Magnitude and size distribution of long-period comets in Earth-crossing or approaching orbits

Abstract

We analyse the population of near-Earth long-period comets (LPCs; perihelion distances q < 1.3 au and orbital periods P > 103yr). We have considered the sample of LPCs discovered during the period 1900-2009 and their estimated absolute total visual magnitudes H. For the period 1900-1970 we have relied upon historical estimates of absolute total magnitudes, while for the more recent period 1970-2009 we have made our own estimates of H based on Green's photometric data base and IAU Circulars. We have also used historical records for the sample of brightest comets (H < 4.5) covering the period: 1500-1899, based mainly on the Vsekhsvyatskii, Hasegawa and Kronk catalogues. We find that the cumulative distribution of H can be represented by a three-modal law of the form log10N< H=C+αH, where the C values are constants for the different legs, and α≃ 0.28 ± 0.10 for H < 4.0, α≃ 0.56 ± 0.10 for 4.0 ≤H < 5.8, and α≃ 0.20 ± 0.02 for 5.8 ≤H < 8.6. The large increase of the slope of the second leg of the H-distribution might be at least partially attributed to splitting of comet nuclei, leading to the creation of two or more daughter comets. The cumulative H-distribution tends to flatten for comets fainter than H≃ 8.6. LPCs fainter than H≃ 12 (or diameters D≲ 0.5 km) are extremely rare, despite several sky surveys of near-Earth objects implemented during the last couple of decades, suggesting a minimum size for an LPC to remain active. We also find that about 30 per cent of all LPCs with q < 1.3 au are new (original bound energies 0 < Eor < 10-4 au-1), and that among the new comets about half come from the outer Oort cloud (energies 0 ≲Eor≲ 0.3 × 10-4 au-1), and the other half from the inner Oort cloud (energies 0.3 × 10-4≲Eor≲ 10-4 au-1). © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.