David Lien

The Formation of Encke Meteoroids and Dust Trail

Abstract We observed Comet 2P/Encke with the Infrared Space Observatory ISOCAM on July 14, 1997, from a particularly favorable viewing geometry above the comets orbital plane and at a distance of 0.25 AU. A structured coma was observed, along with a long, straight dust trail. For the first time, we are able to observe the path of particles as they evolve from the nucleus to the trail. The particles that produce the infrared coma are large, with a radiation to gravitational force ratio β mm-sized particles). The dust trail follows the orbit of the comet across our image, with a central core that is 2×104 km wide, composed of particles with β 10−3, in marked contrast to other comets like P/Halley and C/Hale-Bopp. The structure of the coma requires anisotropic emission and requires that the spin axis of the nucleus be nearly parallel to the orbital plane, resulting in strong seasonal variations of the particle emission. While most of the infrared coma emission is due to dust produced during the 1997 apparition, the core of the dust trail requires emissions from previous apparitions. The total mass lost during the 1997 apparition is estimated to be 2–6×1013 g. Compared to the gas mass loss from ultraviolet observations, the dust-to-gas mass ratio is 10–30, much higher than has ever been suggested from visible light observations. Using the recently measured nuclear diameter, we find that Encke can last only 3000–10,000 ρN yr (where ρN is the nuclear density in g cm−3) at its present mass loss rate.

ORIGIN OF THE NEAR-ECLIPTIC CIRCUMSOLAR DUST BAND

The zodiacal dust bands are bright infrared (IR) strips produced by thermal emission from circumsolar rings of particles. Two of the three principal dust bands, known as β and γ, were previously linked to the recent asteroid collisions that produced groups of fragments, so-called asteroid families, near the orbits of (832) Karin and (490) Veritas. The origin of the third, near-ecliptic α band has been unknown until now. Here we report the discovery of a recent breakup of a >20 km diameter asteroid near αs originally suspected source location in the Themis family. Numerical modeling and observations of the α-band thermal emission from the Spitzer Space Telescope indicate that the discovered breakup is the source of α-band particles. The recent formation of all principal dust bands implies a significant time variability of the circumstellar debris disks.

The Tempel 2 dust trail

Abstract The Tempel 2 dust trail was observed extensively by the Infrared Astronomical Satellite. Evidence is presented suggesting that the trails is composed of particles larger than 1 mm in diameter with velocities of several meters per second relative to the parent comet, assuming isotropic emission. Trail particles forward of the comets orbital position have a minimum diameter of ∼6 mm. The observed trail consists of emission by the nucleus over a few hundred years. Excess color temperatures relative to a blackbody indicate that the particles either are large enough to support a temperature gradient over their surfaces or that a small particle population exists with diameters A . In the case of the latter, such small particles would have to have originated from the large particlesotherwise their acceleration by the gas outflow to km/sec velocities would have prevented their ejection into trail orbits.

The Dust Trail of Comet 67P/Churyumov-Gerasimenko

We present optical and infrared (24 μm) images of the dust trail of comet 67P/Churyumov-Gerasimenko near aphelion, obtained with the Wide Field Imager at the MPG/ESO 2.2m telescope in La Silla in 2004 and with NASA’s Spitzer Space Telescope in 2005 and 2006, respectively. Based on these images, we constrain the size distribution and material properties of mm-sized dust grains emitted from the comet. These particles remain close to the comet’s orbit because of weak radiation pressure and low emission speeds, thereby forming the comet’s dust trail. We evaluate the size distribution of the trail particles by fitting simulated images to the measured intensities. As far as possible, the parameters of the underlying model are derived from the observed emission history of Churyumov-Gerasimenko. The size distribution is a crucial parameter for estimating the number density of large particles in the neighbourhood of the comet nucleus and for the safety of ESA’s Rosetta spacecraft which will pass through the trail region on its approach to Churyumov-Gerasimenko in 2013.

IRAS images of comet tempel 2

Abstract We present the first of 13 sets of observations of periodic Comet Tempel 2 obtained by the Infrared Astronomical Satellite (IRAS). The images taken on UT 1983 August 28 at the four IRAS bandpasses (12, 25, 60 and 100 μm) have been restored to remove instrumental effects. In order to interpret the spatial structure in these infrared images we have compared them with a series of theoretical models based on different dust properties. We conclude that the outer dust coma of this comet is dominated by intermediate size particles (on the order of several millimeters, assuming a density of 1 g cm−3). A population of even larger particles forms the portion of the “dust trail” seen in these images. Comet Tempel 2 appears to be richer in large dust grains than othercomets observed with similar infrared imaging techniques from the ground.

Infrared imaging of the coma of Comet Wilson

Abstract Thermal infrared images of Comet Wilson were obtained on three consecutive days starting on 1987 March 13 UT. The brightness of the nuclear condensation did not vary appreciably during the observations, in sharp contrast with the behavior observed in Comet Halley. A dynamical analysis of the structure of the dust coma and tail indicates that the particle size distribution in Comet Wilson is clearly different from that observed in Comet Giacobini-Zinner but closer to that in Comet Halley. The dust ejection was nonisotropic with most of the activity concentrated within about a 60°-wide area east of the subsolar point. This result is interpreted as evidence for prograde rotation of the nucleus. We speculate that Wilson, which is a new comet, has not yet developed a dust mantle. The absence of such a mantle would explain the lack of short time scale (Halley-like) variability and the constant position, with respect to the subsolar point, of the area of maximum dust ejection on all 3 days.

Ground-based thermal IR images of comet tempel 2

Abstract We present ground-based thermal infrared images of Comet Tempel 2 obtained on UT September 21.29, 21.15, 23.12, and 24.13 1988. At that time the heliocentric and geocentric distances were approximately 1.4 and 1.0 AU, respectively. A well defined structure in the extended dust emission was observed on all 4 days and can be interpreted in three ways: as a large (centimeter sized) grain tail; as a result of an outburst that occured between 3 and 2.5 AU preperihelion; or as a sunward fan. The brightness of the nuclear condensation showed a temporal variability slightly larger than our observational uncertainty. We estimate that during our observations the dust coma contributed approximately 50% of the brightness in the nuclear pixel.