disk are detected on our ground-based data. In addition, we measured that the surface brightness of the disk is slightly different than that derived by HST observations (at 1.1 and 1.6 μm). We interpret this possible color effect in terms of dust properties and derive a minimal grain size of 0.6 ± 0.2 μm for compact grains and a power-law index for the grain size distribution smaller than -3. Basic dynamical considerations are consistent with the presence of a remnant amount of gas in the disk.

DYNAMICS OF THE TRANS-NEPTUNE REGION: APSIDAL WAVES IN THE KUIPER BELT

The role of apsidal density waves propagating in a primordial trans-Neptune disk (i.e., Kuiper belt) is investigated. It is shown that Neptune launches apsidal waves at its secular resonance near 40 AU that propagate radially outward, deeper into the particle disk. The wavelength of apsidal waves is considerably longer than waves that might be launched at Lindblad resonances, because the pattern speed, gs, resulting from the apsis precesssion of Neptune is much slower than its mean motion, Ωs. If the early Kuiper belt had a sufficient surface density, σ, the disks wave response to Neptunes secular perturbation would have spread the disturbing torque radially over a collective scale λ* ≈ r(2μd Ω/|r dg/dr|)1/2, where μd ≡ πσr2/(1 M⊙) and Ω(r) and g(r) are respectively the mean motion and precession frequency of the disk particles. This results in considerably smaller eccentricities at resonance than had the disk particles been treated as noninteracting test particles. Consequently, particles are less apt to be excited into planet-crossing orbits, implying that the erosion timescales reported by earlier test-particle simulations of the Kuiper belt may be underestimated. It is also shown that the torque the disk exerts upon the planet (due to its gravitational attraction for the disks spiral wave pattern) damps the planets eccentricity and further inhibits the planets ability to erode the disk.

NGC 4314. I. Visible and short-wavelength infrared surface photometry of the nucleus and bar

BVI (TI CCD) and JHK (University of Texas IR Camera) surface photometry of NGC 4314, an SB (rs)ap anemic spiral with a nuclear ring containing recent star formation, is presented. The shortwave IR (SWIR) frames reveal a nuclear bar of length 2 arcsec at PA of 0 deg. The nuclear ring and associated dust were detected in all SWIR color indices. A nuclear spiral was detected in the visible and SWIR just exterior to the ring. Extremely low-amplitude spiral-shaped deficits were found in the stellar distribution in the SWIR in this same region. These are attributed to dust, since CO is detectable at or near these locations. Average minor-axis profiles show this galaxy to have a nuclear bulge obeying the de Vaucouleurs r exp 1/4 law for values in the range 2-7 arcsec. The extinction and scattering characteristics of dust near the sites of recent star formation in the nuclear ring are discussed.

The nature of comet Shoemaker-Levy 9 subnuclei from analysis of preimpact Hubble Space Telescope images

We obtained Hubble Space Telescope (HST) Wide Field Planetary Camera images of comet Shoemaker-Levy 9 (S-L 9) prior to impact with Jupiter in 1994 to observe the effects of the Jovian gravitational field on the approaching fragments. The HST images were used to monitor the absolute brightness in the central regions of the comae, changes in the surface brightness profiles, and general morphological changes in the fragments as they approached final impact with Jupiter. S-L 9 most likely came apart under the influence of the Jovian tidal force near perijove in 1992 [Scotti and Melosh, 1993; Asphaug and Benz, 1994a; Solem, 1994; Sekanina et al., 1994] into approximately 20 fragments [Scotti, 1993; Luu and Jewitt, 1993; Weaver et al., 1994]. In contrast to condensed or solid fragments, we suggested that after perijove, the breakup debris condensed initially as swarms of particles [Rettig et al., 1994a; Olson and Mumma, 1994] and if interparticle collisions were infrequent, some of these swarm-like subnuclei might have remained extended throughout the 2-year orbit. In this paper we discuss a simple analytical model to demonstrate how after the original nucleus came apart, the debris cloud formed 14–20 fragments. We also present temporal magnitudes of all fragments obtained from the HST images from January through July 1994 and an analysis of the preimpact images for four S-L 9 condensations T(4), Q2(7b), P1(8a), and K(12) which suggests that not all of the central subnuclei were consistent with solid objects. The temporal magnitudes and surface brightness profiles of several S-L 9 fragments are consistent with tidal dispersal at distances much greater than the classical ∼2RJ Roche limit.

Spiral Bending Waves Launched at a Vertical Secular Resonance

The excitation of spiral bending waves at a secular vertical resonance in a particle disk is examined. These waves are one-armed spirals of very long wavelength that are launched at sites where a secondarys nodal regression rate matches that of the disk. Nodal bending waves usually propagate radially outward as leading waves from a secular resonance exterior to the perturber, and inward as trailing waves from a secular resonance that lies interior. Their pattern speed is negative, so the spiral pattern rotates in a retrograde sense. The waves carry negative angular momentum but very little energy, and their excitation can damp the inclination of the secondary. Here we apply this theory to the case of two mutually precessing planets orbiting in a particle disk and compare their damping rate with the more familiar inclination excitation due to mean motion vertical resonances. We suggest that under certain circumstances, nodal wave damping may be an important element in maintaining planetary and/or embryo orbits in a nearly coplanar state.

Comet Shoemaker-Levy 9: An Active Comet

Abstract The important elements of the debate over the activity versus dormancy of comet Shoemaker-Levy 9 (S-L-9) are reviewed. It is argued that the circularity of the isophotes in the inner comae of S-L 9 as well as the spatial dependencies of the comae brightness profiles are indicators of sustained dust production by S-L 9. It is also shown that the westward tail orientations, which were formerly interpreted as a sign of the comets dormancy, are not a good indicator of either activity or dormancy. Rather, the tail orientations simply place constraints on the dust production rate for grains smaller than ≈ 5 μm. All the available evidence points to S-L 9 as having been an active, dust-producing comet. Synthetic images of an active comet are fitted to Hubble Space Telescope images of the S-L 9 fragment K, and its grain size and outflow velocity distributions are extracted. These findings show that the appearance of the dust coma was dominated by large grains having radii between ≈ 30 μm and ≈ 3 mm, produced at a rate of M ≈ 22 kg s−1, and ejected at outflow velocities of ≈ 0.5 m s−1. Only upper limits on the production rates of smaller grains are obtained. The nucleus of fragment K was not observed directly but its size is restricted to lie within a rather narrow interval 0.4 ≲ Rf ≲ 1.2 km.