Laura M. Woodney

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.

Analyses of dust coma morphology of Comet Hyakutake (1996 B2) near perigee: outburst behavior, jet motion, source region locations, and nucleus pole orientation

Abstract We present inner-coma dust imaging of Comet Hyakutake (1996 B2) obtained on 11 consecutive nights in late March 1996, an interval including a major outburst and the comet’s closest approach to Earth. The evolution of the outburst morphology is followed, along with the motion along the tail of several outburst fragments. Two spiral dust jets—a primary jet, along with a much weaker secondary jet—are visible throughout the interval and are produced by two source regions on a rotating nucleus. These are examined as a function of rotational phase and viewing geometry, with their appearance changing from a nearly face-on view on March 18 to side-on by March 28. The dust outflow velocity as a function of distance from the nucleus is derived, with the dust continuing to accelerate to a distance of 4000 km or more and reaching an average outflow velocity of 0.38 km s −1 between 3000 and 8000 km. We present details of our Monte Carlo modeling of the jets and our methodology of fitting the model to the images. The modeling yields the pole orientation of the nucleus, with an obliquity of approximately 108°, corresponding to an RA of 13 h 41 m and a Dec of −1.1°. For an assumed spherical nucleus, the primary active region is centered at approximately −66° latitude, has a radius of about 56°, and therefore covers about 22% of the surface. The source of the secondary jet is at a latitude of −28°, has a radius of about 16°, and is located at a longitude nearly 180° away from the primary source. Estimated uncertainties for the pole orientation and the source locations and sizes are each about 3°. This solution for the nucleus orientation and source locations explains the strong asymmetry in measured production rates before and after perihelion in radio observations (Biver et al., 1999 , Astron. J. 118, 1850–1872). The modeling also tightly constrains the sidereal rotation period as 0.2618 ± 0.0001 day, completely consistent with the expected +0.0003 day difference from the observed solar rotation period of 0.2614 ± 0.0004 day determined by Schleicher and Osip (2002 , Icarus 159, 210–233), given the pole orientation and position of the comet in its orbit.