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Astron. Astrophys. 363, 323-334 (2000)

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7. The nucleus radius

The upper limit on the comet radius can be obtained from photometry of the nuclear condensation. If there is no coma around the cometary nucleus then the absolute magnitude [FORMULA] given by Rickman (1992):


allows directly to estimate the radius of the bare nucleus knowing its geometric albedo [FORMULA]. The absolute magnitude of a comet can be obtained from the observed nuclear magnitude [FORMULA] reduced to unit geocentic, [FORMULA], and heliocentric distance and zero degree phase i.e.


The [FORMULA] is the phase angle at the time of observation and the phase coefficient, [FORMULA], is usually taken to be 0.035 Jewitt & Luu (1992).

The nuclear magnitudes of the comet 43P/Wolf-Harrington from the period 1951-1978 employed in this study come from the Kamel's Catalogue. A large number of nuclear magnitudes were reported by Roemer and her collaborators (33 data points from the 1958, 1965 and 1971 apparitions) and a few ones by Mrkos & Schwarz. The five observations in 1951-1952 were obtained by Cuningham et al. The data have been supplemented with estimates made by Hainaut et al. (MPC 27955), Offut (MPC 27955), Scotti (MPC 27955, ICQ 107), Hergenrother (ICQ 100) & Garradd (MPC 31329) in 1996 and 1998. The collected nuclear magnitudes have been normalized to a geocentric distance of 1 AU and plotted against [FORMULA] in Fig. 6.

[FIGURE] Fig. 6. The nuclear brightnesses of the comet in 1952, 1958, 1965, 1971, 1977 and 1997 apparition versus log r.

The magnitude estimates follows an inverse fourth power law heliocentric distance therefore they are inconsistent with reflection from a bare nucleus, suggesting that the coma is still present. To avoid possible contamination by products of the comet's activity the observations at a large heliocentric distance should be taken into account. Employing three estimations of the comet's magnitude at a heliocentric distance of 3.87 AU, made during the last apparition, the [FORMULA] were calculated from Eq. (12) and next assuming a geometric albedo of 0.04, the mean nucleus radius was computed from the Eq. (11) as being equal to 2.09 km. Slightly smaller radius of 1.64 km was obtained on the basis on the observations at 3.4 AU and 2.6 AU. On the base photometric observations made at a heliocentric distance of 4.87 AU Lowry et al. (1999) estimated the nuclear radius of the comet on 3.3 km.

In the further analysis of the constraints on the comet nucleus, the nongravitational acceleration was involved. The equation: [FORMULA] allows directly to calculate the mass of the comet nucleus, knowing values of the [FORMULA]. Assuming effective outflow velocity of the pure [FORMULA] as equal to 0.3 km s-1, the nucleus mass amounted to [FORMULA] kg (Model A) and [FORMULA] kg (Model B). On the other hand the nucleus mass is related to R by: [FORMULA]. Assuming the bulk density on 0.3 g/cm3 or 0.5 g/cm3 the nucleus radius was evaluated as [FORMULA] km or [FORMULA] km (Model A) and [FORMULA] km or [FORMULA] km (Model B). The reliability of these estimations is limited by the assumed theory of sublimation.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000