Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei

International audienceContext. Sun-grazing comets almost never re-emerge, but their sublimative destruction near the sun has only recently been observed directly, while chromospheric impacts have not yet been seen, nor impact theory developed. Aims: We seek simple analytic models of comet destruction processes near the sun, to enable estimation of observable signature dependence on original incident mass Mo and perihelion distance q. Methods: Simple analytic solutions are found for M(r) versus q and distance r for insolation sublimation and, for the first time, for impact ablation and explosion. Results: Sun-grazers are found to fall into three (Mo,q) regimes: sublimation-, ablation-, and explosion-dominated. Most sun-grazers have Mo too small (1.01R&sun;) to reach atmospheric densities (n > 2.5 × 1011/cm3) where ablation exceeds sublimation. Our analytic results for sublimation are similar to numerical models. For q &sun;,Mo > 1011 g, ablation initially dominates but results are sensitive to nucleus strength Pc = 106P6 dyne/cm2 and entry angle phi to the vertical. Nuclei with Mo ≼ 1010(P6secphi)3 g are fully ablated before exploding, though the hot wake itself explodes. For most sun-impactors secphi ≫ 1 (since q ~ r*), so for q very close to r* the ablation regime applies to moderate Mo ˜ 1013-16P_6^3 g impactors unless P6 ≼ 0.1. For higher masses, or smaller q, nuclei reach densities n > 2.5 × 1014P6/cm3 where ram pressure causes catastrophic explosion. Conclusions: Analytic descriptions define (Mo,q) regimes where sublimation, ablation and explosion dominate sun-grazer/-impactor destruction. For q ≺ 1.01R&sun;,Mo ≽ 1011 g nuclei are destroyed by ablation or explosion (depending on Mocos3phi/Pc) in the chromosphere, producing flare-like events with cometary abundance spectra. For all plausible Mo,q and physical parameters, nuclei are destroyed above the photosphere