David G. Schleicher

Comet Bowell 1980b

Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.

The fluorescence of cometary OH

A new solar spectral atlas, improved oscillator strengths, and additional transitions have been incorporated in calculations of the fluorescent equilibrium of the OH molecule in comets as a function of heliocentric distance and radial velocity. Comparisons of detailed, ultraviolet band profiles have been made with the observed, high-resolution spectra of eight comets. While a purely fluorescent equilibrium calculation is sufficient to reproduce the major variations seen in the band structure from one comet to another, some collisional quenching of the inversion of the X2II3/2 Lambda-doublet ground state is required to match the detailed features. The relative population inversion of the upper and lower levels of the ground state is also calculated as a function of both distance and velocity, with the results being consistent with 18 cm radio observations. Predictions of the infrared OH vibrational bands are made; the 1-0 band should be observable if a sufficiently high spectral resolution is employed. 60 references.

The nucleus of Comet P/Tempel 2

Simultaneous optical photometry and IR radiometry of Comet P/Tempel 2 are presented. Periodic variations of brightness are present and in phase at all wavelengths. Because the optical and thermal rotational light curves are in phase, it is concluded that the variations are caused by the changing apparent cross section of an elongated nucleus rotating with a period near 8.9 h. The variation of flux with aperture makes it possible to separate the contributions of the nucleus and the coma. The contribution by the coma is about 25 percent at maximum light in the optical and undetectable at the level of 10 percent at all times in the thermal IR. 34 refs.

PHOTOMETRY AND IMAGING RESULTS FOR COMET 9P/TEMPEL 1 AND DEEP IMPACT: GAS PRODUCTION RATES, POSTIMPACT LIGHT CURVES, AND EJECTA PLUME MORPHOLOGY

We present Earth-based imaging and photometry results of Deep Impact target comet 9P/Tempel 1 obtained at and in the nights surrounding the time of the spacecrafts impact. These observations establish the baseline behavior of Tempel 1 prior to impact, including a water vaporization rate of 6 × 1027 molecules s-1, thereby enabling us to determine the effects directly caused by this explosive event. The instantaneous fireball was not detected, but a postimpact brightening from ejecta material was prominent and shows evidence of a slowly decreasing optical depth over more than an hour of time. We have successfully reproduced both the general morphology of the ejecta plume and many details of the shape and brightness distribution on successive nights following the impact using a modified Monte Carlo jet model, with an initial impulse event in the shape of an open, thick-walled cone. As seen from Earth, the center of the plume is derived to be at a position angle of about 255° and about 20° this side of the plane of the sky, i.e., near the limb of the nucleus. Most of the observed ejecta material had an initial outflow velocity of less than 0.23 km s-1 and particle sizes of less than 2.5 μm (assuming compact grains). This resulted in the rapid development of a dust tail from ejecta particles as they are pushed away from the Sun by radiation pressure. Each daughter gas species exhibited an increase in production, with peaks occurring 1-2 days after impact, followed by a gradual decay back to baseline values; the shapes of these light curves suggest that the postimpact excess is due to the vaporization of ices within the ejecta over the course of a few days. In nearly all respects, comet Tempel 1 returned to preimpact conditions only 6 days after the event.

Comet 19P/Borrelly at multiple apparitions: seasonal variations in gas production and dust morphology

We present analysis and results from both narrowband photometry and CCD imaging of Comet 19P/Borrelly from multiple apparitions. Production rates for Borrelly a few days prior to the Deep Space 1 spacecraft encounter were Q(OH) = 2.1×1028 molecule s−1, Q(CN) = 5.1×1025 molecule s−1, and A(θ)fρ = 400–500 cm. The equivalent Q(water; vectorial) = 2.5×1028 molecule s−1. We also find that the radial fall-off of the dust is significantly steeper than the canonical 1/ρ for aperture sizes larger than ρ = 2×104 km. In the near-UV, a strong trend in dust colors with aperture size is present. Imaging of Borrelly revealed a strong radial jet in the near-sunward direction that turns off late in the apparition. For the jet to appear radial, it must originate at or very close to the nucleus’ pole. Modeling the measured position angle of this jet as a function of time during the 1994 and 2001 apparitions yields a nucleus in a simple, rather than complex, rotational state with a pole orientation having an obliquity of 102.7° ± 0.5° and an orbital longitude of the pole of 146° ± 1°, corresponding to an RA of 214.1° and a Declination of −5.7° (J2000). There is also evidence for a small (∼8°) precession of the pole over the past century, based on our preferred model solution for jet measurements obtained during the 1911–1932 apparitions. Our solution for the orientation of the rotation axis implies a very strong seasonal effect as the source region for the jet moves from summer to winter. This change in solar illumination quantitatively explains both the nearly level water production measured in the seven weeks preceding perihelion and the extremely large decrease in water production (25×) as Borrelly moved from perihelion to 1.9 AU. A much smaller fall-off in apparent dust production after perihelion can be explained by a population of old, very slowly moving large grains released near peak water production, and therefore not indicative of the actual ongoing release of dust grains late in the apparition. Based on the water vaporization rate, the source region has an area of approximately 3.5 km2 or 4% of the total surface area of the nucleus, and water ice having an effective depth of 3–10 m is released each apparition from this source region.

Detection of CN Emission from (2060) Chiron.

In the past decade there has been a gradual, but substantial change in our understanding of the physical nature of (2060) Chiron. Once thought to be the first known member of a population of asteroids orbiting between Saturn and Uranus, Chiron is now generally regarded as the largest known comet. The detection of CN emission in the spectrum of Chiron is reported. Not only do these observations underscore the cometary nature of Chiron, but, at a heliocentric distance exceeding 11 astronomical units, represent the most distant detection yet of a neutral gas species common in comets. These results are consistent with the outgassing from Chiron being primarily driven by isolated outbursts of CO2 from a very small fraction of Chirons surface. These may be indicative of primordial inhomogeneities.

THE FLUORESCENCE EFFICIENCIES OF THE CN VIOLET BANDS IN COMETS

We have carried out calculations of the fluorescent equilibrium of the CN molecule in the solar radiation field as a function of heliocentric radial velocity and distance. The detailed rotational line intensities of the CN violet 0–0 band have been recalculated for radial velocities between −60 and +60 km s −1 and at 10 distances in steps of √ 2 from 0.25 to 5.7 AU. Comparisons with observed CN violet 0–0 band profiles yield reasonable agreement and remaining discrepancies are well understood. Based on the derived equilibrium rotational level populations of the ground state, 1–1 band rotational line intensities were also calculated. Total band fluorescence efficiencies for the violet Δν = 0 sequence are tabulated for the above range of distances and velocities, permitting the interpolation to any given comet’s observational circumstances. These fluorescence efficiencies, L/N ,o r “g-factors,” needed to reduce CN emission band fluxes to CN abundances, are also accessible via a Web-based service. The values at any heliocentric distance vary by about a factor of two with varying velocities, but the specific structure of this variation with velocity changes dramatically with distance.

Periodic variations in the activity of Comet P/Halley during the 1985/1986 apparition

A search for periodic variation in the production of gas and dust by Comet Halley has been performed using narrowband photometric measurements from four sites - Lowell Observatory, Mauna Kea Observatory, Perth Observatory, and Cerro Tololo Inter-American Observatory. The method of phase dispersion minimization was applied to observations made during 164 observing nights between September 1985 and June 1986. A clear-cut variation, with a period near 7.4 days, was present throughout the postperihelion window. Less conclusive evidence of a similar period has been found in the pre-perihelion data. No indication of a shorter period or of strong sporadic activity exists in the data. The observations require that Halleys nucleus returns to essentially the same orientation with respect to the sun approximately every 7.4 days except for longer-timescale seasonal evolution. This fact precludes certain proposed models of nuclear motion. 53 refs.

Comet Levy (1990c) - Groundbased photometric results

Abstract Narrowband filter photometry of Comet Levy (1990c) was obtained on 51 nights in the interval from 3 June 1990 to 19 April 1991. The emission bands of OH, NH, CN, C 3 , and C 2 were isolated, in addition to continuum points at 3650 and 4845 A. Relative abundances among the species were typical of most comets previously observed. All species showed pronounced asymmetry about perihelion, with production rates for most species being about twice as large before perihelion as after over the heliocentric distance range of 1.0–3.0 AU. The OH production rates asymmetry was nearly double that observed for the other species, with maximum production occuring more than a month before other species. Periodic variations having a single-peaked lightcurve period of 18.9 hr or a double-peaked lightcurve period of 37.8 hr were observed during late-August 1990. Observations by Feldman et al . (1991) only 3 weeks later using the IUE satellite require a period 7% shorter than the value we found. Neither period determination is consistent with both data sets, implying that the rotational period charged between the two sets of observations.

Uncorrelated Volatile Behavior during the 2011 Apparition of Comet C/2009 P1 Garradd

The High Resolution Instrument Infrared Spectrometer (HRI-IR) on board the Deep Impact Flyby spacecraft detected H2O, CO2, and CO in the coma of the dynamically young Oort Cloud comet C/2009 P1 (Garradd) post-perihelion at a heliocentric distance of 2 AU. Production rates were derived for the parent volatiles, Q_(H2O) = 4.6 ± 0.8 × 10^(28), Q_(CO2) = 3.9 ± 0.7 × 10^(27), and Q_(CO) = 2.9 ± 0.8 × 10^(28) molecules s^(–1), and are consistent with the trends seen by other observers and within the error bars of measurements acquired during a similar time period. When compiled with other observations of Garradds dominant volatiles, unexpected behavior was seen in the release of CO. Garradds H_2O outgassing, increasing and peaking pre-perihelion and then steadily decreasing, is more typical than that of CO, which monotonically increased throughout the entire apparition. Due to the temporal asymmetry in volatile release, Garradd exhibited the highest CO to H_2O abundance ratio ever observed for any comet inside the water snow line at ~60% during the HRI-IR observations. Also, the HRI-IR made the only direct measurement of CO_2, giving a typical cometary abundance ratio of CO_2 to H_2O of 8% but, with only one measurement, no sense of how it varied with orbital position.