Stanley F. Dermott

How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and Its Application to the HR 4796 Disk

Recent images of the disks of dust around the young stars HR 4796A and Fomalhaut show, in each case, a double-lobed feature that may be asymmetric (one lobe may be brighter than the other). A symmetric double-lobed structure is that expected from a disk of dust with a central hole that is observed nearly edge-on (i.e., close to the plane of the disk). This paper shows how the gravitational influence of a second body in the system with an eccentric orbit would cause a brightness asymmetry in such a disk by imposing a forced eccentricity on the orbits of the constituent dust particles, thus shifting the center of symmetry of the disk away from the star and causing the dust near the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR 4796 disk shows that its ~5% brightness asymmetry could be the result of a forced eccentricity as small as 0.02 imposed on the disk by either the binary companion HR 4796B or by an unseen planet close to the inner edge of the disk. Since it is likely that a forced eccentricity of 0.01 or higher would be imposed on a disk in a system in which there are planets but no binary companion, the corresponding asymmetry in the disks structure could serve as a sensitive indicator of these planets that might otherwise remain undetected.

Mid-infrared images of β Pictoris and the possible role of planetesimal collisions in the central disk

When viewed in optical starlight scattered by dust, the nearly edge-on debris disk surrounding the A5V star β Pictoris (distance 19.3 pc; ref. 1) extends farther than 1,450 au from the star. Its large-scale complexity has been well characterized, but the detailed structure of the disks central ∼200-au region has remained elusive. This region is of special interest, because planets may have formed there during the stars 10–20-million-year lifetime, perhaps resulting in both the observed tilt of 4.6 degrees relative to the large-scale main disk and the partial clearing of the innermost dust. A peculiarity of the central disk (also possibly related to the presence of planets) is the asymmetry in the brightness of the ‘wings’, in which the southwestern wing is brighter and more extended at 12 µm than the northeastern wing. Here we present thermal infrared images of the central disk that imply that the brightness asymmetry results from the presence of a bright clump composed of particles that may differ in size from dust elsewhere in the disk. We suggest that this clump results from the collisional grinding of resonantly trapped planetesimals or the cataclysmic break-up of a planetesimal.

Deep 10 and 18 Micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles and Tentative Evidence for a Brightness Asymmetry

We present new 10.8 and 18.2 km images of HR 4796A, a young A0 V star that was recently dis- covered to have a spectacular, nearly edge-on, circumstellar disk prominent at D20 km (Jayawardhana and coworkers ; Koerner and coworkers). These new images, obtained with OSCIR (the University of Florida Observatory Spectrometer/Camera for the Infrared) at Keck II, show that the disks size at 10 km is comparable to its size at 18 km. Therefore, the 18 kmemitting dust may also emit some, or all, of the 10 km radiation. Using these multiwavelength images, we determine a ii characteristic ˇˇ diameter of 2¨3 km for the mid-infraredemitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Rather, as inferred in a com- panion paper (Wyatt and coworkers), they are probably products of collisions that dominate both the creation and the destruction of dust in the HR 4796A disk. Dynamical modeling of the disk, the details of which are presented in the companion paper, indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider and coworkers from their NICMOS images. Interior to 70 AU, the model density drops steeply by a factor of 2 between 70 and 60 AU, falling to zero by 45 AU, which corresponds to the edge of the previously discovered central hole ; in the context of the dynamical models, this ii soft ˇˇ edge for the central hole occurs because the dust particle orbits are noncircular. The optical depth of mid-infraredemitting dust in the hole is D3% of the optical depth in the disk, and the hole is therefore relatively very empty. We present evidence (D1.8p signi—cance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This ii pericenter glow,ˇˇ which must still be con—rmed, results from a very small (a few AU) shift of the disks center of symmetry relative to the central star HR 7496A ; one side of the inner bound- ary of the annulus is shifted toward HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical eUects of planets on disks is within reach. Subject headings : circumstellar matterinfrared : starsstars : individual (HR 4796A)

The size-frequency distribution of the zodiacal cloud: evidence from the solar system dust bands

Recent observations of the size–frequency distribution of zodiacal cloud particles obtained from the cratering record on the LDEF satellite are the latest evidence for a significant large particle population (100-μm diameter or greater) near 1 AU. Our previous modeling of the Solar System dust bands, features of the zodiacal cloud associated with the comminution of Hirayama family asteroids, has been limited by the fact that only small particles (25-μm diameter or smaller) have been considered. This was due to the prohibitively large amount of computing power required to numerically analyze the dynamics of larger particles. The recent availability of inexpensive, fast processors has finally made this work possible. Models of the dust bands are created, built from individual dust particle orbits, taking into account a size–frequency distribution of the material and the dynamical history of the constituent particles. These models are able to match both the shapes and amplitudes of the dust band structures observed by IRAS in multiple wavebands. The size–frequency index, q, that best matches the observations is approximately 1.4, a distribution in which the surface area (and hence the infrared emission) is dominated by large particles. However, in order to successfully model the “ten degree” band, which is usually associated with collisional activity within the Eos family, we find that the mean proper inclination of the dust particle orbits has to be approximately 9.35°, significantly different from the mean proper inclination of the Eos family (10.08°).

The contribution of cometary dust to the zodiacal cloud

Abstract Any theory of the origin of the particles that supply the zodiacal cloud must account for two key, well-established observations. These are: (1) the observed plane of symmetry of the cloud; and (2) the observed shape of the cloud, that is, the observed variation of the flux in a given waveband with ecliptic latitude for a given elongation angle. The dynamics of small asteroidal particles appears to account for the first observation (Dermott et al., Chaos, Resonance and Collective Phenomena in the Solar System, pp. 333–347. Kluwer, Dordrecht, 1992). However, asteroidal particles that spiral towards the Sun under the action of drag forces without significant disintegration due to dust-dust collisions do not, on their own, provide an explanation for the observed shape of the cloud, because asteroidal dust models provide insufficient flux at the Earths ecliptic poles (Dermott et al., Chaos, Resonance and Collective Phenomena in the Solar System, pp. 333–347. Kluwer, Dordrecht, 1992). In an attempt to account for this major diserepancy, the dynamics of cometary particles is investigated. The orbital evolution of 9 μm diameter dust particles that originate from Comet Encke is described. The orbits of 5000 particles are integrated numerically in order to determine their spatial distribution in 1983. All planetary perturbations (except those due to Mercury and Pluto), radiation pressure, Poynting-Robertson drag, and solar wind drag are included in the calculation. The SIMUL code (Dermott et al., Comets to Cosmology, pp. 3–18. Springer, Berlin, 1988) is used to calculate the shapes of several model zodiacal clouds consisting of a range of combinations of cometary and asteroidal particles. By comparing the model results with IRAS observations in the 25 μm waveband, it is shown that the observed shape of the zodiacal cloud can be accounted for by a combination of about 1 4 to 1 3 asteroidal dust and about 3 4 to 2 3 cometary dust. This result is consistent with other work on the structure of the IRAS dust bands and the Earths resonant ring (Dermott et al., Nature 369, 719–723, 1994; Asteroids, Comets and Meteors, 1993, pp. 127–142. Kluwer, Dordrecht, 1994). It is also consistent with the conclusions of other workers that the cloud must be heterogeneous (Levasseur-Regourd et al., Icarus 86, 264–272, 1990; Origin and Evolution of Interplanetary Dust, pp. 131–138. Kluwer, Japan, 1991).

Orbital Evolution of Interplanetary Dust

The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical features include the offset of th e center of symmetry of th e cloud from the Sun, the radial gradient of the ecliptic polar brightness at th e Earth, and th e warp of th e cloud. The dust bands provide th e st rongest evidence th at a substantial and possibly dominant fraction of the cloud originate s from aster oids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of th ese large particles towards th e inner Solar System due to Poynting Robert son light drag and their slow passage through secular resonances at the inner edge of the asteroid belt result s in large increases in th eir eccent ricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and comet ary type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between ast eroidal and cometary particles on dynamical grounds alone.

Three years of Galileo dust data. II. 1993-1995

Abstract Between January 1993–December 1995, the Galileo spacecraft traversed interplanetaryspace between Earth and Jupiter and arrived at Jupiter on 7 December 1995. The dust instrumentonboard the spacecraft was operating during most of the time and data from the instrument wereobtained via memory readouts which occurred at rates between twice per day and once per week.All events were classified by an onboard program into 24 categories. Noise events were usuallyrestricted to the lowest categories (class 0). During Galileos passage through Jupiters radiationbelts on 7 December 1995, several of the higher categories (classes 1 and 2) also show evidencefor contamination by noise. The highest categories (class 3) were noise-free all the time. Arelatively constant impact rate of interplanetary and interstellar (big) particles of 0.4 impacts perday was detected over the whole three-year time span. In the outer solar system (outside about2.6 AU) they are mostly of interstellar origin, whereas in the inner solar system they are mostlyinterplanetary particles. Within about 1.7 AU from Jupiter intense streams of small dust particleswere detected with impact rates of up to 20,000 per day whose impact directions are compatiblewith a Jovian origin. Two different populations of dust particles were detected in Jovianmagnetosphere: small stream particles during Galileos approach to the planet and big particlesconcentrated closer to Jupiter between the Galilean satellites. There is strong evidence that thedust stream particles are orders of magnitude smaller in mass and faster than the instrumentscalibration, whereas the calibration is valid for the big particles. Because the data transmissionrate was very low, the complete data set for only a small fraction (2525) of all detected particlescould be transmitted to Earth; the other particles were only counted. Together with the 358particles published earlier, information about 2883 particles detected by the dust instrumentduring Galileos six years journey to Jupiter is now available.

Dust measurements in the Jovian magnetosphere

Dust measurements have been obtained with the dust detector onboard the Galileo spacecraft inside a distance of about 60RJ from Jupiter (Jupiter radius, RJ = 71,492 km) during two periods of about 8 days around Galileos closest approaches to Ganymede on 27 June and on 6 Sept 1996. The impact rate of submicrometer-sized particles fluctuated by a factor of several hundred with a period of about 10 hours, implying that their trajectories are strongly affected by the interaction with the Jovian magnetic field. Concentrations of small dust impacts were detected at the times of Ganymede closest approaches that could be secondary ejecta particles generated upon impact of other particles onto Ganymedes surface. Micrometer-sized dust particles, which could be on bound orbits about Jupiter, are concentrated in the inner Jovian system inside about 20RJ from Jupiter.

An Estimation of the Interstellar Contribution to the Zodiacal Thermal Emission

Impact data from the Ulysses dust detector at 5 AU from the Sun have been interpreted as a flux of submicron interstellar dust particles arriving from ecliptic longitude 252° and ecliptic latitude 25. By following the motions of these particles under the influence of solar gravity, radiation pressure, and electromagnetic forces, we derive a model of the thermal emission from the resultant particle cloud. Since the distributions of the particles are time variable depending on the solar cycle, calculations are performed for the years 1984 and 1990, corresponding, respectively, to the times of the IRAS and COBE observations. We also illustrate how the distributions vary with particle size (or, at a more basic level β, the ratio of the radiation pressure to gravitational force) by presenting results for three different particle sizes. Patches of emission from our test cloud reach peak levels of 0.1 MJy sr–1 in the 12 μm wave band. This represents 10% of the average brightness asymmetry around the sky between the trailing/leading telescope pointing directions seen in the IRAS and COBE data sets. Some of these patches occur at high ecliptic latitudes where the contribution from the Galaxy is negligible and emission from the smooth zodiacal background is low compared to that at low ecliptic latitudes. A strong seasonal variation in the predicted interstellar emission trailing/leading asymmetry is the most obvious signature of the interstellar source, and, in addition, the time variability of the emission will produce different features in the IRAS and COBE data sets and in any subsequent infrared mission. For these reasons, a search of the data for the predicted signatures is certainly justifiable.

Dust Measurements During Galileo's Approach to Jupiter and Io Encounter

About a hundred dust impacts per day were detected during the first week in December 1995 by Galileo during its approach to Jupiter. These impacts were caused by submicrometer-sized particles that were just above the detection limit. After the closest approach to Io on 7 December, impacts of these small particles ceased. This effect is expected for dust grains emitted from Io that exit the field of view of the instrument after the flyby. The impact rate of bigger micrometer-sized dust grains continued to increase toward Jupiter. These dust particles are in orbit about Jupiter or are interplanetary grains that are gravitationally concentrated near Jupiter.