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 > 103 yr). 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).

The end states of long-period comets and the origin of Halley-type comets

“…We may also note that the discovery rate of old LPCs in Earth-crossing orbits (q < 1 au) shows little change in the considered period 1850-2014 (suggesting a near complete record), and unbiased in q. Fernández and Sosa (2012) found that the great majority of LPCs with P > 10 3 yr have absolute total magnitudes H < 12 (or diameters D > ∼ 0.5 km). A check of the total absolute magnitudes of the old LPCs of our sample agrees with the previous observation: it is very rare to find comets fainter than H ∼ 12 among the old LPCs too.…”

“…Therefore, we will assume in the following that our LPC sample contains comets brighter than H = 12. Fainter comets either disintegrate very fast, or do not exist by cosmogonic reasons, or are too faint to be detected (see a discussion by Fernández and Sosa 2012).…”

Giant comets and mass extinctions of life

“…For longperiod comets, Fernández & Sosa (2012) find a cumulative radius R distribution N (R) ∝ R −s , with s = 2.15 ± 0.75, consistent with s = 2 for an evolved population in the diameter range 20-200 km, expected from current models of planetesimal growth (e.g. Kenyon & Bromley 2012).…”

Comets as collisional fragments of a primordial planetesimal disk

“…In that work, as in Duncan & Levison (1997), the number of comet-sized bodies in the scattered disk was evaluated from (a) the number of known Jupiter-family comets in some given range of orbits and magnitudes for which the JFC sample is assumed complete, and (b) the numerical relationship between the scattered disk population and the Jupiter-family population that the former sustains, obtained from numerical simulations. With respect to previous estimates (e.g., Duncan & Levison 1997), the estimate in Brasser and Morbidelli is improved in two respects: it uses the most recent conversion from total magnitude to nuclear size from Fernández & Sosa (2012), and it is based on new simulations deriving Jupiter-family comets from a scattered disk that is excited in inclination (the original work by Duncan and Levison assumed that inclinations in the scattered disk are only of a few degrees, which has then been refuted by observations). Brasser and Morbidelli concluded that there are 2×10 9 bodies in the scattered disk today that are larger than 2.3 km in diameter.…”