Solar and Heliospheric observatory sungrazing comets with prominent tails:Evidence on dust-production peculiarities

A preliminary examination of the tails of 11 Solar and Heliospheric Observa tory (SOHO) sungrazers, observed between 3 and 20 R. on their approach to t he Sun, provides information on dust ejected from these comets and on the f orces to which the microscopic grains are subjected. Images taken at times of the SOHO spacecraft's transit across the orbit planes of sungrazers show their tails to be perfectly straight and extremely narrow and their appare nt position coinciding with that of the projected orbit plane. This result suggests relatively low particle-ejection velocities in the direction norma l to the orbit plane (estimated at less than similar to 100 m s(-1)) and of fers no evidence whatsoever for the Lorentz force on charged dust. When vie wed broadside, the sungrazers' tails are always narrow, either straight or slightly curved, and deviating strikingly from the antisolar direction, an indication that no microscopic dust was ejected during a period of time jus t preceding observation. The tails include a major population of submicron- sized grains that are dielectric in nature, most probably silicates, since the radiation-pressure accelerations are found never to exceed 0.6 of the s olar attraction. The sampled comets show rather consistently that the produ ction of this dust terminated at similar to 20-30 R. from the Sun along the inbound orbit. The tail of one of the brightest SOHO sungrazers is modeled in slightly greater detail, including its curvature, which abruptly increa ses at a point approximately halfway between the head and the far end.