Water release rates, active areas, and minimum nuclear radius derived from visual magnitudes of comets—an application to comet 46P/Wirtanen

Abstract

A semi-empirical photometric method is developed to estimate water release rates from visual magnitude measurements of “normal” comets of abundance ratio C 2:CN ≈ 1.46 ± 0.68 for the heliocentric distance range 2.5-0.5 AU for which the aperture of the instrument, projected on the coma, is larger than the effective scale length of C 2. The method is applied to comets Austin (1982 VI), Bradfield (1979 X), Bradfield (1980 XV), Bradfield (1987 XXIX), P/Brorsen-Metcalf (1989 X), Levy (1990 XX), Liller (1988 V), and 46P/Wirtanen. The water release rates are compared with those derived from observed OH or OI (6300 Å) line fluxes. The H 2O release rates estimated from visual magnitude measurements may have typical errors of about 2–4, while maximum errors may be as high as a factor of 10. The total active area ( AA) on the sunlit side of the nucleus has been estimated from derived water release rates combined with theoretical estimates of H 2O flux rates (molecules cm −2s −1) by vaporization theory for an assumed nuclear visual albedo p v = 0.03. The constraint AA ≤ 2 π( R N) 2 has been used for estimation of the lower limit of the nuclear radius of a comet. For comet 46P/Wirtanen, the variation of water release rates with heliocentric distance ( r), total active area, lower limit of nuclear radius, and fractional active area are: Q(H 2O) = 1.8 × 10 28 r −7.0±0.4mols −1, 6 ± 3 km 2, 1.0 km, and 0.15, respectively.