The rise time of the change of cometary brightness during its outburst

Abstract

The paper presents a new method for determining the change in the brightness of a comet during an outburst in which particles are ejected into the coma at a uniform rate, i.e. via a long-lived jet. In the proposed approach, the mass flow rate by a jet and the time it takes to increase the brightness of the comet play key roles. Absolute values adopted for H2O ice and CO2 ice were taken from measurements by Rosetta of the jets observed on 67P/Churyumov–Gerasimenko. These two parameters significantly affect the amplitude of the change in the brightness of a comet during its outburst. The performed numerical calculations concern a hypothetical comet from the Jupiter family for which it was assumed that its sublimation activity was controlled by the sublimation of water ice and carbon dioxide. Based on numerical simulations, it was shown that the difference between the change in brightness for H2O ice and CO2 ice is about −3.4 magnitude for the same fraction of the area that is active and the same mass flow rate. This value corresponds to the rise time of the amplitude of the brightness change t = 3 h. In addition, it should be emphasized that this difference is a consequence of thermodynamic parameters, such as the speed of sublimating gas molecules, the sublimation rate of given ice, which determine the numerical values of the individual scattering cross-sections C(t1) and C(t2).