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Astron. Astrophys. 339, L25-L28 (1998)

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3. Remarks on physical behavior, and conclusions

Following the major outburst of the companion, its rapid fading relative to the primary was the most prominent systematic change in the comet's physical behavior. One month after the outburst, the two components were of nearly equal brightness, and several months after another, minor flare-up, component A became some 2 magnitudes fainter than B. The magnitude differences, obtained mostly from the same sources as the positional offsets, are presented in Fig. 2. On the whole, a fading of component A by a factor of [FORMULA] 100 or more relative to component B is apparent on a timescale of seven or so months.

[FIGURE] Fig. 2. Magnitude difference between components A and B vs. time and heliocentric distance. The negative values mean that the companion nucleus A was then the brighter of the two.

The comet's light curve, based on a set of selected observations, is shown in Fig. 3. Judging from the preperihelion magnitudes, reported by Hale and Nakamura (1996) and Luethen (1997) and reduced to a consistent system of visual magnitudes, the comet was brightening steadily, its intrinsic brightness varying inversely as about a cube of heliocentric distance. Only very few magnitude estimates of the comet as a whole are available from the post-breakup period of time. Although somewhat uncertain, these data suffice to show that the comet was much brighter than implied by the numerous reported CCD "total" brightness estimates of component B, suggesting that the latter refer only to the primary's nuclear condensation (though not to the true nucleus). The light curve for component B shows interesting temporal variations: its fading proceeded at a moderate rate up to a heliocentric distance of 3.1 AU, at which time it slowed down dramatically, only to resume at a steeper rate again at 4.5 AU from the Sun. These moderate variations contrast with the precipitous drop in the brightness of component A.

[FIGURE] Fig. 3. The normalized light curve of comet Evans-Drinkwater. Circled dots and open circles are, respectively, the visual estimates of the comet's integrated brightness before perihelion (the "pre" curve) and afterwards (the "post" curve). The dots are magnitude estimates for the nuclear condensation of component B. The light curve of component A is schematically depicted by dashes. The brightness is shown to vary inversely as an nth power of heliocentric distance, with n being specified.

In the context of split comets as a category of objects, Evans-Drinkwater is the 30th nontidally split comet on the list (cf. Sekanina 1997b), the seventh new comet (in the Oort sense) known to have split (for the original orbit, see Marsden and Williams 1997), and one of four split comets with the extremely well determined separation parameters. This is the first time that a companion to the nucleus of a new comet was discovered while in outburst and much brighter than the primary. The previously known instances included only two short-period comets: 69P/Taylor, which confused Barnard (1916) into believing that the companion was all that was left from the comet; and 79P/du Toit-Hartley, for which the companion's rapid fading was in fact successfully predicted (Sekanina 1982b).

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998