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Session 19 - Solar & Planetary Systems.
Display session, Monday, January 13
The previously unassuming asteroid 1979 OW7 startled observers in the summer of 1996 by sporting a thin tail stretching several arcminutes --- a manifestly cometary characteristic. This was extremely unusual, given that the object resides in a distinctly un-comet-like orbit in a stable region of the main belt, among members of the Themis asteroid family.
We obtained images and spectra of 1979 OW7 (= Comet P/1996 N2) with the Apache Point Observatory 3.5-m telescope (+DIS) on Sept. 18, 1996. In addition to the main sunward tail noted previously, we detected a faint anti--sunward tail (which was independently discovered and announced by Boehnhardt et. al., IAUC 6495). Preliminary analysis using Finson--Probstein models indicates that the dust emission could not have been confined to a single, short outburst but rather must have occurred over an extended period of at least several weeks. This excludes a collisional origin for the dust emission, and provides circumstantial evidence for the existence of sublimating volatiles on the object.
Our 0.38--1.0 \mum reflectance spectra reveal a neutral continuum absent of emission or strong absorption features, consistent with known cometary nuclei but also similar to a subset of the low--albedo asteroids (notably including the Themis family asteroids).
We explore several questions in this poster:
--- Is it possible for a comet (with a presumed origin in the outer solar system) to become lodged in the main asteroid belt?
--- If one comet has moved into a stable orbit in the main asteroid belt, it seems likely that others may also have done so over the lifetime of the solar system. Could the main asteroid belt constitute a ``graveyard'' for evolved comets?
--- Alternatively, is 1979 OW7 simply a volatile--rich asteroid which has had a pocket of ices recently exposed? If so, can other outer--main--belt asteroids evolve into what look like short--period comets?
Program listing for Monday