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

¹ School of Physics and Astronomy, University of Glasgow, G12 8QQ, UK
e-mail: john.brown@glasgow.ac.uk
² Max-Planck-Institut für Astrophysik, 85748 Garching, Germany
³ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 92190 Meudon, France
⁴ NCAR High Altitude Observatory, Boulder, CO 80303, USA
⁵ Astrophysics Research Group. Trinity College, Dublin (TCD), Ireland

Received: 30 August 2010
Accepted: 12 October 2011

Abstract

Context. 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 M_o 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 (M_o,q) regimes: sublimation-, ablation-, and explosion-dominated. Most sun-grazers have M_o too small (<10¹¹ g) or q too large (>1.01R_⊙) 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 < 1.01R_⊙,M_o > 10¹¹ g, ablation initially dominates but results are sensitive to nucleus strength P_c = 10⁶P₆ dyne/cm² and entry angle φ to the vertical. Nuclei with M_o ≼ 10¹⁰(P₆secφ)³ g are fully ablated before exploding, though the hot wake itself explodes. For most sun-impactors secφ ≫ 1 (since q ~ r_∗), so for q very close to r_∗ the ablation regime applies to moderate  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 (M_o,q) regimes where sublimation, ablation and explosion dominate sun-grazer/-impactor destruction. For q ≺ 1.01R_⊙,M_o ≽ 10¹¹ g nuclei are destroyed by ablation or explosion (depending on M_ocos³φ/P_c) in the chromosphere, producing flare-like events with cometary abundance spectra. For all plausible M_o,q and physical parameters, nuclei are destroyed above the photosphere.