The Physics of Cometary Anti-tails as Observed in 3I/ATLAS

TL;DR

The paper addresses the first robust observation of a cometary anti-tail in an interstellar object, 3I/ATLAS, at 3.8 au. It develops a Haser-type radial outflow model in which sublimating ice grains experience an anisotropic survival length set by the solar illumination angle, with CO2-driven gas sublimation on the nucleus and H2O ice grains in the coma. The key result is that the sunward anti-tail arises naturally from the angular variation of the snow line length, ell(a), affecting grain sizes, lifetimes, and observable surface-brightness profiles; fits to the HST data show different effective snow-line lengths along sunward and perpendicular directions. The model supports a volatile-dominated coma and suggests that the anti-tail is most visible at larger heliocentric distances and with high-resolution imaging, offering insights into the composition and evolution of interstellar comets.

Abstract

Observations of interstellar comet 3I/ATLAS at 3.8 au show an elongated coma similar to a cometary tail but pointing in the direction of the Sun, possibly the first instance of this type of anti-tail which is not a result of perspective. We explain the anti-tail as an anisotropic extension of the snow line, or survival radius of a sublimating ice grain, in the direction of the Sun. The anisotropy is due to the difference in the sublimation mass flux in the solar and perpendicular directions caused by the change in the illumination angle of the cometary surface. The stronger sublimation mass flux in the solar direction results in ice grains with larger sizes, longer sublimation lifetimes, and a snow line at a larger radial distance with respect to other directions. The observed radial surface brightness profiles as a function of illumination angle are well reproduced by a Haser-type radial outflow with constant velocity and sublimating ice grains with angularly dependent survival lengths.

Figures (8)

• Figure 1: Left: HST image in log$_{10}$ counts (electrons s$^{-1}$).
• Figure 2: Radial profiles of the surface brightness at angles of 10$^\circ$, 100$^\circ$, and 190$^\circ$ where the angles are measured with respect to the $x,y$ coordinate axes. The observed profiles are overlaid with fits to a model Haser-type surface brightness. The $x,y$ axes have units of pixels (0.04") and counts (electrons s$^{-1}$).
• Figure 3: HST image filtered by the rotational-gradient transform with ($\Delta r, \Delta \alpha) = (3,10)$ (equation \ref{['RG']} showing an absence of small scale anisotropy.
• Figure 4: Surface temperature of the nucleus as a function of angle for H$_2$O, CO$_2$, and mixed 80% H$_2$O 20% CO$_2$.
• Figure 5: Ratio $\psi$ (equation \ref{['psi']})