Implications of evaporative cooling by H$_2$ for 1I/`Oumuamua

Abstract: The first interstellar object observed in our solar system, 1I/`Oumuamua, exhibited several peculiar properties, including extreme elongation and non-gravitational acceleration. \cite{Bergner.2023} proposed that evaporation of trapped H$_2$ created by cosmic rays (CRs) can explain the non-gravitational acceleration. However, their calculation of surface temperature ignored the crucial cooling effect of evaporating H$_2$. By taking into account the cooling by H$_2$ evaporation, we show that the surface temperature of H$_2$-water ice is lower than the temperature obtained by Bergner and Seligman (2023) by a factor of 9. As a result, the thermal speed of outgassing H$_2$ is decreased by a factor of 3, which requires that all H$_2$ from water ice is dissociated by CRs in the interstellar medium, making the model untenable as an explanation for the properties of 1I/`Oumuamua. Moreover, the lower surface temperature also influences the thermal annealing of water ice, a key process that is appealed to by Bergner and Seligman (2023) as a mechanism to release H$_2$.