Masses of long-period comets derived from non-gravitational effects - analysis of the computed results and the consistency and reliability of the non-gravitational parameters

Abstract

We estimate masses for a selected sample of long-period comets (LPCs) (with orbital periods P > 1000 yr and perihelion distances q < 2 au), with good photometric visual light curves and known non-gravitational parameters. We follow a procedure similar to that developed by Szutowicz et al. to estimate the masses of comets C/1995 O1 (Hale–Bopp) and C/1996 B2 (Hyakutake). The method also requires the knowledge of the water production rates Q ,f or which we find a new correlation between Q and the visual total heliocentric magnitude mh of LPCs, which can be expressed as log10Q = 30.53 − 0.234mh. The computed masses are in the range [0.5, 10] × 10 12 kg. From these masses, and by assuming a cometary bulk density ρ of 0.4 g cm −3 , we estimate the effective nuclear radii RN for the studied LPCs to be in the range [0.7, 1.8] km. From the computed values of RN ,w e find that the active surface areas are (in most cases) greater than the geometric ones (fractions f ranging from 0.8 to 2.4). We argue that this could be understood as a state of hyperactivity, which can be explained if a significant fraction of the sublimated water molecules come from the sublimation of icy grains in the coma. Hyperactivity would make LPC nuclei smaller (i.e. less massive) than expected from their coma brightness. Other effects, like extremely low densities or overestimated non-gravitational parameters, might also contribute to the rather high computed value of f . For comet C/1995 O1 (Hale–Bopp), we find a seeming inconsistency between some size estimates and some computed non-gravitational parameters. We analyse some alternatives to this apparent paradox. We also compute absolute total magnitudes and photometric indices for the studied sample of LPCs and find a new correlation between masses and absolute total visual magnitudes H. By assuming ρ = 0.4 g cm −3 , we can convert masses to diameters D, leading to a relation log10D(km) = 1.5 − 0.13H.